CVE-2026-76835

OAuth2 Proxy honours a client-supplied X-Forwarded-Uri header when deciding whether a request may skip authentication, because the guard added for CVE-2026-40575 is inert in the default reverse-proxy configuration. GetRequestURI prefers that header over the real request URI whenever CanTrustForwardedHeaders returns true, and isAllowedPath matches the skip_auth_routes and skip_auth_regex allow list against the resulting path. CanTrustForwardedHeaders grants that trust when the caller's address is in the trusted proxy set, and buildTrustedProxyNetSet falls back to defaultTrustedProxyIPs, which is 0.0.0.0/0 and ::/0, whenever reverse proxy mode is enabled without trusted_proxy_ip configured. Every client is therefore treated as a trusted proxy. An unauthenticated attacker can request a protected upstream path while setting X-Forwarded-Uri to a value matching an allow-listed route, so the skip-auth decision is made against the spoofed value while the upstream receives the protected path unchanged.

An unauthenticated attacker can bypass authentication and access protected resources, potentially leading to confidentiality and integrity breaches.

Configure trusted_proxy_ip with a list of trusted proxy addresses or update OAuth2 Proxy to a version that fixes the issue, preventing trust in the X-Forwarded-Uri header in default configuration.