OAuth2 Proxy is a reverse proxy that provides authentication using OAuth2 providers. Versions 7.5.0 through 7.15.1 have a configuration-dependent authentication bypass. Deployments are affected when all of the following are true: Use of skipauthroutes or the legacy skipauthregex; use of patterns that can be widened by attacker-controlled suffixes, such as ^/foo/./bar$ causing potential exposure of /foo/secret; and protected upstream applications that interpret # as a fragment delimiter or otherwise route the request to the protected base path. In deployments that rely on these settings, an unauthenticated attacker can send a crafted request containing a number sign in the path, including the browser-safe encoded form %23, so that OAuth2 Proxy matches a public allowlist rule while the backend serves a protected resource. Deployments that do not use these skip-auth options, or that only allow exact public paths with tightly scoped method and path rules, are not affected. A fix has been implemented in version 7.15.2 to normalize request paths more conservatively before skip-auth matching so fragment content does not influence allowlist decisions. Users who cannot upgrade immediately can reduce exposure by tightening or removing skipauthroutes and skipauthregex rules, especially patterns that use broad wildcards across path segments. Recommended mitigations include replacing broad rules with exact, anchored public paths and explicit HTTP methods; rejecting requests whose path contains %23 or # at the ingress, load balancer, or WAF level; and/or avoiding placing sensitive application paths behind broad skipauthroutes rules.

Impact

An authorization bypass exists in OAuth2 Proxy as part of the emaildomain enforcement option. An attacker may be able to authenticate with an email claim such as attacker@evil.com@company.com and satisfy an allowed domain check for company.com, even though the claim is not a valid email address.

The issue ONLY affects deployments that rely on emaildomain restrictions and accept email claim values from identity providers or claim mappings that do not strictly enforce normal email syntax. The practical risk ONLY exists in self-hosted or custom OIDC environments and federated setups where unexpected claim values can reach oauth2-proxy. Standard hosted providers that enforce valid email formatting ARE NOT effected.

Patches Users should upgrade to v7.15.2 or later once available.

Workarounds The most effective workaround is to ensure the configured identity provider cannot emit malformed or attacker-controlled email claim values.

Impact

A configuration-dependent authentication bypass exists in OAuth2 Proxy.

Deployments are affected when all of the following are true:

OAuth2 Proxy is configured with --reverse-proxy and at least one rule is defined with --skipauthroutes or the legacy --skip-auth-regex

OAuth2 Proxy may trust a client-supplied X-Forwarded-Uri header when --reverse-proxy is enabled and --skip-auth-route or --skip-auth-regex is configured. An attacker can spoof this header so OAuth2 Proxy evaluates authentication and skip-auth rules against a different path than the one actually sent to the upstream application.

This can result in an unauthenticated remote attacker bypassing authentication and accessing protected routes without a valid session.

Patches This issue is addressed as part of the newly introduced --trusted-proxy-ip flag in v7.15.2. If you leave it unset, OAuth2 Proxy will continue to trust ALL source IPs (0.0.0.0/0) for backwards compatibility, which means a client may still be able to spoof forwarded headers. Therefore after upgrading we urge you to use the new --trusted-proxy-ip flag to set the IPs or CIDR ranges of the reverse proxies that are allowed to send X-Forwarded- headers and furthermore implement the mitigation steps outlined below to properly configure your load balancer infrastructure.

Mitigation

- Strip any client-provided X-Forwarded-Uri header at the reverse proxy or load balancer level - Explicitly overwrite X-Forwarded-Uri with the actual request URI before forwarding requests to OAuth2 Proxy

Example nginx mitigation for the auth subrequest: location /internal-auth/ { internal; # Ensure external users can't access this path # Make sure the OAuth2 Proxy knows where the original request came from. proxysetheader Host $host; proxysetheader X-Real-IP $remoteaddr; # set the value to the actual $requesturi and therefore strip any user provided X-Forwarded-Uri proxysetheader X-Forwarded-Uri $requesturi; proxypass http://oauth2-proxy:4180/; } - Restrict direct client access to OAuth2 Proxy so it can only be reached through a trusted reverse proxy - Remove or narrow --skip-auth-route / --skip-auth-regex rules where possible

Impact A configuration-dependent authentication bypass exists in OAuth2 Proxy.

Deployments are affected when all of the following are true:

- OAuth2 Proxy is used with an authrequest-style integration (for example, nginx authrequest) - --ping-user-agent is set or --gcp-healthchecks is enabled

In affected configurations, OAuth2 Proxy will treat a request with the configured health check User-Agent value as a successful health check regardless of the requested path. This allows an unauthenticated remote attacker to bypass authentication and access protected upstream resources without completing the normal login flow.

This issue does not affect deployments that do not use authrequest-style subrequests, or that do not enable --ping-user-agent/--gcp-healthchecks.

Patches Users should upgrade to v7.15.2 or later once available. Deployments running versions prior to v7.15.2 should be considered affected if they use authrequest-style authentication together with --ping-user-agent or --gcp-healthchecks.

Workarounds Users can mitigate this issue by:

- disabling --gcp-healthchecks - removing any configured --ping-user-agent - ensuring the reverse proxy does not forward client-controlled User-Agent headers to the OAuth2 Proxy auth subrequest - using path-based health checks only, on dedicated health check endpoints

Example nginx mitigation for the auth subrequest:

nginx location = /oauth2/auth { internal; proxypass http://127.0.0.1:4180; proxypassrequestbody off; proxysetheader Content-Length ""; proxysetheader Host $host; # set to value that isn't the same as your configured PingUserAgent or GCPs "GoogleHC/1.0" proxysetheader User-Agent "oauth2-proxy-auth-request"; }

Impact A regression introduced in v7.11.0 is preventing OAuth2 Proxy from clearing the session cookie when rendering the sign-in page.

This only impacts deployments that rely on the sign-in page as part of their logout flow. In those setups, a user may be shown the sign-in page while the existing session cookie remains valid, so the browser session is not actually logged out. On shared workstations be it browsers or devices, a subsequent user could continue to use the previous user's authenticated session.

Deployments that use a dedicated logout/sign-out endpoint to terminate sessions are not affected.

Patches This issue is fixed in v7.15.2.

Workarounds Do not rely on the sign-in page to clear an existing session. Instead:

- Use the dedicated logout/sign-out endpoint of OAuth2 Proxy - Ensure your application logout flow explicitly clears the OAuth2 Proxy session cookie before redirecting users to the sign-in page - If needed, clear the session cookie at the reverse proxy or application layer as a temporary mitigation

OAuth2 Proxy before 5.0 has an open redirect vulnerability. Authentication tokens could be silently harvested by an attacker. This has been patched in version 5.0.