| Web Authorization Protocol | T. Lodderstedt |
| Internet-Draft | yes.com |
| Intended status: Best Current Practice | J. Bradley |
| Expires: August 13, 2020 | Yubico |
| A. Labunets | |
| D. Fett | |
| yes.com | |
| February 10, 2020 |
OAuth 2.0 Security Best Current Practice
draft-ietf-oauth-security-topics-14
This document describes best current security practice for OAuth 2.0. It updates and extends the OAuth 2.0 Security Threat Model to incorporate practical experiences gathered since OAuth 2.0 was published and covers new threats relevant due to the broader application of OAuth 2.0.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 13, 2020.
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.
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Since its publication in [RFC6749] and [RFC6750], OAuth 2.0 ("OAuth" in the following) has gotten massive traction in the market and became the standard for API protection and the basis for federated login using OpenID Connect [OpenID]. While OAuth is used in a variety of scenarios and different kinds of deployments, the following challenges can be observed:
This document provides updated security recommendations to address these challenges. It does not supplant the security advice given in [RFC6749], [RFC6750], and [RFC6819], but complements those documents.
The remainder of this document is organized as follows: The next section summarizes the most important recommendations of the OAuth working group for every OAuth implementor. Afterwards, the updated the OAuth attacker model is presented. Subsequently, a detailed analysis of the threats and implementation issues that can be found in the wild today is given along with a discussion of potential countermeasures.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
This specification uses the terms "access token", "authorization endpoint", "authorization grant", "authorization server", "client", "client identifier" (client ID), "protected resource", "refresh token", "resource owner", "resource server", and "token endpoint" defined by OAuth 2.0 [RFC6749].
This section describes the set of security mechanisms the OAuth working group recommends to OAuth implementers.
When comparing client redirect URIs against pre-registered URIs, authorization servers MUST utilize exact string matching. This measure contributes to the prevention of leakage of authorization codes and access tokens (see Section 4.1). It can also help to detect mix-up attacks (see Section 4.4).
Clients MUST NOT expose URLs that forward the user’s browser to arbitrary URIs obtained from a query parameter ("open redirector"). Open redirectors can enable exfiltration of authorization codes and access tokens, see Section 4.9.1.
Clients MUST prevent Cross-Site Request Forgery (CSRF). In this context, CSRF refers to requests to the redirection endpoint that do not originate at the authorization server, but a malicious third party (see Section 4.4.1.8. of [RFC6819] for details). Clients that have ensured that the authorization server supports PKCE [RFC7636] MAY rely the CSRF protection provided by PKCE. In OpenID Connect flows, the nonce parameter provides CSRF protection. Otherwise, one-time use CSRF tokens carried in the state parameter that are securely bound to the user agent MUST be used for CSRF protection (see Section 4.7.1).
In order to prevent mix-up attacks (see Section 4.4), clients MUST only process redirect responses of the authorization server they sent the respective request to and from the same user agent this authorization request was initiated with. Clients MUST store the authorization server they sent an authorization request to and bind this information to the user agent and check that the authorization request was received from the correct authorization server. Clients MUST ensure that the subsequent token request, if applicable, is sent to the same authorization server. Clients SHOULD use distinct redirect URIs for each authorization server as a means to identify the authorization server a particular response came from.
An AS that redirects a request potentially containing user credentials MUST avoid forwarding these user credentials accidentally (see Section 4.10 for details).
Clients MUST prevent injection (replay) of authorization codes into the authorization response by attackers. The use of PKCE [RFC7636] is RECOMMENDED to this end. The OpenID Connect nonce parameter and ID Token Claim [OpenID] MAY be used as well. 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.
Note: although PKCE so far 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].
Authorization servers MUST provide a way to detect their support for PKCE. To this end, they MUST either (a) publish the element code_challenge_methods_supported in their AS metadata ([RFC8418]) containing the supported PKCE challenge methods (which can be used by the client to detect PKCE support) or (b) provide a deployment-specific way to ensure or determine PKCE support by the AS.
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 Section 4.1, Section 4.2, Section 4.3, and Section 4.6.
Moreover, no viable mechanism exists to cryptographically bind access tokens issued in the authorization response to a certain client as it is recommended in Section 2.2. This makes replay detection for such access tokens at resource servers impossible.
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" (aka 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 next section).
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 prerequisite for the acceptance of that token at the recipient (e.g., a resource server).
Authorization and resource servers SHOULD use mechanisms for sender-constrained access tokens to prevent token replay as described in Section 4.8.1.1.2. The use of Mutual TLS for OAuth 2.0 [RFC8705] is RECOMMENDED. Refresh tokens MUST be sender-constrained or use refresh token rotation as described in Section 4.12.
It is RECOMMENDED to use end-to-end TLS. If TLS traffic needs to be terminated at an intermediary, refer to Section 4.11 for further security advice.
The privileges associated with an access token SHOULD be restricted to the minimum required for the particular application or use case. This prevents clients from exceeding the privileges authorized by the resource owner. It also prevents users from exceeding their privileges authorized by the respective security policy. Privilege restrictions also help to reduce the impact of access token leakage.
In particular, access tokens SHOULD be restricted to certain resource servers (audience restriction), preferably to a single resource server. To put this into effect, the authorization server associates the access token with certain resource servers and every resource server is obliged to verify, for every request, whether the access token sent with that request was meant to be used for that particular resource server. If not, the resource server MUST refuse to serve the respective request. Clients and authorization servers MAY utilize the parameters scope or resource as specified in [RFC6749] and [I-D.ietf-oauth-resource-indicators], respectively, to determine the resource server they want to access.
Additionally, access tokens SHOULD be restricted to certain resources and actions on resource servers or resources. To put this into effect, the authorization server associates the access token with the respective resource and actions and every resource server is obliged to verify, for every request, whether the access token sent with that request was meant to be used for that particular action on the particular resource. If not, the resource server must refuse to serve the respective request. Clients and authorization servers MAY utilize the parameter scope as specified in [RFC6749] and authorization_details as specified in [I-D.ietf-oauth-rar] to determine those resources and/or actions.
The resource owner password credentials grant MUST NOT be used. This grant type insecurely exposes the credentials of the resource owner to the client. Even if the client is benign, this results in an increased attack surface (credentials can leak in more places than just the AS) and users are trained to enter their credentials in places other than the AS.
Furthermore, adapting the resource owner password credentials grant to two-factor authentication, authentication with cryptographic credentials (cf. WebCrypto [webcrypto], WebAuthn [webauthn]), and authentication processes that require multiple steps can be hard or impossible.
Authorization servers SHOULD use client authentication if possible.
It is RECOMMENDED to use asymmetric (public-key based) methods for client authentication such as mTLS [RFC8705] or private_key_jwt [OpenID]. When asymmetric methods for client authentication are used, authorization servers do not need to store sensitive symmetric keys, making these methods more robust against a number of attacks.
Authorization servers SHOULD NOT allow clients to influence their client_id or sub value or any other claim if that can cause confusion with a genuine resource owner (see Section 4.13).
In [RFC6819], an attacker model is laid out that describes the capabilities of attackers against which OAuth deployments must be protected. In the following, this attacker model is updated to account for the potentially dynamic relationships involving multiple parties (as described in Section 1), to include new types of attackers and to define the attacker model more clearly.
OAuth MUST ensure that the authorization of the resource owner (RO) (with a user agent) at the authorization server (AS) and the subsequent usage of the access token at the resource server (RS) is protected at least against the following attackers:
While an example for a web attacker would be a customer of an internet service provider, network attackers could be the internet service provider itself, an attacker in a public (wifi) network using ARP spoofing, or a state-sponsored attacker with access to internet exchange points, for instance.
These attackers conform to the attacker model that was used in formal analysis efforts for OAuth [arXiv.1601.01229]. This is a minimal attacker model. Implementers MUST take into account all possible attackers in the environment in which their OAuth implementations are expected to run. Previous attacks on OAuth have shown that OAuth deployments SHOULD in particular consider the following, stronger attackers in addition to those listed above:
(A3), (A4) and (A5) typically occur together with either (A1) or (A2).
Note that in this attacker model, an attacker (see A1) can be a RO or act as one. For example, an attacker can use his own browser to replay tokens or authorization codes obtained by any of the attacks described above at the client or RS.
This document focusses on threats resulting from these attackers. Attacks in an even stronger attacker model are discussed, for example, in [arXiv.1901.11520].
This section gives a detailed description of attacks on OAuth implementations, along with potential countermeasures. Attacks and mitigations already covered in [RFC6819] are not listed here, except where new recommendations are made.
Some authorization servers allow clients to register redirect URI patterns instead of complete redirect URIs. The authorization servers then match the redirect URI parameter value at the authorization endpoint against the registered patterns at runtime. This approach allows clients to encode transaction state into additional redirect URI parameters or to register a single pattern for multiple redirect URIs.
This approach turned out to be more complex to implement and more error prone to manage than exact redirect URI matching. Several successful attacks exploiting flaws in the pattern matching implementation or concrete configurations have been observed in the wild . Insufficient validation of the redirect URI effectively breaks client identification or authentication (depending on grant and client type) and allows the attacker to obtain an authorization code or access token, either
These attacks are shown in detail in the following subsections.
For a client using the grant type code, an attack may work as follows:
Assume the redirect URL pattern https://*.somesite.example/* is registered for the client with the client ID s6BhdRkqt3. The intention is to allow any subdomain of somesite.example to be a valid redirect URI for the client, for example https://app1.somesite.example/redirect. A naive implementation on the authorization server, however, might interpret the wildcard * as "any character" and not "any character valid for a domain name". The authorization server, therefore, might permit https://attacker.example/.somesite.example as a redirect URI, although attacker.example is a different domain potentially controlled by a malicious party.
The attack can then be conducted as follows:
First, the attacker needs to trick the user into opening a tampered URL in his browser that launches a page under the attacker's control, say https://www.evil.example (see Attacker A1.)
This URL initiates the following authorization request with the client ID of a legitimate client to the authorization endpoint (line breaks for display only):
GET /authorize?response_type=code&client_id=s6BhdRkqt3&state=9ad67f13
&redirect_uri=https%3A%2F%2Fattacker.example%2F.somesite.example
HTTP/1.1
Host: server.somesite.example
The authorization server validates the redirect URI and compares it to the registered redirect URL patterns for the client s6BhdRkqt3. The authorization request is processed and presented to the user.
If the user does not see the redirect URI or does not recognize the attack, the code is issued and immediately sent to the attacker's domain. If an automatic approval of the authorization is enabled (which is not recommended for public clients according to [RFC6749]), the attack can be performed even without user interaction.
If the attacker impersonated a public client, the attacker can exchange the code for tokens at the respective token endpoint.
This attack will not work as easily for confidential clients, since the code exchange requires authentication with the legitimate client's secret. The attacker can, however, use the legitimate confidential client to redeem the code by performing an authorization code injection attack, see Section 4.5.
Note: Vulnerabilities of this kind can also exist if the authorization server handles wildcards properly. For example, assume that the client registers the redirect URL pattern https://*.somesite.example/* and the authorization server interprets this as "allow redirect URIs pointing to any host residing in the domain somesite.example". If an attacker manages to establish a host or subdomain in somesite.example, he can impersonate the legitimate client. This could be caused, for example, by a subdomain takeover attack [subdomaintakeover], where an outdated CNAME record (say, external-service.somesite.example) points to an external DNS name that does no longer exist (say, customer-abc.service.example) and can be taken over by an attacker (e.g., by registering as customer-abc with the external service).
The attack described above works for the implicit grant as well. If the attacker is able to send the authorization response to a URI under his control, he will directly get access to the fragment carrying the access token.
Additionally, implicit clients can be subject to a further kind of attack. It utilizes the fact that user agents re-attach fragments to the destination URL of a redirect if the location header does not contain a fragment (see [RFC7231], Section 9.5). The attack described here combines this behavior with the client as an open redirector (see Section 4.9.1) in order to get access to access tokens. This allows circumvention even of very narrow redirect URI patterns, but not strict URL matching.
Assume the registered URL pattern for client s6BhdRkqt3 is https://client.somesite.example/cb?*, i.e., any parameter is allowed for redirects to https://client.somesite.example/cb. Unfortunately, the client exposes an open redirector. This endpoint supports a parameter redirect_to which takes a target URL and will send the browser to this URL using an HTTP Location header redirect 303.
The attack can now be conducted as follows:
First, and as above, the attacker needs to trick the user into opening a tampered URL in his browser that launches a page under the attacker's control, say https://www.evil.example.
Afterwards, the website initiates an authorization request that is very similar to the one in the attack on the code flow. Different to above, it utilizes the open redirector by encoding redirect_to=https://attacker.example into the parameters of the redirect URI and it uses the response type "token" (line breaks for display only):
GET /authorize?response_type=token&state=9ad67f13
&client_id=s6BhdRkqt3
&redirect_uri=https%3A%2F%2Fclient.somesite.example
%2Fcb%26redirect_to%253Dhttps%253A%252F
%252Fattacker.example%252F HTTP/1.1
Host: server.somesite.example
Now, since the redirect URI matches the registered pattern, the authorization server permits the request and sends the resulting access token in a 303 redirect (some response parameters omitted for readability):
HTTP/1.1 303 See Other
Location: https://client.somesite.example/cb?
redirect_to%3Dhttps%3A%2F%2Fattacker.example%2Fcb
#access_token=2YotnFZFEjr1zCsicMWpAA&...
At example.com, the request arrives at the open redirector. The endpoint will read the redirect parameter and will issue an HTTP 303 Location header redirect to the URL https://attacker.example/.
HTTP/1.1 303 See Other Location: https://attacker.example/
Since the redirector at client.somesite.example does not include a fragment in the Location header, the user agent will re-attach the original fragment #access_token=2YotnFZFEjr1zCsicMWpAA&... to the URL and will navigate to the following URL:
https://attacker.example/#access_token=2YotnFZFEjr1z...
The attacker's page at attacker.example can now access the fragment and obtain the access token.
The complexity of implementing and managing pattern matching correctly obviously causes security issues. This document therefore advises to simplify the required logic and configuration by using exact redirect URI matching only. This means the authorization server MUST compare the two URIs using simple string comparison as defined in [RFC3986], Section 6.2.1.
Additional recommendations:
If the origin and integrity of the authorization request containing the redirect URI can be verified, for example when using [I-D.ietf-oauth-jwsreq] or [I-D.ietf-oauth-par] with client authentication, the authorization server MAY trust the redirect URI without further checks.
The contents of the authorization request URI or the authorization response URI can unintentionally be disclosed to attackers through the Referer HTTP header (see [RFC7231], Section 5.5.2), by leaking either from the AS's or the client's web site, respectively. Most importantly, authorization codes or state values can be disclosed in this way. Although specified otherwise in [RFC7231], Section 5.5.2, the same may happen to access tokens conveyed in URI fragments due to browser implementation issues as illustrated by Chromium Issue 168213 [bug.chromium].
Leakage from the OAuth client requires that the client, as a result of a successful authorization request, renders a page that
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).
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.
An attacker that learns a valid code or access token through a Referer header can perform the attacks as described in Section 4.1.1, Section 4.5, and Section 4.6. If the attacker learns state, the CSRF protection achieved by using state is lost, resulting in CSRF attacks as described in [RFC6819], 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 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.
Countermeasures:
An access token may end up in the browser history if a client or a web site that already has a token deliberately navigates to a page like provider.com/get_user_profile?access_token=abcdef. [RFC6750] discourages this practice and advises to transfer tokens via a header, but in practice web sites often pass access tokens in query parameters.
In case of the implicit grant, a URL like client.example/redirection_endpoint#access_token=abcdef may also end up in the browser history as a result of a redirect from a provider's authorization endpoint.
Countermeasures:
Mix-up is an attack on scenarios where an OAuth client interacts with two or more authorization servers and at least one authorization server is under the control of the attacker. This can be the case, for example, if the attacker uses dynamic registration to register the client at his own authorization server or if an authorization server becomes compromised.
The goal of the attack is to obtain an authorization code or an access token for an uncompromised authorization server. This is achieved by tricking the client into sending those credentials to the compromised authorization server (the attacker) instead of using them at the respective endpoint of the uncompromised authorization/resource server.
The description here closely follows [arXiv.1601.01229], with variants of the attack outlined below.
Preconditions: For this variant of the attack to work, we assume that
The latter ability can, for example, be the result of a man-in-the-middle attack on the user's connection to the client. Note that an attack variant exists that does not require this ability, see below.
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:
Variants:
In scenarios where an OAuth client interacts with multiple authorization servers, clients MUST prevent mix-up attacks.
To this end, clients SHOULD use distinct redirect URIs for each AS (with alternatives listed below). Clients MUST store, for each authorization request, the AS they sent the authorization request to and bind this information to the user agent. Clients MUST check that the authorization request was received from the correct authorization server and ensure that the subsequent token request, if applicable, is sent to the same authorization server.
Unfortunately, distinct redirect URIs per AS do not work for all kinds of OAuth clients. They are effective for web and JavaScript 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.
If clients cannot use distinct redirect URIs for each AS, the following options exist:
In an authorization code injection attack, the attacker attempts to inject a stolen authorization code into the attacker's own session with the client. The aim is to associate the attacker's session at the client with the victim's resources or identity.
This attack is useful if the attacker cannot exchange the authorization code for an access token himself. Examples include:
In the following attack description and discussion, we assume the presence of a web (A1) or network attacker (A2).
The attack works as follows:
Obviously, the check in step (5.) will fail if the code was issued to another client ID, e.g., a client set up by the attacker. The check will also fail if the authorization code was already redeemed by the legitimate user and was one-time use only.
An attempt to inject a code obtained via a manipulated redirect URI should also be detected if the authorization server stored the complete redirect URI used in the authorization request and compares it with the redirect_uri parameter.
[RFC6749], Section 4.1.3, requires the AS to "... ensure that the redirect_uri parameter is present if the redirect_uri parameter was included in the initial authorization request as described in Section 4.1.1, and if included ensure that their values are identical.". In the attack scenario described above, the legitimate client would use the correct redirect URI it always uses for authorization requests. But this URI would not match the tampered redirect URI used by the attacker (otherwise, the redirect would not land at the attackers page). So the authorization server would detect the attack and refuse to exchange the code.
Note: this check could also detect attempts to inject an authorization code which had been obtained from another instance of the same client on another device, if certain conditions are fulfilled:
But this approach conflicts with the idea to enforce exact redirect URI matching at the authorization endpoint. Moreover, it has been observed that providers very often ignore the redirect_uri check requirement at this stage, maybe because it doesn't seem to be security-critical from reading the specification.
Other providers just pattern match the redirect_uri parameter against the registered redirect URI pattern. This saves the authorization server from storing the link between the actual redirect URI and the respective authorization code for every transaction. But this kind of check obviously does not fulfill the intent of the specification, since the tampered redirect URI is not considered. So any attempt to inject an authorization code obtained using the client_id of a legitimate client or by utilizing the legitimate client on another device will not be detected in the respective deployments.
It is also assumed that the requirements defined in [RFC6749], Section 4.1.3, increase client implementation complexity as clients need to store or re-construct the correct redirect URI for the call to the token endpoint.
This document therefore recommends to instead bind every authorization code to a certain client instance on a certain device (or in a certain user agent) in the context of a certain transaction using one of the mechanisms described next.
There are two good technical solutions to achieve this goal:
Other solutions, like binding state to the code, using token binding for the code, or per-instance client credentials are conceivable, but lack support and bring new security requirements.
PKCE is the most obvious solution for OAuth clients as it is available today (originally intended for OAuth native apps) whereas nonce is appropriate for OpenID Connect clients.
An attacker can circumvent the countermeasures described above if he can modify the nonce or code_challenge values that are used in the victim's authorization request. The attacker can modify these values to be the same ones as those chosen by the client in his own session in Step 2 of the attack above. (This requires that the victim's session with the client begins after the attacker started his session with the client.) If the attacker is then able to capture the authorization code from the victim, the attacker will be able to inject the stolen code in Step 3 even if PKCE or nonce are used.
This attack is complex and requires a close interaction between the attacker and the victim's session. Nonetheless, measures to prevent attackers from reading the contents of the authorization response still need to be taken, as described in Section 4.1, Section 4.2, Section 4.3, Section 4.4, and Section 4.9.
In an access token injection attack, the attacker attempts to inject a stolen access token into a legitimate client (that is not under the attacker's control). This will typically happen if the attacker wants to utilize a leaked access token to impersonate a user in a certain client.
To conduct the attack, the attacker starts an OAuth flow with the client using the implicit grant and modifies the authorization response by replacing the access token issued by the authorization server or directly makes up an authorization server response including the leaked access token. Since the response includes the state value generated by the client for this particular transaction, the client does not treat the response as a CSRF attack and uses the access token injected by the attacker.
There is no way to detect such an injection attack on the OAuth protocol level, since the token is issued without any binding to the transaction or the particular user agent.
The recommendation is therefore to use the authorization code grant type instead of relying on response types issuing acess tokens at the authorization endpoint. Authorization code injection can be detected using one of the countermeasures discussed in Section 4.5.
An attacker might attempt to inject a request to the redirect URI of the legitimate client on the victim's device, e.g., to cause the client to access resources under the attacker's control. This is a variant of an attack known as Cross-Site Request Forgery (CSRF).
The traditional countermeasure are CSRF tokens that are bound to the user agent and passed in the state parameter to the authorization server as described in [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:
AS therefore MUST provide a way to detect their support for PKCE either via AS metadata according to [RFC8414] or provide a deployment-specific way to ensure or determine PKCE support.
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 that are valid for other resource servers (see Attackers A1 and A5). If the client sends a valid access token to this counterfeit resource server, the attacker in turn may use that token to access other services on behalf of the resource owner.
This attack assumes the client is not bound to one specific resource server (and its URL) at development time, but client instances are provided with the resource server URL at runtime. This kind of late binding is typical in situations where the client uses a service implementing a standardized API (e.g., for e-Mail, calendar, health, or banking) and where the client is configured by a user or administrator for a service which this 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 non-standard metadata parameter resource_servers:
HTTP/1.1 200 OK
Content-Type: application/json
{
"issuer":"https://server.somesite.example",
"authorization_endpoint":
"https://server.somesite.example/authorize",
"resource_servers":[
"email.somesite.example",
"storage.somesite.example",
"video.somesite.example"
]
...
}
The AS could also return the URL(s) an access token is good for in the token response, illustrated by the example and non-standard return parameter access_token_resource_server:
HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8
Cache-Control: no-store
Pragma: no-cache
{
"access_token":"2YotnFZFEjr1zCsicMWpAA",
"access_token_resource_server":
"https://hostedresource.somesite.example/path1",
...
}
This mitigation strategy would rely on the client to enforce the security policy and to only send access tokens to legitimate destinations. Results of OAuth related security research (see for example [oauth_security_ubc] and [oauth_security_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 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:
There exist several proposals to demonstrate the proof of possession in the scope of the OAuth working group:
At the time of writing, OAuth Mutual TLS is the most widely implemented and the only standardized sender-constraining method. The use of OAuth Mutual TLS therefore is RECOMMENDED.
Note that the security of sender-constrained tokens is undermined when an attacker gets access to the token and the key material. This is in particular the case for corrupted client software and cross-site scripting attacks (when the client is running in the browser). If the key material is protected in a hardware or software security module or only indirectly accessible (like in a TLS stack), sender-constrained tokens at least protect against a use of the token when the client is offline, i.e., when the security module or interface is not available to the attacker. This applies to access tokens as well as to refresh tokens (see Section 4.12).
Audience restriction essentially restricts access tokens to a particular resource server. The authorization server associates the access token with the particular resource server and the resource server SHOULD verify the intended audience. If the access token fails the intended audience validation, the resource server must refuse to serve the respective request.
In general, audience restrictions limit the impact of token leakage. In the case of a counterfeit resource server, it may (as described below) also prevent abuse of the phished access token at the legitimate resource server.
The audience can be expressed using logical names or physical addresses (like URLs). 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 SHOULD tell the authorization server the intended resource server. The proposed mechanism [I-D.ietf-oauth-resource-indicators] could be used or by encoding the information in the scope value.
Instead of the URL, it is also possible to utilize the fingerprint of the resource server's X.509 certificate as audience value. This variant would also allow to detect an attempt to spoof the legitimate resource server's URL by using a valid TLS certificate obtained from a different CA. It might also be considered a privacy benefit to hide the resource server URL from the authorization server.
Audience restriction may seem easier to use since it does not require any crypto on the client-side. Still, since every access token is bound to a specific resource server, the client also needs to obtain a single RS-specific access token when accessing several resource servers. (Resource indicators, as specified in [I-D.ietf-oauth-resource-indicators], can help to achieve this.) [I-D.ietf-oauth-token-binding] has the same property since different token binding ids must be associated with the access token. Using [RFC8705], 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 to gain access to the resources of the respective deployment. Such a compromise may range from partial access to the system, e.g., its log files, to full control of the respective server.
If the attacker were able to gain full control, including shell access, all controls can be circumvented and all resources be accessed. The attacker would also be able to obtain other access tokens held on the compromised system that would potentially be valid to access other resource servers.
Preventing server breaches by hardening and monitoring server systems is considered a standard operational procedure and, therefore, out of the scope of this document. This section focuses on the impact of OAuth-related breaches and the replaying of captured access tokens.
The following measures should be taken into account by implementers in order to cope with access token replay by malicious actors:
The first and second recommendation also apply to other scenarios where access tokens leak (see Attacker A5).
The following attacks can occur when an AS or client has an open redirector. An open redirector is an endpoint that forwards a user’s browser to an arbitrary URI obtained from a query parameter.
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 follow it in their browser. This can be abused for phishing.
In order to prevent open redirection, clients should only redirect if the target URLs are whitelisted or if the origin and integrity of a request can be authenticated. Countermeasures against open redirection are described by OWASP [owasp_redir].
Just as with clients, attackers could try to utilize a user's trust in the authorization server (and its URL in particular) for performing phishing attacks. OAuth authorization servers regularly redirect users to other web sites (the clients), but must do so in a safe way.
[RFC6749], Section 4.1.2.1, already prevents open redirects by stating that the AS MUST NOT automatically redirect the user agent in case of an invalid combination of client_id and redirect_uri.
However, an attacker could also utilize a correctly registered redirect URI to perform phishing attacks. The attacker could, for example, register a client via dynamic client registration [RFC7591] and intentionally send an erroneous authorization request, e.g., by using an invalid scope value, thus instructing the AS to redirect the user agent to its phishing site.
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 and SHOULD only automatically redirect the user agent if it trusts the redirect URI. If the URI is not trusted, the AS MAY inform the user and rely on the user to make the correct decision.
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 [RFC6749], the HTTP status code 302 is used for this purpose, but "any other method available via the user-agent to accomplish this redirection is allowed". When the status code 307 is used for redirection instead, the user agent will send the user credentials via HTTP POST to the client.
This discloses the sensitive credentials to the client. If the relying party is malicious, it can use the credentials to impersonate the user at the AS.
The behavior might be unexpected for developers, but is defined in [RFC7231], Section 6.4.7. This status code does not require the user agent to rewrite the POST request to a GET request and thereby drop the form data in the POST request body.
In the HTTP standard [RFC7231], 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 hide the application server behind a reverse proxy that terminates the TLS connection and dispatches the incoming requests to the respective application server nodes.
This section highlights some attack angles of this deployment architecture with relevance to OAuth and gives recommendations for security controls.
In some situations, the reverse proxy needs to pass security-related data to the upstream application servers for further processing. Examples include the IP address of the request originator, token binding ids, and authenticated TLS client certificates. This data is usually passed in custom HTTP headers added to the upstream request.
If the reverse proxy would pass through any header sent from the outside, an attacker could try to directly send the faked header values through the proxy to the application server in order to circumvent security controls that way. For example, it is standard practice of reverse proxies to accept X-Forwarded-For headers and just add the origin of the inbound request (making it a list). Depending on the logic performed in the application server, the attacker could simply add a whitelisted IP address to the header and render a IP whitelist useless.
A reverse proxy must therefore sanitize any inbound requests to ensure the authenticity and integrity of all header values relevant for the security of the application servers.
If an attacker was able to get access to the internal network between proxy and application server, the attacker could also try to circumvent security controls in place. It is, therefore, essential to ensure the authenticity of the communicating entities. Furthermore, the communication link between reverse proxy and application server must be protected against eavesdropping, injection, and replay of messages.
Refresh tokens are a convenient and user-friendly way to obtain new access tokens after the expiration of access tokens. Refresh tokens also add to the security of OAuth since they allow the authorization server to issue access tokens with a short lifetime and reduced scope thus reducing the potential impact of access token leakage.
Refresh tokens are an attractive target for attackers since they represent the overall grant a resource owner delegated to a certain client. If an attacker is able to exfiltrate and successfully replay a refresh token, the attacker will be able to mint access tokens and use them to access resource servers on behalf of the resource owner.
[RFC6749] already provides a robust baseline protection by requiring
[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 behavior.
This specification gives recommendations beyond the scope of [RFC6749] and clarifications.
Authorization servers SHOULD determine, based on a risk assessment, whether to issue refresh tokens to a certain client. If the authorization server decides not to issue refresh tokens, the client MAY refresh access tokens by utilizing other grant types, such as the authorization code grant type. In such a case, the authorization server may utilize cookies and persistent grants to optimize the user experience.
If refresh tokens are issued, those refresh tokens MUST be bound to the scope and resource servers as consented by the resource owner. This is to prevent privilege escalation by the legitimate client and reduce the impact of refresh token leakage.
Authorization server MUST utilize one of these methods to detect refresh token replay by malicious actors for public clients:
Authorization servers MAY revoke refresh tokens automatically in case of a security event, such as:
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).
Resource servers may make access control decisions based on the identity of the resource owner as communicated in the sub claim returned by the authorization server in a token introspection response [RFC7662] or other mechanisms. If a client is able to choose its own client_id during registration with the authorization server, then there is a risk that it can register with the same sub value as a privileged user. A subsequent access token obtained under the client credentials grant may be mistaken for an access token authorized by the privileged user if the resource server does not perform additional checks.
Authorization servers SHOULD NOT allow clients to influence their client_id or sub value or any other claim if that can cause confusion with a genuine resource owner. Where this cannot be avoided, authorization servers MUST provide other means for the resource server to distinguish between access tokens authorized by a resource owner from access tokens authorized by the client itself.
As described in Section 4.4.1.9 of [RFC6819], the authorization request is susceptible to clickjacking. An attacker can use this vector to obtain the user's authentication credentials, change the scope of access granted to the client, and potentially access the user's resources.
Authorization servers MUST prevent clickjacking attacks. Multiple countermeasures are described in [RFC6819], including the use of the X-Frame-Options HTTP response header field and frame-busting JavaScript. In addition to those, authorization servers SHOULD also use Content Security Policy (CSP) level 2 [CSP-2] or greater.
To be effective, CSP must be used on the authorization endpoint and, if applicable, other endpoints used to authenticate the user and authorize the client (e.g., the device authorization endpoint, login pages, error pages, etc.). This prevents framing by unauthorized origins in user agents that support CSP. The client MAY permit being framed by some other origin than the one used in its redirection endpoint. For this reason, authorization servers SHOULD allow administrators to configure allowed origins for particular clients and/or for clients to register these dynamically.
Using CSP allows authorization servers to specify multiple origins in a single response header field and to constrain these using flexible patterns (see [CSP-2] for details). Level 2 of this standard provides a robust mechanism for protecting against clickjacking by using policies that restrict the origin of frames (using frame-ancestors) together with those that restrict the sources of scripts allowed to execute on an HTML page (by using script-src). A non-normative example of such a policy is shown in the following listing:
HTTP/1.1 200 OK Content-Security-Policy: frame-ancestors https://ext.example.org:8000 Content-Security-Policy: script-src 'self' X-Frame-Options: ALLOW-FROM https://ext.example.org:8000 ...
Because some user agents do not support [CSP-2], this technique SHOULD be combined with others, including those described in [RFC6819], unless such legacy user agents are explicitly unsupported by the authorization server. Even in such cases, additional countermeasures SHOULD still be employed.
We would like to thank Jim Manico, Phil Hunt, Nat Sakimura, Christian Mainka, Doug McDorman, Johan Peeters, Joseph Heenan, Brock Allen, Vittorio Bertocci, David Waite, Nov Matake, Tomek Stojecki, Dominick Baier, Neil Madden, William Dennis, Dick Hardt, Petteri Stenius, Annabelle Richard Backman, Aaron Parecki, George Fletscher, Brian Campbell, Konstantin Lapine, Tim Würtele, Guido Schmitz, Hans Zandbelt, Jared Jennings, Michael Peck, Pedram Hosseyni, Michael B. Jones, and Travis Spencer for their valuable feedback.
This draft includes no request to IANA.
All relevant security considerations have been given in the functional specification.
[[ To be removed from the final specification ]]
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