{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 3.6.2"
      },
      "package": {
        "ecosystem": "Go",
        "name": "github.com/traefik/traefik/v3"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.5.0"
            },
            {
              "fixed": "3.6.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2025-66491"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-295"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2025-12-08T16:43:06Z",
    "nvd_published_at": "2025-12-09T01:16:55Z",
    "severity": "MODERATE"
  },
  "details": "## Impact\n\nThere is a potential vulnerability in Traefik NGINX provider managing the `nginx.ingress.kubernetes.io/proxy-ssl-verify` annotation.\n\nThe provider inverts the semantics of the `nginx.ingress.kubernetes.io/proxy-ssl-verify` annotation. Setting the annotation to `\"on\"` (intending to enable backend TLS certificate verification) actually disables verification, allowing man-in-the-middle attacks against HTTPS backends when operators believe they are protected.\n\n## Patches\n\n- https://github.com/traefik/traefik/releases/tag/v3.6.3\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\nA logic error in Traefik\u0027s experimental ingress-nginx provider inverts the semantics of the `nginx.ingress.kubernetes.io/proxy-ssl-verify` annotation. Setting the annotation to `\"on\"` (intending to enable backend TLS certificate verification) actually disables verification, allowing man-in-the-middle attacks against HTTPS backends when operators believe they are protected.\n\n### Details\n\nIn `pkg/provider/kubernetes/ingress-nginx/kubernetes.go` at line 512, the `InsecureSkipVerify` field is set using inverted logic:\n\n```go\nnst := \u0026namedServersTransport{\n    Name: provider.Normalize(namespace + \"-\" + name),\n    ServersTransport: \u0026dynamic.ServersTransport{\n        ServerName:         ptr.Deref(cfg.ProxySSLName, ptr.Deref(cfg.ProxySSLServerName, \"\")),\n        InsecureSkipVerify: strings.ToLower(ptr.Deref(cfg.ProxySSLVerify, \"off\")) == \"on\",\n    },\n}\n```\n\nThe expression `== \"on\"` evaluates to `true` when the annotation is `\"on\"`, setting `InsecureSkipVerify: true`. In Go\u0027s `crypto/tls`, `InsecureSkipVerify: true` means \"do not verify the server\u0027s certificate\" \u2014 the opposite of what `proxy-ssl-verify: \"on\"` should do according to NGINX semantics.\n\n**Current behavior:**\n| Annotation Value | InsecureSkipVerify | Actual Result |\n|------------------|-------------------|---------------|\n| `\"on\"` | `true` | Verification **disabled** \u274c |\n| `\"off\"` (default) | `false` | Verification **enabled** |\n\n**Expected behavior (per NGINX semantics):**\n| Annotation Value | InsecureSkipVerify | Expected Result |\n|------------------|-------------------|-----------------|\n| `\"on\"` | `false` | Verification **enabled** |\n| `\"off\"` (default) | `true` | Verification **disabled** |\n\nThe test in `pkg/provider/kubernetes/ingress-nginx/kubernetes_test.go` lines 397-403 confirms this inverted behavior is codified as \"expected\":\n\n```go\nServersTransports: map[string]*dynamic.ServersTransport{\n    \"default-ingress-with-proxy-ssl\": {\n        ServerName:         \"whoami.localhost\",\n        InsecureSkipVerify: true,  // Wrong: should be false when annotation is \"on\"\n        RootCAs:            []types.FileOrContent{\"-----BEGIN CERTIFICATE-----\"},\n    },\n},\n```\n\n**Affected versions:** v3.5.0 through current master (introduced in commit `9bd5c617820f2a8d23b50b68d114bb7bc464eccd`)\n\nPavel Kohout\nAisle Research\n\u003c/details\u003e\n\n-",
  "id": "GHSA-7vww-mvcr-x6vj",
  "modified": "2025-12-09T16:32:29Z",
  "published": "2025-12-08T16:43:06Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/traefik/traefik/security/advisories/GHSA-7vww-mvcr-x6vj"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-66491"
    },
    {
      "type": "WEB",
      "url": "https://github.com/traefik/traefik/commit/14a1aedf5704673d875d210d7bacf103a43c77e4"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/traefik/traefik"
    },
    {
      "type": "WEB",
      "url": "https://github.com/traefik/traefik/releases/tag/v3.6.3"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Traefik Inverted TLS Verification Logic in ingress-nginx Provider"
}