CVE-2026-76835
Received Received - Intake

OAuth2 Proxy Authentication Bypass via X-Forwarded-Uri Header

Publication date: 2026-08-24

Last updated on: 2026-08-24

Assigner: VulnCheck

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.

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Meta Information

Published
2026-08-24
Last Modified
2026-08-24
Generated
2026-08-24
AI Q&A
2026-08-24
EPSS Evaluated
N/A
NVD
CVE-2026-76835
EUVD
EUVD-2026-65061

Affected Vendors & Products

Showing 5 associated CPEs
Vendor Product Version / Range
oauth2_proxy oauth2_proxy From 7.15.2 (inc) to 7.15.4 (inc)
oauth2_proxy oauth2_proxy From 7.15.0 (inc) to 7.15.2 (exc)
oauth2_proxy oauth2_proxy 7.15.4
oauth2_proxy oauth2_proxy 7.15.3
oauth2_proxy oauth2_proxy From 7.5.0 (inc) to 7.15.2 (exc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-290 This attack-focused weakness is caused by incorrectly implemented authentication schemes that are subject to spoofing attacks.

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Executive Summary

This vulnerability is an authentication bypass in OAuth2 Proxy versions 7.15.2 through 7.15.4. It occurs because the default configuration treats all clients as trusted proxies, allowing attackers to spoof the X-Forwarded-Uri header. By setting this header to a value matching an allow-listed route, an unauthenticated attacker can bypass authentication checks while the actual protected path is still forwarded to the upstream server.

Detection Guidance

Check if OAuth2 Proxy is running in reverse-proxy mode without trusted_proxy_ip configured. Inspect logs for requests with X-Forwarded-Uri headers matching skip-auth routes. Use curl to test if authentication can be bypassed by spoofing the header. Example: curl -H 'X-Forwarded-Uri: /public' http://target/admin

Verify if the server responds with protected content when X-Forwarded-Uri points to an allow-listed route. Check middleware configuration files for defaultTrustedProxyIPs settings.

Impact Analysis

This vulnerability allows unauthenticated attackers to access protected upstream paths by spoofing the X-Forwarded-Uri header. This could lead to unauthorized access to admin panels, APIs, or internal endpoints, potentially exposing sensitive data or allowing further attacks.

Compliance Impact

This vulnerability allows unauthenticated attackers to bypass authentication and access protected routes by spoofing the X-Forwarded-Uri header. This could lead to unauthorized access to sensitive data, violating confidentiality requirements in GDPR and HIPAA. Non-compliance risks include data breaches, unauthorized disclosures, and failure to implement proper access controls.

Mitigation Strategies

Upgrade to OAuth2 Proxy version 7.15.4 or later. Configure --trusted-proxy-ip with specific IP ranges or CIDR blocks instead of default 0.0.0.0/0. Strip or overwrite X-Forwarded-Uri headers at reverse proxy level before forwarding to OAuth2 Proxy.

Restrict direct client access to OAuth2 Proxy, ensuring it is only reachable through trusted reverse proxies. Minimize or remove --skip-auth-route and --skip-auth-regex rules where possible.

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