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