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High, CVSS 7.8amended

Traefik: Incomplete fix for CVE-2026-33433 + CVE-2026-39858 cross-cohort: headerField underscore-variant identity spoofing in BasicAuth / DigestAuth / ForwardAuth

CVE-2026-54763GHSA-x677-9fxg-v5c5

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
    <= 3.6.21>= 3.7.0, <= 3.7.5
    Fixed in
    3.6.22, 3.7.6

  • Go github.com/traefik/traefik/v2

    Affected
    <= 2.11.50
    Fixed in
    2.11.51

  • Go github.com/traefik/traefik

    Affected
    every version
    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-2026-54763: 0.24% estimated exploitation activity in the next 30 days; percentile 15.08 (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 vulnerability in Traefik's BasicAuth, DigestAuth, and ForwardAuth middlewares. The fix for CVE-2026-33433 stripped canonical-cased spoofed identity headers (e.g. X-Auth-User) before writing Traefik's own value, but did not account for underscore-variant header names (e.g. XAuthUser), which many backends normalize identically to the dashed form. An attacker able to reach a protected route could inject an underscore-variant header that survives Traefik's stripping and reaches the backend alongside — or, on the unauthenticated ForwardAuth authResponseHeaders path, instead of — the value Traefik intended to set, spoofing identity or authorization context. This is fixed by setting the new allowHeadersWithUnderscores: false entry point option, which strips all headers with underscores in their names before routing. Patches - https://github.com/traefik/traefik/releases/tag/v2.11.51 - https://github.com/traefik/traefik/releases/tag/v3.6.22 -…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 4 · amended ·

    Amended from ghsa record GHSA-x677-9fxg-v5c5

  2. Revision 3 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 2 · amended ·

    Amended from osv record GO-2026-6211

  4. Revision 1 · created ·

    Published from osv record GHSA-x677-9fxg-v5c5

Agent brief

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# Patch Next advisory brief: CVE-2026-54763

Advisory: Traefik: Incomplete fix for CVE-2026-33433 + CVE-2026-39858 cross-cohort: headerField underscore-variant identity spoofing in BasicAuth / DigestAuth / ForwardAuth
IDs: CVE-2026-54763, GHSA-x677-9fxg-v5c5
Record: https://patchnext.com/advisories/cve-2026-54763 (revision 4, source checked 26 Oct 2026, 15:28 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 3.7.6, 3.6.22; github.com/traefik/traefik/v2 2.11.51. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/traefik/traefik/v3: affected <= 3.6.21; >= 3.7.0, <= 3.7.5; fixed in 3.6.22, 3.7.6
- Go github.com/traefik/traefik/v2: affected <= 2.11.50; fixed in 2.11.51
- Go github.com/traefik/traefik: affected every version

## 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://osv.dev/vulnerability/GHSA-x677-9fxg-v5c5 (OSV record GHSA-x677-9fxg-v5c5 lists affected ranges: Go github.com/traefik/traefik/v2 >=0 =0 =3.7.0 <3.7.6 (fixed))
- Verified: https://osv.dev/vulnerability/GO-2026-6211 (OSV record GO-2026-6211 lists affected ranges: Go github.com/traefik/traefik >=0; Go github.com/traefik/traefik/v2 >=0 =0 =3.7.0 <3.7.6 (fixed))
- Verified: https://github.com/advisories/GHSA-x677-9fxg-v5c5 (GitHub-reviewed advisory GHSA-x677-9fxg-v5c5 lists affected ranges: Go github.com/traefik/traefik/v2 >=0 =0 =3.7.0 <=3.7.5)

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