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High, CVSS 7.8

Traefik: Pre-authentication decision bypass due to forwarded alias spoofing

CVE-2026-39858GHSA-5m6w-wvh7-57vm

What to do

Partial vendor fix. Follow the primary source's mitigation guidance and check back for a fix.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • Go github.com/traefik/traefik/v3

    Affected
    >= 3.7.0-ea.1, < 3.7.0-rc.2>= 3.0.0-beta1, < 3.6.14
    Fixed in
    3.7.0-rc.2, 3.6.14

  • Go github.com/traefik/traefik/v2

    Affected
    < 2.11.43
    Fixed in
    2.11.43

  • Go github.com/traefik/traefik

    Affected
    <= 1.7.34
    Fixed in
    no fixed version recorded

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
7.8 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N · score source
EPSS
  • CVE-2026-39858: 0.48% estimated exploitation activity in the next 30 days; percentile 41.42 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary 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., XForwardedProto). 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. Patches - https://github.com/traefik/traefik/releases/tag/v2.11.43 - https://github.com/traefik/traefik/releases/tag/v3.6.14 - https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2 For more information If there are any questions or comments about this advisory, please open an issue. <details…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 1 · created ·

    Published from ghsa record GHSA-5m6w-wvh7-57vm

Agent brief

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# Patch Next advisory brief: CVE-2026-39858

Advisory: Traefik: Pre-authentication decision bypass due to forwarded alias spoofing
IDs: CVE-2026-39858, GHSA-5m6w-wvh7-57vm
Record: https://patchnext.com/advisories/cve-2026-39858 (revision 1, source checked 26 Oct 2026, 15:20 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Partial vendor fix; fixed versions by package: github.com/traefik/traefik/v3 3.7.0-rc.2, 3.6.14; github.com/traefik/traefik/v2 2.11.43. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/traefik/traefik/v3: affected >= 3.7.0-ea.1, < 3.7.0-rc.2; >= 3.0.0-beta1, < 3.6.14; fixed in 3.7.0-rc.2, 3.6.14
- Go github.com/traefik/traefik/v2: affected < 2.11.43; fixed in 2.11.43
- Go github.com/traefik/traefik: affected <= 1.7.34

## Suggested inspection (read-only)
- Go github.com/traefik/traefik/v3: check the installed version with your package manager.
- Go github.com/traefik/traefik/v2: check the installed version with your package manager.
- Go github.com/traefik/traefik: check the installed version with your package manager.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/traefik/traefik/v3, github.com/traefik/traefik/v2, github.com/traefik/traefik.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-5m6w-wvh7-57vm (GitHub-reviewed advisory GHSA-5m6w-wvh7-57vm lists affected ranges: Go github.com/traefik/traefik/v3 >=3.7.0-ea.1 =3.0.0-beta1 =0 =0 <=1.7.34)

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