CVE-2025-66491

EUVD-2025-201730 · Published Dec 8, 2025

Traefik Inverted TLS Verification Logic in 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:

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/kubernetes_test.go lines 397-403 confirms this inverted behavior is codified as "expected":

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>

Technologies

Traefik Traefik

Weaknesses

CWE-295

CVSS scores

v3.15.9CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

MITRE ATT&CK

Inferred

Where this vulnerability fits in an attack

ReconImpact

Initial Access

T1190Exploit Public-Facing Application

References

Potentially impacted assets

See if this affects your attack surface

Latest trending attack

Criticalyesterday

Google Brotli decoder - Remote Code Execution via compressed requests (Exploitation Check)

This control is the second step of the Patrowl response to CVE-2026-111497 (Node2Shell). It complements the first control of 22 October 2026, which focused on the Node.js runtime, and does not replace it. Why a second control CVE-2026-111497 is an out-of-bounds write in the Google Brotli decoder, the library that decompresses Brotli ("br") data. It was first published as a Node.js vulnerability, because Node.js bundles this decoder. Later publications showed that the flaw is in the library itself. Every product that embeds an affected version of the decoder is concerned, whatever its language or vendor. A public proof of concept demonstrates code execution. Any server that decompresses request bodies sent with "Content-Encoding: br" exposes the decoder to unauthenticated attackers. Known cases include Express 5 and NestJS 11 on Node.js, Koa and Fastify setups, Bun, Envoy and Istio with request decompression enabled, and any application or reverse proxy that decodes Brotli request bodies with an affected library. The flaw also works in the other direction. A malicious server can compromise an unpatched client that decodes a Brotli response, for example Node.js fetch(), Python clients (requests, httpx, aiohttp, Scrapy) with the Brotli or brotlicffi package, or libcurl builds with Brotli support. What this control checks Patrowl sends small, valid Brotli-compressed requests to each asset and observes whether the server decodes them. The check does not depend on the technology: it detects the behaviour, whatever the stack behind it. These requests are what any legitimate client could send. They never reach the vulnerable code path and cannot affect the availability of the service. How to read the result Potentially impacted means the asset decodes Brotli request bodies: the attack surface is exposed. It does not confirm that the asset is vulnerable. The version of the Brotli decoder in use is not visible from the outside. This check covers applications that parse request bodies on their root path. A Not impacted result does not rule out exposure on other routes, nor on the client side. Checking the versions on the servers remains the only exhaustive verification. Affected and fixed versions Google Brotli 1.0.3 to 1.2.0 are affected, fixed in 1.0.10, 1.1.1 and 1.2.1. Fixed releases of the main products embedding it: Node.js 22.23.4, 24.22.1 and 26.12.1 (end-of-life lines 20.x and older and 25.x receive no fix), Bun 1.4.4, Envoy 1.37.8, 1.38.6, 1.39.3 and 1.40.0, Istio 1.29.9, 1.30.6 and 1.31.2, Python Brotli 1.2.1 and brotlicffi 1.2.1.0. Products that link against the system library depend on the Brotli package of the Linux distribution. Recommendations Update every component that embeds the Brotli decoder (only real fix). Start with the assets flagged by this control. Restart the processes after the update. Rebuild containers, single-executable applications and appliances that bundle an affected runtime or library. Reduce the exposure until the update is deployed. Reject requests with "Content-Encoding: br" at the reverse proxy, load balancer or WAF, case-insensitively and including lists such as "gzip, br". On Envoy and Istio, disable request decompression for Brotli. Review outbound calls. List the features that fetch content from servers you do not control, and patch the services that host them first.