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CVE-2026-39858

Traefik authentication bypass

10.0 CRITICALPublic exploitEPSS 0.60%
  1. Published30 Apr 2026

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 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeChanged
ConfidentialityHigh
IntegrityHigh
AvailabilityNone
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

Affected products

Traefik≥ 3.0.0 < 3.6.14 · < 2.11.43 · 3.7.0

Remediation

Fixed in
Traefik≥ 3.6.14

References

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Frequently asked

Is CVE-2026-39858 being actively exploited?

CVE-2026-39858 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 0.60%.

How severe is CVE-2026-39858?

CVE-2026-39858 has a CVSS 3.1 base score of 10.0, rated critical. EPSS estimates a 0.60% probability of exploitation.

Which products are affected by CVE-2026-39858?

Affected products include Traefik.

How do I fix CVE-2026-39858?

Upgrade affected software to a fixed version, for example Traefik 3.6.14 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.