Traefik: Forwarded alias spoofing top pre-auth decision bypass

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Summary

CVECVE-2026-39858
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-04-30 21:16:32 UTC
Updated2026-07-15 02:20:53 UTC
DescriptionTraefik 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.

Risk And Classification

Primary CVSS: v4.0 7.8 HIGH from [email protected]

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

EPSS: 0.000510000 probability, percentile 0.157220000 (date 2026-05-05)

Problem Types: CWE-290 | CWE-306 | CWE-289 | CWE-290 CWE-290: Authentication Bypass by Spoofing | CWE-306 CWE-306: Missing Authentication for Critical Function | CWE-289 Authentication Bypass by Alternate Name

VersionSourceTypeScoreSeverityVector
4.0[email protected]Secondary7.8HIGHCVSS: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/C...
4.0CNADECLARED7.8HIGHCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N
3.1[email protected]Primary10CRITICALCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N
3.1ADPCVSS8.2HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N
3.10b0ca135-0b70-47e7-9f44-1890c2a1c46cSecondary8.2HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N

CVSS v4.0 Breakdown

Attack Vector
Network
Attack Complexity
Low
Attack Requirements
None
Privileges Required
None
User Interaction
None
Confidentiality
None
Integrity
None
Availability
None
Sub Conf.
High
Sub Integrity
High
Sub Availability
None

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

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Traefik Traefik All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Traefik Traefik affected < 2.11.43 Not specified
CNA Traefik Traefik affected >= 3.0.0-beta1, < 3.6.14 Not specified
CNA Traefik Traefik affected >= 3.7.0-ea.1, < 3.7.0-rc.2 Not specified
ADP Red Hat Red Hat OpenShift Dev Spaces 3.28 unaffected 1779786779 * rpm Not specified

References

ReferenceSourceLinkTags
github.com/traefik/traefik/releases/tag/v2.11.43 [email protected] github.com Product, Release Notes
security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-39858.json 0b0ca135-0b70-47e7-9f44-1890c2a1c46c security.access.redhat.com
github.com/traefik/traefik/releases/tag/v3.6.14 [email protected] github.com Product, Release Notes
bugzilla.redhat.com/show_bug.cgi 0b0ca135-0b70-47e7-9f44-1890c2a1c46c bugzilla.redhat.com
access.redhat.com/errata/RHSA-2026:21772 0b0ca135-0b70-47e7-9f44-1890c2a1c46c access.redhat.com
access.redhat.com/security/cve/CVE-2026-39858 0b0ca135-0b70-47e7-9f44-1890c2a1c46c access.redhat.com
github.com/traefik/traefik/releases/tag/v3.7.0-rc.2 [email protected] github.com Product, Release Notes
github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm [email protected] github.com Exploit, Mitigation, Patch, Vendor Advisory
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Additional Advisory Data

SourceTimeEvent
ADP2026-04-30T21:01:23.755ZReported to Red Hat.
ADP2026-04-30T20:26:26.300ZMade public.

Solutions

ADP: RHSA-2026:21772: Red Hat OpenShift Dev Spaces 3.28

Workarounds

ADP: Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.

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