CVE-2026-76835: oauth2-proxy Authentication Bypass by Spoofing

Vulnerability summary

CVE-2026-76835 is a critical (CVSS 9.3) Authentication Bypass by Spoofing vulnerability in oauth2-proxy. EPSS estimates a 0.45% chance of exploitation in the next 30 days, higher than 38% of scored vulnerabilities.

Timeline

1. 24-08-2026
   PublishedVulnerability first disclosed.
2. 08-10-2026
   Last modifiedVulnerability information updated.

Description

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 in pkg/requests/util/util.go prefers that header over the real request URI whenever CanTrustForwardedHeaders returns true, and isAllowedPath in oauthproxy.go matches the skip_auth_routes and skip_auth_regex allow list against the resulting path. CanTrustForwardedHeaders in pkg/apis/middleware/scope.go 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.

• CWE-290Authentication Bypass by SpoofingThis attack-focused weakness is caused by incorrectly implemented authentication schemes that are subject to spoofing attacks.