Echo static file middleware route protection bypass via encoded dot-dot traversal

Summary

Echo v5.2.0 contains a bypass of the fix for GHSA-vfp3-v2gw-7wfq. The fix rejects encoded path separators such as %2F and %5C before static file path unescaping, but two traversal forms still bypass route-level middleware protections for static files.

The primary bypass uses encoded dot-dot segments. A request such as /public/%2E%2E/admin/secret.txt passes the encoded-separator guard, is unescaped to public/../admin/secret.txt, then path.Clean resolves it to admin/secret.txt. If /admin/* is protected by middleware but the static filesystem is mounted at /, the protected file is returned through the public static route.

A second bypass exists when UseEscapedPathForMatching=true. In that mode, the router normalizes encoded separators before the static handler checks the wildcard. The handler receives a decoded wildcard with no % characters, so the encoded-separator guard does not run on the original encoded path.

Impact

An unauthenticated attacker can read files that an application intended to protect with route-level middleware, when those files are also reachable through a static filesystem mount after path normalization.

The high-severity variant works with the default router configuration and requires only encoded dot-dot path segments. The direct protected route returns 403, while the public static route returns the protected file content.

The UseEscapedPathForMatching=true variant is lower severity because it requires a non-defau…