CVE-2026-39858
Traefik: Forwarded alias spoofing top pre-auth decision bypass
Description
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.
INFO
Published Date :
April 30, 2026, 9:16 p.m.
Last Modified :
Nov. 11, 2026, 9:10 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
Affected Products
The following products are affected by CVE-2026-39858
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | CRITICAL | [email protected] | ||||
| CVSS 3.1 | HIGH | 0b0ca135-0b70-47e7-9f44-1890c2a1c46c | ||||
| CVSS 4.0 | HIGH | [email protected] |
Solution
- Update Traefik to version 2.11.43 or later.
- Update Traefik to version 3.6.14 or later.
- Update Traefik to version 3.7.0-rc.2 or later.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-39858.
| URL | Resource |
|---|---|
| https://github.com/traefik/traefik/releases/tag/v2.11.43 | Product Release Notes |
| https://github.com/traefik/traefik/releases/tag/v3.6.14 | Product Release Notes |
| https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2 | Product Release Notes |
| https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm | Exploit Mitigation Patch Vendor Advisory |
| https://access.redhat.com/errata/RHSA-2026:21772 | |
| https://access.redhat.com/security/cve/CVE-2026-39858 | |
| https://bugzilla.redhat.com/show_bug.cgi?id=2464234 | |
| https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-39858.json |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-39858 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-39858
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-39858 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-39858 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Translated by [email protected]
Nov. 11, 2026
Action Type Old Value New Value Added Translation Title: Traefik, Description: Traefik es un proxy inverso HTTP y un balanceador de carga. Antes de las versiones 2.11.43, 3.6.14 y 3.7.0-rc.2, existe una vulnerabilidad de omisión de autenticación de alta gravedad en el middleware de autenticación ForwardAuth y basado en snippets de Traefik. La lógica de saneamiento de encabezados reenviados de Traefik se dirige solo a nombres de encabezado canónicos (por ejemplo, X-Forwarded-Proto) y no elimina ni normaliza variantes de alias que usan guiones bajos en lugar de guiones (por ejemplo, X_Forwarded_Proto). Estos encabezados de alias no saneados se reenvían intactos al backend de autenticación. Cuando el backend normaliza las formas de encabezado con guiones bajos y guiones de manera equivalente, un atacante puede inyectar contexto de confianza falsificado - como un esquema o host de confianza - a través de los encabezados de alias y omitir la autenticación en rutas protegidas sin credenciales válidas. Este problema ha sido parcheado en las versiones 2.11.43, 3.6.14 y 3.7.0-rc.2. -
CVE Modified by 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
Jul. 15, 2026
Action Type Old Value New Value Changed Affected [{'cpes': ['cpe:/a:redhat:openshift_devspaces:3.28::el9'], 'vendor': 'Red Hat', 'product': 'Red Hat OpenShift Dev Spaces 3.28', 'defaultStatus': 'affected'}] [{'cpes': ['cpe:/a:redhat:openshift_devspaces:3.28::el9'], 'vendor': 'Red Hat', 'product': 'Red Hat OpenShift Dev Spaces 3.28', 'versions': [{'status': 'unaffected', 'version': '1779786779', 'lessThan': '*', 'versionType': 'rpm'}], 'packageName': 'devspaces/traefik-rhel9', 'collectionURL': 'https://catalog.redhat.com/software/containers/', 'defaultStatus': 'affected'}] -
CVE Modified by 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
Jun. 30, 2026
Action Type Old Value New Value Added Affected [{'cpes': ['cpe:/a:redhat:openshift_devspaces:3.28::el9'], 'vendor': 'Red Hat', 'product': 'Red Hat OpenShift Dev Spaces 3.28', 'defaultStatus': 'affected'}] Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:L/A:N Added CWE CWE-289 Added Reference https://access.redhat.com/errata/RHSA-2026:21772 Added Reference https://access.redhat.com/security/cve/CVE-2026-39858 Added Reference https://bugzilla.redhat.com/show_bug.cgi?id=2464234 Added Reference https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-39858.json -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jun. 17, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-39858', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'yes'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-05-04T16:57:57.062955Z'} -
CVE Modified by [email protected]
Jun. 17, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'traefik', 'product': 'traefik', 'versions': [{'status': 'affected', 'version': '< 2.11.43'}, {'status': 'affected', 'version': '>= 3.0.0-beta1, < 3.6.14'}, {'status': 'affected', 'version': '>= 3.7.0-ea.1, < 3.7.0-rc.2'}]}] -
Initial Analysis by [email protected]
May. 01, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N Added CPE Configuration OR *cpe:2.3:a:traefik:traefik:3.7.0:ea1:*:*:*:*:*:* *cpe:2.3:a:traefik:traefik:3.7.0:ea2:*:*:*:*:*:* *cpe:2.3:a:traefik:traefik:*:*:*:*:*:*:*:* versions up to (excluding) 2.11.43 *cpe:2.3:a:traefik:traefik:*:*:*:*:*:*:*:* versions from (including) 3.0.0 up to (excluding) 3.6.14 *cpe:2.3:a:traefik:traefik:3.7.0:ea3:*:*:*:*:*:* *cpe:2.3:a:traefik:traefik:3.7.0:rc1:*:*:*:*:*:* Added Reference Type GitHub, Inc.: https://github.com/traefik/traefik/releases/tag/v2.11.43 Types: Product, Release Notes Added Reference Type GitHub, Inc.: https://github.com/traefik/traefik/releases/tag/v3.6.14 Types: Product, Release Notes Added Reference Type GitHub, Inc.: https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2 Types: Product, Release Notes Added Reference Type GitHub, Inc.: https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm Types: Exploit, Mitigation, Patch, Vendor Advisory -
New CVE Received by [email protected]
Apr. 30, 2026
Action Type Old Value New Value Added Description 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. Added CVSS V4.0 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 Added CWE CWE-306 Added CWE CWE-290 Added Reference https://github.com/traefik/traefik/releases/tag/v2.11.43 Added Reference https://github.com/traefik/traefik/releases/tag/v3.6.14 Added Reference https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2 Added Reference https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm