CVE-2026-34457: OAuth2 Proxy: Health Check User-Agent Matching Bypasses Authentication in auth_request Mode

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"; }