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Critical Authentication Bypass Vulnerability Found in OAuth2 Proxy, No Patch Released

A severe authentication bypass vulnerability has been identified in the OAuth2 Proxy authentication proxy, tracked as CVE-2026-76835. The issue allows attackers to circumvent authentication by sending crafted X-Forwarded-Uri headers when the trusted_proxy_ip setting is not configured. This affects default reverse proxy configurations and enables unauthorized access to protected paths. Although version 7.15.2 addressed a related flaw under CVE-2026-40575, the vulnerability persists in standard reverse proxy setups. VulnCheck rated the flaw with a CVSS v4.0 base score of 9.3, classifying it as Critical. No patched release is currently available despite ongoing work in the source repository.

Security NEXTTranslated from Japanese

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A serious authentication bypass vulnerability has been disclosed in the popular authentication proxy OAuth2 Proxy. The flaw, identified as CVE-2026-76835, can be exploited when the proxy operates in reverse proxy mode without the trusted_proxy_ip configuration enabled.

Attackers can abuse a crafted X-Forwarded-Uri header to bypass authentication checks entirely. This allows unauthenticated access to paths that should be protected, even under default configuration settings.

The root cause lies in the proxy’s trust of forwarded headers sent by clients in reverse proxy environments. Security researchers note that this behavior enables direct access to restricted resources without valid credentials.

While version 7.15.2 introduced a fix for a related issue tracked as CVE-2026-40575, the vulnerability remains present in default reverse proxy deployments. The project repository shows active development toward a fix, yet no patched release has been published.

VulnCheck assigned the flaw a CVSS v4.0 base score of 9.3, rating it Critical. Organizations using OAuth2 Proxy in reverse proxy mode are strongly advised to implement compensating controls until an official update becomes available.

OAuth2 ProxyCVE-2026-76835CVE-2026-40575X-Forwarded-Uri
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