CVE-2026-40912: Traefik has an StripPrefixRegex Middleware Authorization Bypass via Path/RawPath Desync

There is a high severity authentication bypass vulnerability in Traefik’s StripPrefixRegex middleware when used in combination with ForwardAuth, BasicAuth, or DigestAuth.

The middleware matches the regex against the decoded URL path but uses the resulting byte length to slice the percent-encoded raw path. When a dot (or multiple dots) appears in the prefix portion of the URL, the raw path after stripping becomes a dot-segment (e.g. /./admin/secret).

ForwardAuth receives this dot-segment path in X-Forwarded-Uri, which does not match the protected path patterns and therefore allows the request through. The backend then normalizes the dot-segment to the real path per RFC 3986 and serves the protected content

An unauthenticated attacker can exploit this against any backend that performs dot-segment normalization.

StripPrefixRegex uses the byte length of a decoded Path match to slice the encoded RawPath. When percent-encoded characters are in the prefix region, this produces a wrong RawPath. ForwardAuth then receives this wrong path in X-Forwarded-Uri, sees a path that doesn’t match its protection rules, and approves the request. The backend serves protected content.