OAuth 2.0 Security Best Current Practice

This section gives a detailed description of attacks on OAuth implementations, along with potential countermeasures. This section complements and enhances the description given in .

Some authorization servers allow clients to register redirect URI patterns instead of complete redirect URIs. In those cases, the authorization server, at runtime, matches the actual redirect URI parameter value at the authorization endpoint against this pattern. This approach allows clients to encode transaction state into additional redirect URI parameters or to register just a single pattern for multiple redirect URIs. As a downside, it turned out to be more complex to implement and error prone to manage than exact redirect URI matching. Several successful attacks have been observed in the wild, which utilized flaws in the pattern matching implementation or concrete configurations. Such a flaw 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.

For a public client using the grant type code, an attack would look as follows: Let's assume the redirect URL pattern "https://*.somesite.example/*" had been registered for the client "s6BhdRkqt3". This pattern allows redirect URIs pointing to any host residing in the domain somesite.example. So if an attacker manages to establish a host or subdomain in somesite.example he can impersonate the legitimate client. Assume the attacker sets up the host "evil.somesite.example". The attacker needs to trick the user into opening a tampered URL in his browser, which launches a page under the attacker's control, say "https://www.evil.example". This URL initiates an authorization request with the client id of a legitimate client to the authorization endpoint. This is the example authorization request (line breaks are for display purposes only):

The authorization server validates the redirect URI in order to identify the client. Since the pattern allows arbitrary domains host names in "somesite.example", the authorization request is processed under the legitimate client's identity. This includes the way the request for user consent is presented to the user. If auto-approval is allowed (which is not recommended for public clients according to ), the attack can be performed even easier. If the user does not recognize the attack, the code is issued and directly sent to the attacker's client. Since the attacker impersonated a public client, it can directly exchange the code for tokens at the respective token endpoint. Note: This attack will not directly work for confidential clients, since the code exchange requires authentication with the legitimate client's secret. The attacker will need to impersonate or utilize the legitimate client to redeem the code (e.g., by performing a code injection attack). This kind of injections is covered in Section .

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 attacks. 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 in order to get access to access tokens. This allows circumvention even of strict redirect URI patterns (but not strict URL matching!). Assume the 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. Same as above, the attacker needs to trick the user into opening a tampered URL in his browser, which launches a page under the attacker's control, say "https://www.evil.example". The URL initiates an authorization request, which is very similar to the attack on the code flow. As differences, it utilizes the open redirector by encoding "redirect_to=https://client.evil.example" into the redirect URI and it uses the response type "token" (line breaks are for display purposes only):

Since the redirect URI matches the registered pattern, the authorization server allows the request and sends the resulting access token with a 303 redirect (some response parameters are omitted for better readability)

At example.com, the request arrives at the open redirector. It will read the redirect parameter and will issue an HTTP 303 Location header redirect to the URL "https://client.evil.example/cb".

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:

The attacker's page at client.evil.example can access the fragment and obtain the access token.

The complexity of implementing and managing pattern matching correctly obviously causes security issues. This document therefore proposes to simplify the required logic and configuration by using exact redirect URI matching only. This means the authorization server shall compare the two URIs using simple string comparison as defined in , Section 6.2.1.. Additional recommendations: Servers on which callbacks are hosted must not expose open redirectors (see ). Clients may drop fragments via intermediary URLs with "fix fragments" (see ) to prevent the user agent from appending any unintended fragments. As an alternative to exact redirect URI matching, the AS could also authenticate clients, e.g., using .

Authorization codes or values of state can unintentionally be disclosed to attackers through the referrer header, by leaking either from a client's web site or from an AS's web site. Leakage from OAuth client: This 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 (ads, faq, ...) and a user clicks on such a link, or includes third-party content (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. Leakage from AS: In a similar way, an attacker can learn state if the authorization endpoint at the authorization server contains links or third-party content as above. Consequences: An attacker that learns a valid code through a referrer header can perform the same attacks as described in Section . 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..

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: Authorization codes SHOULD 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.) 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 referrer 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.) Bind authorization code to a confidential client or PKCE challenge. In this case, the attacker lacks the secret to request the code exchange. Suppress the referrer header by adding the attribute rel="noreferrer" to HTML links or by applying an appropriate Referrer Policy to the document (either as part of the "referrer" meta attribute or by setting a Referrer-Policy header). Use form post response mode instead of redirect for authorization response (see ).

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

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

An access token may end up in the browser history if a a client or just a web site, which already has a token, deliberately navigates to a page like "provider.com/get_user_profile?access_token=abcdef.". Actually discourages this practice and asks to transfer tokens via a header, but in practice web sites often just pass access token in query parameters. In 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. Proposed countermeasures: Replace implicit flow with postmessage communication or the authorization code grant Never pass access tokens in URL query parameters

Mix-up is an attack on scenarios where an OAuth client interacts with multiple authorization servers, as is usually the case when dynamic registration is used. The goal of the attack is to obtain an authorization code or an access token by tricking the client into sending those credentials to the attacker instead of using them at the respective endpoint at the authorization/resource server.

For a detailed attack description, refer to and . The description here closely follows , with variants of the attack outlined below. Preconditions: For the attack to work, we assume 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), 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, and the attacker can 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). Some of the attack variants described below require different preconditions. In the following, we assume 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). Attack on the authorization code grant: The user selects to start the grant using H-AS (e.g., by clicking on a button at the client's website). The attacker intercepts this request and changes the user's selection to "A-AS". The client stores in the user's session that the user selected "A-AS" and redirects the user to A-AS's authorization endpoint by sending the following response:

Now the attacker intercepts this response and changes the redirection such that the user is being redirected to H-AS. The attacker also replaces the client id of the client at A-AS with the client's id at H-AS, resulting in the following response being sent to the browser:

Now, the user authorizes the client to access her resources at H-AS. 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 a code injection attack as described in Section . Variants: In the implicit grant, the attacker receives an access token instead of the code; the rest of the attack works as above. A variant of the above attack works even if the first request/response pair cannot be intercepted (for example, because TLS is used to protect these messages): Here, we assume that the user wants to start the grant using A-AS (and not H-AS). After the client redirected the user to the authorization endpoint at A-AS, the attacker immediately redirects the user to H-AS (changing the client id "7ZGZldHQ"). (A vigilant user might at this point detect that she intended to use A-AS instead of H-AS.) The attack now proceeds exactly as in step of the attack description above. 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". We refer to for details. There are several variants that can be used to attack OpenID Connect. They are described in detail in , Appendix A, and , Section 6 ("Malicious Endpoints Attacks").

In scenarios where an OAuth client interacts with multiple authorization servers, clients MUST prevent mix-up attacks. Potential countermeasures: Configure authorization servers to return an AS identitifier (iss) and the client_id for which a code or token was issued in the authorization response. This enables clients to compare this data to their own client id and the iss identifier of the AS it believed it sent the user agent to. This mitigation is discussed in detail in . In OpenID Connect, if an ID token is returned in the authorization response, it carries client id and issuer. It can be used for this mitigation. As it can be seen in the preconditions of the attacks above, clients can prevent mix-up attack by (1) using AS-specific redirect URIs with exact redirect URI matching, (2) storing, for each authorization request, the intended AS, and (3) comparing the intended AS with the actual redirect URI where the authorization response was received.

In such an attack, the adversary attempts to inject a stolen authorization code into a legitimate client on a device under his control. In the simplest case, the attacker would want to use the code in his own client. But there are situations where this might not be possible or intended. Examples are: 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 code is bound to a particular confidential client and the attacker is unable to obtain the required client credentials to redeem the code himself. The authorization or resource servers are limited to certain networks, the attackers is unable to access directly. How does an attack look like? The attacker obtains an authorization code by performing any of the attacks described above. It performs a regular OAuth authorization process with the legitimate client on his device. The attacker injects the stolen authorization code in the response of the authorization server to the legitimate client. The client sends the code to the authorization server's token endpoint, along with client id, client secret and actual redirect_uri. 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 ). If all checks succeed, the authorization server issues access and other tokens to the client, so now the attacker is able to impersonate the legitimate user. 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 malware pretending to be the legitimate client 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 attempt to inject a code, which 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. 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 spec. 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 spec, since the tampered redirect URI is not considered. So any attempt to inject a code obtained using the client_id of a legitimate client or by utilizing the legitimate client on another device won't 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 memorize or re-construct the correct redirect URI for the call to the tokens 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.

There are multiple technical solutions to achieve this goal: OpenID Connect's existing nonce parameter could be used for this purpose. 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 associates the nonce to the authorization code and attests this binding in the ID token, which 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 victims device, where he has stolen the respective authorization code. The main advantage of this option is that Nonce is an existing feature used in the wild. On the other hand, leveraging Nonce by the broader OAuth community would require AS and client to adopt ID Tokens. The state parameter as specified in could be used similarly to what is described above. This would require to add a further parameter state to the code exchange token endpoint request. The authorization server would then compare the state value it associated with the code and the state value in the parameter. If those values do not match, it is considered an attack and the request fails. The advantage of this approach would be to utilize an existing OAuth parameter. But it would also mean to re-interpret the purpose of state and to extend the token endpoint request. The PKCE parameter challenge along with the corresponding verifier as specified in could be used in the same way as nonce or state. In contrast to its original intention, the verifier check would fail although the client uses its correct verifier but the code is associated with a challenge, which does not match. PKCE is a deployed OAuth feature, even though it is used today to secure native apps, only. Token binding could also be used. In this case, the code would need to be bound to two legs, between user agent and AS and the user agent and the client. This requires further data (extension to response) to manifest binding id for particular code. Token binding is promising as a secure and convenient mechanism (due to its browser integration). As a challenge, it requires broad browser support and use with native apps is still under discussion. One could use per instance client_id and secrets and bind the code to the respective client_id. Unfortunately, this does not fit into the web application programming model (would need to use per user client ids). PKCE seems to be the most obvious solution for OAuth clients as it available and effectively used today for similar purposes for OAuth native apps whereas nonce is appropriate for OpenId Connect clients. Note on pre-warmed secrets: An attacker can circumvent the countermeasures described above if he is able to create or capture the respective secret or code_challenge on a device under his control, which is then used in the victim's authorization request. Exact redirect URI matching of authorization requests can prevent the attacker from using the pre-warmed secret in the faked authorization transaction on the victim's device. Unfortunately, it does not work for all kinds of OAuth clients. It is effective for web and JS apps and for native apps with claimed URLs. Attacks on native apps using custom schemes or redirect URIs on localhost cannot be prevented this way, except if the AS enforces one-time use for PKCE verifier or nonce values.

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.

Standard CSRF defenses should be used to protect the redirection endpoint, for example: Use of CSRF tokens which are bound to the user agent and passed in the state parameter to the authorization server. The Origin header can be used to detect and prevent CSRF attacks. Since this feature, at the time of writing, is not consistently supported by all browsers, CSRF tokens should be used in addition to Origin header checking. For more details see .

Access tokens can leak from a resource server under certain circumstances.

An attacker may setup his own resource server and trick a client into sending access tokens to it, which are valid for other resource servers. 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 a certain resource server (and the respective URL) at development time, but client instances are configured with an resource server's URL at runtime. This kind of late binding is typical in situations where the client uses a standard API, e.g., for e-Mail, calendar, health, or banking and is configured by an user or administrator for the standard-based service, this particular user or company uses. There are several potential mitigation strategies, which will be discussed in the following sections.

An authorization server could provide the client with additional information about the location 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 metadata parameter resource_servers:

The AS could also return the URL(s) an access token is good for in the token response, illustrated by the example return parameter access_token_resource_server:

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 in the next sections, which provide a better balance between the involved parties.

As the name suggests, sender constrained access token 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 which 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 happens 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, it may utilize capabilities of the transport layer (e.g., TLS). Note: replay prevention is required as well! There exists several proposals to demonstrate the proof of possession in the scope of the OAuth working group: : In this approach, an access tokens is, via the so-called 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 than 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). : The approach as specified in this document allow use of mutual TLS for both client authentication and sender constraint access tokens. For the purpose of sender constraint 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. specifies an approach to sign HTTP requests. It 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. : 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 RFC 7800. 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. and are built on top of TLS and this way continue the successful OAuth 2.0 philosophy to leverage TLS to secure OAuth wherever possible. Both mechanisms allow prevention of access token leakage in a fairly client developer friendly way. There are some differences between both approaches: To start with, in all key material is automatically managed by the TLS stack whereas requires the developer to create and maintain the key pairs and respective certificates. Use of self-signed certificates, which is supported by the draft, significantly reduce the complexity of this task. Furthermore, allows to use different key pairs for different resource servers, which is a privacy benefit. On the other hand, only requires widely deployed TLS features, which means it might be easier to adopt in the short term. Application level signing approaches, like and have been debated for a long time in the OAuth working group without a clear outcome. As one advantage, application-level signing allows for end-to-end protection including non-repudiation even if the TLS connection is terminated between client and resource server. But deployment experiences have revealed challenges regarding robustness (e.g., reproduction of the signature base string including correct URL) as well as state management (e.g., replay prevention). This document therefore recommends implementors to consider one of TLS-based approaches wherever possible.

An audience restriction essentially restricts the resource server a particular access token can be used at. The authorization server associates the access token with a certain resource server and every resource server is obliged to verify for every request, whether the access token sent with that request was meant to be used at the particular resource server. If not, the resource server must refuse to serve the respective request. In the general case, audience restrictions limit the impact of a token leakage. In the case of a counterfeit resource server, it may (as described see below) also prevent abuse of the phished access token at the legitimate resource server. The audience can basically be expressed using logical names or physical addresses (like URLs). In order 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 needs to tell the authorization server, at which URL it will use the access token it is requesting. It could use the mechanism proposed or encode 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 legit 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 seems easy to use since it does not require any crypto on the client side. But since every access token is bound to a certain resource server, the client also needs to obtain different RS-specific access tokens, if it wants to access several resource services. has the same property, since different token binding ids must be associated with the access token. on the other hand allows a client to use the access token at multiple resource servers. It shall 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 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.

An attacker may compromise a resource server in order to get access to its resources and other resources of the respective deployment. Such a compromise may range from partial access to the system, e.g., its logfiles, to full control of the respective server. If the attacker was able to take over full control including shell access it will be able to circumvent all controls in place and access resources without access control. It will also get access to access tokens, which are sent to the compromised system and which potentially are valid for access to other resource servers as well. Even if the attacker "only" is able to access logfiles or databases of the server system, it may get access to valid access tokens. Preventing server breaches by way of hardening and monitoring server systems is considered a standard operational procedure and therefore out of scope of this document. This section will focus on the impact of such breaches on OAuth-related parts of the ecosystem, which is the replay of captured access tokens on the compromised resource server and other resource servers of the respective deployment. The following measures shall be taken into account by implementors in order to cope with access token replay: The resource server must treat access tokens like any other credentials. It is considered good practice to not log them and not to store them in plain text. Sender constraint access tokens as described in will prevent the attacker from replaying the access tokens on other resource servers. Depending on the severity of the penetration, it will also prevent replay on the compromised system. Audience restriction as described in may be used to prevent replay of captured access tokens on other resource servers.

The following attacks can occur when an AS or client has an open redirector, i.e., a URL which causes an HTTP redirect to an attacker-controlled web site.

Attackers could try to utilize a user's trust in the authorization server (and its URL in particular) for performing phishing attacks. , Section 4.1.2.1, already prevents open redirects by stating the AS MUST NOT automatically redirect the user agent in case of an invalid combination of client_id and redirect_uri. However, as described in , an attacker could also utilize a correctly registered redirect URI to perform phishing attacks. It could for example register a client via dynamic client registration and intentionally send an erroneous authorization request, e.g., by using an invalid scope value, to cause the AS to automatically redirect the user agent to its phishing site. The AS MUST take precautions to prevent this threat. Based on its risk assessment the AS needs to decide whether it can trust the redirect URI or not and SHOULD only automatically redirect the user agent, if it trusts the redirect URI. If not, it MAY inform the user that it is about to redirect her to the another site and rely on the user to decide or MAY just inform the user about the error.

Client MUST NOT expose URLs which could be utilized as open redirector. Attackers may use an open redirector to produce URLs which appear to point to the client, which might trick users to trust the URL and follow it in her browser. Another abuse case is to produce URLs pointing to the client and utilize them to impersonate a client with an authorization server. In order to prevent open redirection, clients should only expose such a function, if the target URLs are whitelisted or if the origin of a request can be authenticated.

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". However, when the status code 307 is used for redirection, the user agent will send the form data (user credentials) via HTTP POST to the client since this status code does not require the user agent to rewrite the POST request to a GET request (and thereby dropping the form data in the POST request body). If the relying party is malicious, it can use the credentials to impersonate the user at the AS. 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 credentials to the client. (In practice, however, most user agents will only show this behaviour for 307 redirects.) AS which redirect a request that potentially contains user 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, AS SHOULD use HTTP status code 303 "See Other".

A common deployment architecture for HTTP applications is to have the application server sitting behind a reverse proxy, which 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, which are relevant to OAuth, and give 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. 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 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 would be able to get access to the internal network between proxy and application server, it could also try to circumvent security controls in place. It is therefore important to ensure the authenticity of the communicating entities. Furthermore, the communication link between reverse proxy and application server must therefore be protected against tapping and injection (including replay prevention).