CVE-2026-48491: Traefik: SNICheck ignores wildcard TLSOptions mappings, allowing domain-fronted mTLS bypass

There is a high severity vulnerability in Traefik’s domain-fronting protection (SNICheck) that allows an unauthenticated client to bypass mutual TLS enforced through wildcard router TLSOptions. When a router uses a wildcard host rule such as Host(*.example.com) with stricter TLS options (for example RequireAndVerifyClientCert), SNICheck resolves the TLS options for the HTTP Host header using exact map lookups only and never applies wildcard matching. If another permissive SNI is served on the same entrypoint, an attacker can complete the TLS handshake under the permissive options and then send an HTTP Host header targeting the wildcard-protected backend, reaching it without presenting a client certificate. This affects the regular HTTPS / HTTP-2 path and does not require HTTP/3.

Traefik’s SNICheck domain-fronting protection ignores wildcard TLSOptions mappings. A wildcard router such as Host("*.example.com") can require mTLS for direct access, but an unauthenticated client can complete the TLS handshake with another permissive SNI on the same entrypoint and then send Host: api.example.com / HTTP request authority api.example.com to reach the wildcard-protected backend.

This issue does not require HTTP/3. The PoC uses the regular HTTPS/HTTP2 path and abuses the domain-fronting consistency check between TLS SNI and the HTTP Host header.

For HTTP/2, this corresponds to the request authority / Host value as exposed to Traefik’s HTTP request handling.