CVE-2021-21411

OAuth2-Proxy is an open source reverse proxy that provides authentication with Google, Github or other providers. The --gitlab-group flag for group-based authorization in the GitLab provider stopped working in the v7.0.0 release. Regardless of the flag settings, authorization wasnt restricted. Additionally, any authenticated users had whichever groups were set in --gitlab-group added to the new X-Forwarded-Groups header to the upstream application. While adding GitLab project based authorization support in #630, a bug was introduced where the user sessions groups field was populated with the --gitlab-group config entries instead of pulling the individual users group membership from the GitLab Userinfo endpoint. When the session groups where compared against the allowed groups for authorization, they matched improperly (since both lists were populated with the same data) so authorization was allowed. This impacts GitLab Provider users who relies on group membership for authorization restrictions. Any authenticated users in your GitLab environment can access your applications regardless of --gitlab-group membership restrictions. This is patched in v7.1.0. There is no workaround for the Group membership bug. But --gitlab-project can be set to use Project membership as the authorization checks instead of groups; it is not broken.

Weakness

The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. This allows attackers to bypass intended access restrictions.

Affected Software

Name	Vendor	Start Version	End Version
Oauth2_proxy	Oauth2_proxy_project	7.0.0 (including)	7.1.0 (excluding)

Extended Description

Assuming a user with a given identity, authorization is the process of determining whether that user can access a given resource, based on the user’s privileges and any permissions or other access-control specifications that apply to the resource. When access control checks are incorrectly applied, users are able to access data or perform actions that they should not be allowed to perform. This can lead to a wide range of problems, including information exposures, denial of service, and arbitrary code execution.

Potential Mitigations

Divide the product into anonymous, normal, privileged, and administrative areas. Reduce the attack surface by carefully mapping roles with data and functionality. Use role-based access control (RBAC) [REF-229] to enforce the roles at the appropriate boundaries.
Note that this approach may not protect against horizontal authorization, i.e., it will not protect a user from attacking others with the same role.
Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
For example, consider using authorization frameworks such as the JAAS Authorization Framework [REF-233] and the OWASP ESAPI Access Control feature [REF-45].
For web applications, make sure that the access control mechanism is enforced correctly at the server side on every page. Users should not be able to access any unauthorized functionality or information by simply requesting direct access to that page.
One way to do this is to ensure that all pages containing sensitive information are not cached, and that all such pages restrict access to requests that are accompanied by an active and authenticated session token associated with a user who has the required permissions to access that page.

NVD	https://nvd.nist.gov/vuln/detail/CVE-2021-21411
CWE	https://cwe.mitre.org/data/definitions/863.html

Incorrect Authorization

Weakness

Affected Software

Extended Description

Potential Mitigations

References