CVE-2026-39858
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedTraefik 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceTraefik's forwarded-header sanitization only removes canonical header names (X-Forwarded-Proto) but fails to strip underscore variants (X_Forwarded_Proto). Attackers can inject spoofed trust context through these alias headers to bypass ForwardAuth and snippet-based authentication middleware when backends normalize underscore and dash header forms equivalently.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 2.11.43>= 3.0.0, < 3.6.14= 3.7.0CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- 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
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify Traefik versionRun `traefik version` or check the container/image version. Compare against affected ranges: < 2.11.43, >= 3.0.0 to < 3.6.14, or exactly 3.7.0Affected if The installed version falls within any of the affected version ranges
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Locate ForwardAuth middleware configurationsSearch Traefik configuration files (static and dynamic) for `forwardAuth` middleware definitions. Check for files like traefik.yml, traefik.toml, or dynamic configs referencing ForwardAuth.Affected if ForwardAuth middleware is configured and the Traefik version is affected
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Identify snippet-based authentication middlewareSearch for `plugin` or `snippet` references in Traefik config that handle authentication or authorization logic, particularly those that process incoming request headersAffected if Authentication middleware using plugins/snippets is active and the Traefik version is affected
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Inspect trusted header configurationExamine Traefik's entrypoint configuration for `forwardedHeaders` or similar trust settings. Look for configurations that specify which incoming headers are trusted from downstream clients.Affected if Trust headers configuration allows client-supplied headers (insecure setting) and the Traefik version is affected
You are affected if Traefik version is in the vulnerable range AND you use ForwardAuth or snippet-based authentication middleware with any configuration that trusts incoming headers containing X-Forwarded-* variants.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped2.11.433.6.14
Upgrade Traefik to versions 2.11.43, 3.6.14, or 3.7.0-rc.2 or later; additionally audit backend applications to ensure they do not treat underscore and dash header name variants as equivalent for trust decisions.
Upgrade to Traefik 2.11.43 (for 2.x users), 3.6.14 (for 3.0.0-3.6.13 users), or 3.7.0-rc.2+/3.7.1+ (for 3.7.0 users)
- 1. Identify your current Traefik version using `traefik version` or checking your deployment manifests
- 2. If running Traefik 2.x series (< 2.11.43): upgrade to version 2.11.43 or later
- 3. If running Traefik 3.0.0 - 3.6.13: upgrade to version 3.6.14 or later
- 4. If running Traefik 3.7.0: upgrade to version 3.7.0-rc.2 or later
- 5. After upgrade, verify the authentication middleware still functions correctly with your protected routes
- 6. Monitor logs for any authentication errors during the transition
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation3.0 h
- Implementation2.0 h
- Testing6.0 h
- Review / QA3.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-39858 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-39858 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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