Executive brief
Traefik is a popular open-source tool used to route and manage web traffic. A security flaw in its domain-fronting protection allows unauthorized users to bypass security requirements, such as digital certificates (mTLS), when accessing certain protected websites. By using a "permissive" connection to one site and then switching the request to a "protected" site that uses wildcard rules (like *.example.com), an attacker can gain access to sensitive internal services without the required credentials.
Technical details
A vulnerability exists in Traefik's SNICheck middleware where it resolves TLS options for the HTTP Host header using exact map lookups only, failing to account for wildcard host rules (e.g., Host(*.example.com)). If a router is configured with a wildcard host and strict TLSOptions (such as RequireAndVerifyClientCert), an attacker can perform a 'domain-fronting' attack. By completing a TLS handshake using a different, permissive SNI on the same entrypoint and then sending an HTTP Host header targeting the wildcard-protected backend, the attacker bypasses the mTLS requirement. This occurs because SNICheck incorrectly classifies the wildcard-protected host as using default (permissive) TLS options. The issue affects HTTPS/HTTP-2 paths in versions 3.7.0 through 3.7.2 and is fixed in version 3.7.3.
Affected products
- Traefik Proxy Traefik >= 3.7.0, < 3.7.3
Timeline
- 2026-06-05: advisory: GitHub Security Advisory published
- 2026-06-04: patched: Version 3.7.3 released
- 2026-06-23: disclosed: CVE-2026-48491 published to NVD