# CVE-2026-39858

> Vulnerability · severity: **CRITICAL** (CVSS 10).

## Description

Traefik is an HTTP reverse proxy and load balancer. Prior to versions 2.11.43, 3.6.14, and 3.7.0-rc.2, there is a high severity authentication bypass vulnerability in Traefik's ForwardAuth and snippet-based authentication middleware. Traefik's forwarded-header sanitization logic targets only canonical header names (e.g., X-Forwarded-Proto) and does not strip or normalize alias variants that use underscores instead of dashes (e.g., X_Forwarded_Proto). These unsanitized alias headers are forwarded intact to the authentication backend. When the backend normalizes underscore and dash header forms equivalently, an attacker can inject spoofed trust context — such as a trusted scheme or host — through the alias headers and bypass authentication on protected routes without valid credentials. This issue has been patched in versions 2.11.43, 3.6.14, and 3.7.0-rc.2.

## Key facts

- **CVE ID:** CVE-2026-39858
- **Published:** 2026-04-30
- **CVSS severity:** CRITICAL
- **CVSS base score:** 10
- **CWE codes:** CWE-290, CWE-306

## Affected products

- `traefik:traefik`

## Primary sources

- NIST NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39858

## Citation

> AI Analytics. CVE-2026-39858. Retrieved 2026-07-20 from https://api.ai-analytics.org/cve/CVE-2026-39858. Derived from NIST NVD. Licensed CC0.

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