Critical [CVE-2026-88877] Traefik v3.7.0 Authentication Bypass via from-to-www-redirect
This critical-severity Red Hat Linux advisory covers CVE-2026-88877.
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Summary
A flaw was found in Traefik's Kubernetes ingress-nginx provider. A remote attacker, by sending a specially crafted request with a malformed Host header (e.g., containing a non-numeric port), can bypass security middlewares, including authentication and IP allowlisting.
This leads to unauthorized access to protected backend services. The provider generates a sibling router with a RedirectRegex middleware that matches on host alone.
Requests containing non-numeric or empty ports in the Host header (e.g., Host: www.example.com:x) bypass net. SplitHostPort redirect logic while matching the sibling router, proxying requests directly to backend services without evaluating any annotation-derived security middlewares, including BasicAuth and source-IP allowlists.
Red Hat severity: Critical — CVSS 9.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N). Weakness: CWE-639.
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative — check it before change work.
Official advisory · high-confidence parse· fetched 1 hour ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists — confirm against the vendor advisory.
Official advisory · high-confidence parse· fetched 1 hour ago·verify at source
Mitigation checklist
- Remove the nginx.ingress.kubernetes.io/from-to-www-redirect annotation from Ingress resources that enforce authentication or access control middlewares. Alternatively, implement host redirects using explicit Traefik RedirectScheme middlewares or enforce authentication upstream.
Official advisory · high-confidence parse· fetched 1 hour ago·verify at source
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