CVE-2026-39858

What it is

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.

At a glance

Exploitability

Can be exploited over the network without logging in. Limited direct impact.

Exploit odds

0.5% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2026-04-30 21:16 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-07-15 02:20 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 0.5% in the next 30 days (EPSS).
• Severity: 7.8 HIGH (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2026-04-30 21:16 UTC

Last modified

2026-07-15 02:20 UTC

Exploit probability

0.5% · EPSS, next 30 days

Weakness

CWE-290

Record status

Modified

Affected

• Traefik < 2.11.43
• Traefik < 3.6.14
• Traefik 3.7.0

Attack profile CVSS 4.0 · 7.8

Attack vector

Network

Complexity

Low

Attack requirements

None

Privileges needed

None

User interaction

None

Confidentiality

None

Integrity

None

Availability

None

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2026-39858.md and follow it. Check whether this project is affected by CVE-2026-39858. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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