CVE-2025-66490: Github.com/traefik/traefik Path Normalization Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-gm3x-23wp-hc2c",
  "modified" : "2025-12-17T00:44:50Z",
  "published" : "2025-12-08T16:42:30Z",
  "aliases" : [ "CVE-2025-66490" ],
  "summary" : "Path Normalization Bypass in Traefik Router + Middleware Rules",
  "details" : "## Impact\n\nThere is a potential vulnerability in Traefik managing the requests using a `PathPrefix`, `Path` or `PathRegex` matcher.\n\nWhen 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.\n\n## Example\n\n```yaml\napiVersion: traefik.io/v1alpha1\nkind: IngressRoute\nmetadata:\n  name: my-service\nspec:\n  routes:\n    - match: PathPrefix(‘/admin/’)\n      kind: Rule\n      services:\n        - name: service-a\n          port: 8080\n      middlewares:\n        - name: my-security-middleware\n    - match: PathPrefix(‘/’)\n      kind: Rule\n      services:\n        - name: service-a\n          port: 8080\n```\n\nIn 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.\n\n## Patches\n\n- https://github.com/traefik/traefik/releases/tag/v2.11.32\n- https://github.com/traefik/traefik/releases/tag/v3.6.4\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<details>\n<summary>Original Description</summary>### Summary\nA 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\n\n### Details\nTraefik 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.\n\n### PoC\nTraefik configuration used in this issue :\n```[http.routers.flask-router-report-deny]\n  entryPoints = [\"web\"]\n  rule = \"PathPrefix(`/report_note`)\"\n  priority = 10\n  middlewares = [\"block-access\"]\n  service = \"flask-service\"\n\n[http.middlewares.block-access.replacePathRegex]\n  regex = \".*\"\n  replacement = \"/blocked\"\n```\nThe intention is to block all access to /report_note.\n\nHowever, the following request bypasses the block:\n```\nPOST /%2freport_note HTTP/1.1\nHost: localhost:62814\n\n\n```\n### Impact\nAccess Control Bypass:\nAny endpoint intended to be blocked (e.g., admin/debug/beta APIs) can be accessed by URL-encoding slashes or other characters.\n\nThis could lead to:\n\n- Unauthorized access to restricted endpoints\n- Execution of protected internal functionality\n- Potential privilege escalation\n- Bypass of security policies enforced via Traefik routing rules\n</details>",
  "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:L/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/traefik/traefik"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "1.7.34"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/traefik/traefik/v2"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.11.32"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/traefik/traefik/v3"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.6.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/traefik/traefik/security/advisories/GHSA-gm3x-23wp-hc2c"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2025-66490"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/traefik/traefik"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/traefik/traefik/releases/tag/v2.11.32"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/traefik/traefik/releases/tag/v3.6.4"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-436" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2025-12-08T16:42:30Z",
    "nvd_published_at" : "2025-12-09T01:16:55Z"
  }
}