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Vulnerability record · CVE-2026-39858 · published 30 April 2026

CVE-2026-39858: Traefik authentication bypass by spoofing vulnerability

Traefik · Traefik

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.

7.8 CVSS 4.0 High EPSS 0.61% · top 53.2% CWE-290 · Authentication bypass by spoofingCWE-306 · Missing authentication for critical function
7.8CVSS 4.0 base score
0.61%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
8References, 1 tagged exploit
15 Jul 2026Last modified by NVD

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.

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:

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/traefik/traefik/releases/tag/v2.11.43 ProductRelease Notes
https://github.com/traefik/traefik/releases/tag/v3.6.14 ProductRelease Notes
https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2 ProductRelease Notes
https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm ExploitMitigationPatchVendor 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

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

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Source: NIST National Vulnerability Database (record CVE-2026-39858), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.