CVE-2025-66491

Traefik Inverted TLS Verification Logic in ingress-nginx Provider

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",
    },
}

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-----"},
    },
},

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.

CVE-2025-66491 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.6.3); upgrading removes the vulnerable code path.

Kodem intelligence

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

1. What is CVE-2025-66491? CVE-2025-66491 is a medium-severity security vulnerability in github.com/traefik/traefik/v3 (go), affecting versions >= 3.5.0, <= 3.6.2. It is fixed in 3.6.3.
2. How severe is CVE-2025-66491? CVE-2025-66491 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/traefik/traefik/v3 are affected by CVE-2025-66491? github.com/traefik/traefik/v3 (go) versions >= 3.5.0, <= 3.6.2 is affected.
4. Is there a fix for CVE-2025-66491? Yes. CVE-2025-66491 is fixed in 3.6.3. Upgrade to this version or later.
5. Is CVE-2025-66491 exploitable, and should I be worried? Whether CVE-2025-66491 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-66491 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-66491? Upgrade github.com/traefik/traefik/v3 to 3.6.3 or later.

Other vulnerabilities in github.com/traefik/traefik/v3