{"schema_version":"1.7.3","id":"GHSA-7vww-mvcr-x6vj","published":"2025-12-08T16:43:06Z","modified":"2026-07-02T14:44:23.840812270Z","aliases":["CVE-2025-66491","GO-2025-4205"],"related":["CGA-6h5r-m494-hr3j"],"summary":"Traefik Inverted TLS Verification Logic in ingress-nginx Provider","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<details>\n<summary>Original Description</summary>\n\n### Summary\n\nA logic error in Traefik's 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 := &namedServersTransport{\n    Name: provider.Normalize(namespace + \"-\" + name),\n    ServersTransport: &dynamic.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's `crypto/tls`, `InsecureSkipVerify: true` means \"do not verify the server's certificate\" — 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** ❌ |\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</details>\n\n-","affected":[{"package":{"name":"github.com/traefik/traefik/v3","ecosystem":"Go","purl":"pkg:golang/github.com/traefik/traefik/v3"},"ranges":[{"type":"SEMVER","events":[{"introduced":"3.5.0"},{"fixed":"3.6.3"}]}],"database_specific":{"last_known_affected_version_range":"<= 3.6.2","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2025/12/GHSA-7vww-mvcr-x6vj/GHSA-7vww-mvcr-x6vj.json"}}],"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"}],"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"},"severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N"}]}