GHSA-7x63-xv5r-3p2x OAuth2 Proxy has an Authentication Bypass via X-Forwarded-Uri Header Spoofing

• Go github.com/oauth2-proxy/oauth2-proxy/v7: >= 7.5.0, < 7.15.2 → fixed in 7.15.2

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 `--skip_auth_routes` 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. proxy_set_header Host $host; proxy_set_header X-Real-IP $remote_addr; # set the value to the actual $request_uri and therefore strip any user provided X-Forwarded-Uri proxy_set_header X-Forwarded-Uri $request_uri; proxy_pass 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