GHSA-cxjq-mrr5-89rv

Summary

A path traversal vulnerability in the ReplacePathRegex middleware allows an unauthenticated remote attacker to bypass authentication middleware and access protected routes by sending a single crafted HTTP request. The vulnerability exists because ReplacePathRegex does not perform post-replacement path normalization validation - the same check added to StripPrefix in the fix for CVE-2026-48020 was not applied to ReplacePathRegex.

Details

When ReplacePathRegex is configured with a regex that captures user-controlled path segments without a mandatory path separator (e.g., regex: "^/api(.*)", replacement: "/$1"), an attacker can inject implicit traversal sequences into the capture group.

Root cause: pkg/middlewares/replacepathregex/replace_path_regex.go, function ServeHTTP (lines 56-74). After the regex substitution produces a new path, the middleware forwards it to the backend without checking whether the path normalizes differently - unlike StripPrefix which rejects such paths with HTTP 400 after the CVE-2026-48020 fix.

Attack flow:

1. Attacker sends GET /api../admin
2. sanitizePath passes it unchanged (api.. is a valid segment name, not a dot-segment)
3. Router matches PathPrefix(/api) → selects the public router (no auth middleware)
4. ReplacePathRegex applies ^/api(.*) → captures ../admin → replacement produces /../admin
5. No normalization check exists → path forwarded to backend as-is
6. Backend framework (Express, Flask, Django, Spring, ASP.NET) normalizes /../admin to /admin
7. Attacker receives protected content without authentication

Suggested fix: Add the same JoinPath equality check after line 67:

PoC

Prerequisites: Docker Engine 20.10+, Docker Compose v2, curl

1. Create docker-compose.yml:

2. Create dynamic.yml:

3. Create server.js:

4. Run and exploit:

Configuration note: The regex ^/api(.*) (without slash separator before the capture group) is the exploitable pattern. This is the natural way to write a prefix-strip equivalent with ReplacePathRegex and is functionally identical to StripPrefix("/api") for legitimate traffic. The pattern ^/api/(.*) (with mandatory slash) is not exploitable - the same structural narrowing as CVE-2026-48020 where StripPrefix("/api") was vulnerable but StripPrefix("/api/") was not.

Impact

Authentication bypass. Any route protected by auth middleware on a separate router (BasicAuth, ForwardAuth, DigestAuth) can be accessed without credentials by an unauthenticated network attacker via a single HTTP request. Both read and write operations (GET/POST/PUT/DELETE) bypass authentication. The vulnerability affects deployments using ReplacePathRegex for prefix stripping - a common, documented configuration pattern.