CVE-2026-39858
Traefik: Pre-authentication decision bypass due to forwarded alias spoofing
Impact
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.
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| go | github.com/traefik/traefik/v3 | >= 3.7.0-ea.1, < 3.7.0-rc.2 | 3.7.0-rc.2 |
| go | github.com/traefik/traefik/v2 | < 2.11.43 | 2.11.43 |
| go | github.com/traefik/traefik | <= 1.7.34 | none recorded |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Traefik | Traefik | before 2.11.43 from 3.0.0 before 3.6.14 Version 3.7.0 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/advisories/GHSA-5m6w-wvh7-57vm
- https://github.com/traefik/traefik/security/advisories/GHSA-5m6w-wvh7-57vm
- https://github.com/traefik/traefik/releases/tag/v2.11.43
- https://github.com/traefik/traefik/releases/tag/v3.6.14
- https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2
- https://nvd.nist.gov/vuln/detail/CVE-2026-39858
- 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