---
id: CVE-2025-66491
title: Traefik is an HTTP reverse proxy and load balancer
summary: >-
  Traefik is an HTTP reverse proxy and load balancer. Versions 3.5.0 through
  3.6.2 have inverted TLS verification logic in the
  nginx.ingress.kubernetes.io/proxy-ssl-verify annotation. Setting the
  annotation to "on" (intending to enable bac…
severity: medium
cvss: 5.9
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N'
cwe:
  - CWE-295
vendor: traefik
product: traefik
affected:
  - 'traefik >= 3.5.0, < 3.6.3'
patched:
  - traefik 3.6.3
published: '2025-12-09'
updated: '2026-10-14'
sourceUpdated: '2026-10-14T22:10:00.210'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2025-66491'
references:
  - url: >-
      https://github.com/traefik/traefik/commit/14a1aedf5704673d875d210d7bacf103a43c77e4
    label: security-advisories@github.com
  - url: 'https://github.com/traefik/traefik/releases/tag/v3.6.3'
    label: security-advisories@github.com
  - url: 'https://github.com/traefik/traefik/security/advisories/GHSA-7vww-mvcr-x6vj'
    label: security-advisories@github.com
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2025-66491'
  - url: 'https://github.com/traefik/traefik'
tags:
  - nvd
  - osv
  - go
epss: 0.00221
epssPercentile: 0.12554
aliases:
  - GHSA-7vww-mvcr-x6vj
  - GO-2025-4205
ecosystem: go
ingestedAt: '2026-09-12T03:13:01.765Z'
---

## Overview

Traefik is an HTTP reverse proxy and load balancer. Versions 3.5.0 through 3.6.2 have inverted TLS verification logic in 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. This issue is fixed in version 3.6.3.

## Affected

- `traefik >= 3.5.0, < 3.6.3`

## Remediation

Upgrade past the affected range:

- `traefik 3.6.3`

## Package advisory (CVE-2025-66491)

Affected packages:

- `github.com/traefik/traefik/v3 >= 3.5.0, < 3.6.3`

Patched in:

- `github.com/traefik/traefik/v3 3.6.3`

Source: https://osv.dev/vulnerability/GHSA-7vww-mvcr-x6vj