CVE-2025-66490: Traefik doesn't Prevent Path Normalization Bypass in Router + Middleware Rules

Impact

There is a potential vulnerability in Traefik managing the requests using a PathPrefix, Path or PathRegex matcher.

When Traefik is configured to route the requests to a backend using a matcher based on the path; if the request path contains an encoded restricted character from the following set ('/', '\', 'Null', ';', '?', '#'), it’s possible to target a backend, exposed using another router, by-passing the middlewares chain.

Example

yaml apiVersion: traefik.io/v1alpha1 kind: IngressRoute metadata: name: my-service spec: routes: - match: PathPrefix(‘/admin/’) kind: Rule services: - name: service-a port: 8080 middlewares: - name: my-security-middleware - match: PathPrefix(‘/’) kind: Rule services: - name: service-a port: 8080

In such a case, the request http://mydomain.example.com/admin%2F will reach the backend service-a without operating the middleware my-security-middleware and passing the security put in place for the /admin/ path.

Patches

- https://github.com/traefik/traefik/releases/tag/v2.11.32 - https://github.com/traefik/traefik/releases/tag/v3.6.4

For more information

If you have any questions or comments about this advisory, please open an issue.

<details> <summary>Original Description</summary>### Summary A vulnerability exists in Traefik’s path matching logic that allows attackers to bypass access-control middleware (e.g., blocking rules) by using URL-encoded paths. I found this vulnerability while playing PwnSec CTF 2025 with my team @0xL4ugh

Details Traefik evaluates router rules before decoding or normalizing the request path, but forwards the request after decoding to the backend service. As a result, routes meant to block access to sensitive endpoints (such as internal, beta, or admin endpoints) can be trivially bypassed.

PoC Traefik configuration used in this issue : [http.routers.flask-router-report-deny] entryPoints = ["web"] rule = "PathPrefix(/reportnote)" priority = 10 middlewares = ["block-access"] service = "flask-service"

[http.middlewares.block-access.replacePathRegex] regex = "." replacement = "/blocked" The intention is to block all access to /reportnote.

However, the following request bypasses the block: POST /%2freportnote HTTP/1.1 Host: localhost:62814

Impact Access Control Bypass: Any endpoint intended to be blocked (e.g., admin/debug/beta APIs) can be accessed by URL-encoding slashes or other characters.

This could lead to:

- Unauthorized access to restricted endpoints - Execution of protected internal functionality - Potential privilege escalation - Bypass of security policies enforced via Traefik routing rules </details>