CVE-2025-66491: Traefik has Inverted TLS Verification Logic in its ingress-nginx Provider

Impact

There is a potential vulnerability in Traefik NGINX provider managing the nginx.ingress.kubernetes.io/proxy-ssl-verify annotation.

The 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.

Patches

- https://github.com/traefik/traefik/releases/tag/v3.6.3

For more information

If you have any questions or comments about this advisory, please open an issue.

<details> <summary>Original Description</summary>

Summary

A 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.

Details

In pkg/provider/kubernetes/ingress-nginx/kubernetes.go at line 512, the InsecureSkipVerify field is set using inverted logic:

go nst := &namedServersTransport{ Name: provider.Normalize(namespace + "-" + name), ServersTransport: &dynamic.ServersTransport{ ServerName: ptr.Deref(cfg.ProxySSLName, ptr.Deref(cfg.ProxySSLServerName, "")), InsecureSkipVerify: strings.ToLower(ptr.Deref(cfg.ProxySSLVerify, "off")) == "on", }, }

The 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.

Current behavior: | Annotation Value | InsecureSkipVerify | Actual Result | |------------------|-------------------|---------------| | "on" | true | Verification disabled ❌ | | "off" (default) | false | Verification enabled |

Expected behavior (per NGINX semantics): | Annotation Value | InsecureSkipVerify | Expected Result | |------------------|-------------------|-----------------| | "on" | false | Verification enabled | | "off" (default) | true | Verification disabled |

The test in pkg/provider/kubernetes/ingress-nginx/kubernetestest.go lines 397-403 confirms this inverted behavior is codified as "expected":

go ServersTransports: map[string]dynamic.ServersTransport{ "default-ingress-with-proxy-ssl": { ServerName: "whoami.localhost", InsecureSkipVerify: true, // Wrong: should be false when annotation is "on" RootCAs: []types.FileOrContent{"-----BEGIN CERTIFICATE-----"}, }, },

Affected versions: v3.5.0 through current master (introduced in commit 9bd5c617820f2a8d23b50b68d114bb7bc464eccd)

Pavel Kohout Aisle Research </details>

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