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Fencer Vulns
high 8.2

CVE-2026-39858 json

Traefik: Forwarded alias spoofing top pre-auth decision bypass

published 2026-04-24 6 mo ago modified 2026-07-15 3 mo ago also EUVD-2026-26427 , GHSA-5m6w-wvh7-57vm , GO-2026-5149

cvss
8.2
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.5%
EPSS 0.5% chance of exploitation in 30 days, percentile 41%
public exploits
0
none known

Summary

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.

Patch / fix

  • go github.com/traefik/traefik/v3 3.6.14, 3.7.0-rc.2
  • go github.com/traefik/traefik/v2 2.11.43
  • Red Hat Red Hat OpenShift Dev Spaces 3.28 1779786779
  • govulncheck-vulndb openSUSE Leap 16.0 0.0.20260723T184607-160000.1.1
  • traefik openSUSE Tumbleweed 3.6.15-1.1
  • traefik2 openSUSE Tumbleweed 2.11.44-1.1

opensuse advisories openSUSE-SU-2026:21483-1openSUSE-SU-2026:10697-1openSUSE-SU-2026:10698-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-39858 updated cvss 8.2

traefik: Traefik: Authentication bypass via unsanitized alias headers

  • Red Hat OpenShift Dev Spaces 3.28 devspaces/traefik-rhel9 1779786779
    fixed: vendor fix RHSA-2026:21772
  • Red Hat OpenShift Dev Spaces 3.28 devspaces/udi-rhel9 1779829736, devspaces/code-rhel9 1779814592, devspaces/server-rhel9 1779342423, devspaces/openvsx-rhel9 1779528224, devspaces/udi-base-rhel9 1779741066, devspaces/code-sshd-rhel9 1779813519, +9 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
6 mo ago

Exploit prediction

0.48% chance of exploitation in the next 30 days, higher than 41% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 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
CISA ADP 3.1 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N
GitHub 3.1 10.0 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N
CNA 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
NVD 3.1 10.0 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N
CNA 3.1 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N

Affected packages

ecosystem package affected fixed in
go github.com/traefik/traefik/v3
>=3.7.0-ea.1, <3.7.0-rc.2
>=3.0.0-beta1, <3.6.14
3.7.0-rc.2
3.6.14
go github.com/traefik/traefik/v2
<2.11.43
2.11.43
go github.com/traefik/traefik
<=1.7.34
all versions
 no fix listed

Affected products

  • traefik traefik
  • Red Hat Red Hat OpenShift Dev Spaces 3.28

Classification

Weakness
  • CWE-290 Authentication Bypass by Spoofing
  • CWE-306 Missing Authentication for Critical Functionality
  • CWE-289 Authentication Bypass by Alternate Name

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.