CVE-2026-76835 in OAuth2 Proxy
Summary
by MITRE • 08/24/2026
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.
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Analysis
by VulDB Data Team • 08/24/2026
The vulnerability in OAuth2 Proxy stems from a critical flaw in how the application handles forwarded headers when determining authentication bypass conditions. Specifically, the system honors the client-supplied X-Forwarded-Uri header to decide whether a request should skip authentication checks. This behavior is driven by the GetRequestURI function located in pkg/requests/util/util.go, which prioritizes this header over the actual request URI whenever CanTrustForwardedHeaders returns true. The logic for trusting these headers relies on checking if the caller's address exists within a trusted proxy set defined by buildTrustedProxyNetSet. However, when reverse proxy mode is enabled without an explicit configuration of trusted_proxy_ip, the system falls back to defaultTrustedProxyIPs, which includes 0.0.0.0/0 and ::/0. This effectively treats every client as a trusted proxy, thereby allowing any incoming request to be considered trustworthy for header manipulation purposes.
This architectural oversight creates a significant security gap where an unauthenticated attacker can bypass authentication controls by spoofing the X-Forwarded-Uri header. The isAllowedPath function in oauthproxy.go matches skip_auth_routes and skip_auth_regex allow lists against the path derived from this potentially manipulated header rather than the actual request URI. Consequently, if an attacker sets the X-Forwarded-Uri to a value that matches an allow-listed route, the system decides to skip authentication based on this spoofed value. Meanwhile, the upstream server receives the original, protected path unchanged because only the internal decision-making process is deceived by the header manipulation. This discrepancy allows unauthorized access to resources that should require strict authentication verification.
From a technical classification perspective, this vulnerability aligns with CWE-287 Improper Authentication and CWE-436 Interpretation Conflict. The core issue lies in the inconsistent interpretation of input data where the application logic trusts client-supplied metadata over actual request context due to misconfigured trust boundaries. In terms of attack vectors, this flaw facilitates unauthorized access by exploiting header injection techniques similar to those described in MITRE ATT&CK technique T1584.002 Compromise Client Software Dependencies or more broadly under input validation failures that lead to privilege escalation within the application layer. The lack of strict source verification for forwarded headers is a common pitfall in reverse proxy configurations but becomes severe here due to the default fallback behavior.
To mitigate this vulnerability, it is imperative to explicitly configure trusted_proxy_ip with specific IP addresses or CIDR ranges corresponding only to legitimate load balancers or proxies that are authorized to set these headers. Relying on the default 0.0.0.0/0 and ::/0 settings in production environments effectively disables any meaningful trust boundary for forwarded header validation. Additionally, developers should review the implementation of GetRequestURI to ensure it strictly validates the source of trusted headers or defaults to using the actual request URI when the client address is not explicitly verified as a known proxy. Implementing strict allow-lists for X-Forwarded-* headers and ensuring that authentication decisions are always based on the canonical request path rather than manipulated metadata will significantly reduce the risk of such bypasses. Regular security audits focusing on header handling logic in reverse proxy configurations can help identify similar misconfigurations before they are exploited.