## Summary There is a high severity vulnerability in Traefik's Kubernetes Ingress NGINX provider. When an Ingress uses the `nginx.ingress.kubernetes.io/rewrite-target` annotation with a regular expression that captures attacker-controlled text without requiring a path separator (for example path `/api(.*)` with rewrite target `/$1`), the generated `RewriteTarget` middleware can turn an initially safe request path into a dot-segment traversal path after the router has already been selected. ## Patches - https://github.com/traefik/traefik/releases/tag/v3.7.8 ## For more information If you have any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues). <details> <summary>Original Description</summary> ## Summary Traefik's Kubernetes Ingress NGINX provider creates an internal `RewriteTarget` middleware for the `nginx.ingress.kubernetes.io/rewrite-target` annotation. When an Ingress path captures attacker-controlled text without requiring a path separator, the middleware can turn an initially safe path into a dot-segment traversal path after Traefik has already selected the router. For example, with Ingress path `/api(.*)` and rewrite target `/$1`, an unauthenticated request to `/api../admin` follows this flow: 1. The default entry-point path sanitizer leaves `/api../admin` unchanged because `api..` is one ordinary segment. 2. The public router's `PathRegexp("(?i)^/api(.*)")` rule matches. 3. `RewriteTarget` captures `../admin` and creates `/../admin`. 4. The middleware forwards `/../admin` without checking whether path normalization changes it. 5. A backend that normalizes paths resolves `/../admin` to `/admin`. 6. The request reaches content intended to be reachable only through a separate `/admin` router with BasicAuth, DigestAuth, or ForwardAuth. This is an unpatched sibling of [GHSA-cxjq-mrr5-89rv](https://github.com/traefik/traefik/security/advisories/GHSA-cxjq-mrr5-89rv), which added post-replacement normalization validation to `ReplacePathRegex`. The separate ingress-nginx `RewriteTarget` implementation did not receive the same validation. The bypass remains exploitable in the patched Traefik v3.7.7 release. ## Severity **Proposed severity:** Critical **CVSS 3.1:** 9.1 — `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N` - Attack vector: Network - Attack complexity: Low once the affected routing pattern exists - Privileges required: None - User interaction: None - Scope: Unchanged - Confidentiality: High - Integrity: High - Availability: None **Primary weakness:** CWE-22 — Improper Limitation of a Pathname to a Restricted Directory (Path Traversal) **Secondary weakness:** CWE-288 — Authentication Bypass Using an Alternate Path or Channel The practical impact depends on the protected backend paths. If they are read-only or low sensitivity, environmental severity may be lower. ### Exploitation Preconditions - The Kubernetes Ingress NGINX provider is enabled. - A public Ingress uses `rewrite-target` with a regex that can capture `..` adjacent to the matched prefix, such as `/api(.*)` with `/$1`. - A protected router exposes another path on the same backend, such as `/admin`, and relies on a Traefik authentication or authorization middleware. - The backend normalizes dot segments before dispatching the request. These are deployment prerequisites; the remote attacker needs no credentials or special timing. ## Affected Components ### Confirmed versions - Traefik v3.7.0 through v3.7.7 - Current `master` at commit `b93f02cd07b79490fb8c8f02e301a7a1ec553195` - Current `v3.7` branch at `69259c3acc9d4bdc065cb2e3b83336f7de3e7038` The vulnerable middleware is present in every stable v3.7 release checked. The v2.11 and v3.6 branches do not contain this ingress-nginx `RewriteTarget` implementation. ### Code locations - `pkg/provider/kubernetes/ingress-nginx/middleware.go:257-274` - Converts the Ingress path and `rewrite-target` annotation directly into `dynamic.RewriteTarget` configuration. - `pkg/middlewares/ingressnginx/rewritetarget/rewrite_target.go:85-157` - Performs capture-based path rewriting and forwards the rewritten path without normalization validation. - `pkg/server/middleware/middlewares.go:346-353` - Instantiates the vulnerable middleware in the live HTTP chain. ## Root Cause The provider passes the route regex and annotation replacement into the middleware: ```go loc.RewriteTarget = &dynamic.RewriteTarget{ Regex: loc.Path, Replacement: rewrite, } ``` `RewriteTarget.ServeHTTP` then derives a path from attacker-controlled capture groups: ```go newTarget = rt.regexp.ReplaceAllString(currentPath, rt.replacement) req.URL.RawPath = newTarget req.URL.Path, err = url.PathUnescape(req.URL.RawPath) req.RequestURI = req.URL.RequestURI() rt.next.ServeHTTP(rw, req) ``` There is no invariant check between `PathUnescape` and forwarding to ensure that `req.URL.Path` equals its normalized form. Because routing happen

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