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CVE-2026-33433 traefik CVE debrief

CVE-2026-33433 is a medium-severity vulnerability in Traefik, an HTTP reverse proxy and load balancer. An authenticated attacker can inject their own canonical version of a non-canonical HTTP header to impersonate any identity to the backend. The vulnerability exists because Traefik writes non-canonical header names, allowing an attacker to override them with their own canonical version. This issue affects Traefik versions prior to 2.11.42, 3.6.11, and 3.7.0-ea.3. Users of affected versions should update to a patched version as soon as possible.

Vendor
traefik
Product
Unknown
CVSS
MEDIUM 5.1
CISA KEV
Not listed in stored evidence
Original CVE published
2026-03-27
Original CVE updated
2026-07-15
Advisory published
2026-03-27
Advisory updated
2026-07-15

Who should care

Users of Traefik, an HTTP reverse proxy and load balancer, should be aware of this vulnerability if they are using versions prior to 2.11.42, 3.6.11, or 3.7.0-ea.3. This includes administrators and security teams responsible for maintaining Traefik installations, especially in environments where authentication and header handling are critical.

Technical summary

The vulnerability arises from Traefik's handling of HTTP headers. When a non-canonical HTTP header name is configured (e.g., 'x-auth-user' instead of 'X-Auth-User'), an authenticated attacker can inject their own canonical version of that header. This allows the attacker to impersonate any identity to the backend, as the backend reads the attacker-injected canonical header first, overriding Traefik's non-canonical write. The CVSS score for this vulnerability is 5.1, indicating a medium severity level.

Defensive priority

Defenders should prioritize patching Traefik installations to versions 2.11.42, 3.6.11, or 3.7.0-ea.3. In the meantime, they should monitor for suspicious header activity and ensure that only authorized users can configure header fields.

Recommended defensive actions

  • Update Traefik to version 2.11.42, 3.6.11, or 3.7.0-ea.3 or later.
  • Monitor Traefik logs for suspicious header activity.
  • Restrict access to configure header fields to authorized users only.
  • Review and update configurations to use canonical HTTP header names where possible.
  • Implement additional monitoring to detect potential impersonation attempts.

Evidence notes

The CVE record and NVD detail provide official information about the vulnerability. Vendor advisories and release notes from Traefik offer patch details. Red Hat errata and security advisories provide additional context for affected users.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-33433 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-33433

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-33433 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-33433

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.