CVE-2026-53622: Traefik: HTTP/3 mTLS bypass via exact SNI TLSOptions lookup for wildcard and mixed-case hosts

There is a critical vulnerability in Traefik’s HTTP/3 (QUIC) TLS configuration selection that allows unauthenticated clients to bypass router-specific mTLS enforcement. When HTTP/3 is enabled on an entrypoint, the TLS handshake selects the applicable TLS configuration through an exact, case-sensitive lookup on the SNI value, which fails to match wildcard host patterns (e.g., *.example.com) or case variants of the configured hostname. Because the handshake falls back to the default TLS configuration — which may not require client certificates — a client can complete the QUIC handshake without presenting a certificate, while the subsequent HTTP routing layer still dispatches the request to a backend protected by a router-specific mTLS policy. The issue affects deployments where HTTP/3 is enabled, a router uses a wildcard Host rule or case-insensitive hostname matching, a router-specific TLSOptions enforces client certificate authentication, and UDP access to the entrypoint is reachable by an attacker.

Traefik’s HTTP/3 TLS configuration selection can ignore router-specific TLSOptions and allow unauthenticated clients to bypass mTLS. The QUIC/HTTP3 path resolves TLS configuration with Router.GetTLSGetClientInfo(), which performs a direct, case-sensitive map lookup on hostHTTPTLSConfig[info.ServerName].

This is inconsistent with the later HTTP host routing semantics, where the same request host can still match wildcard or case-insensitive Host rules after the HTTP/3 TLS handshake has already fallen back to the default TLS configuration. Two exploit paths are confirmed:

1. Host("*.example.com") with tls.options=mtls: HTTP/2 requires a client certificate, but HTTP/3 reaches the protected backend without one.
2. Host("api.example.com") with tls.options=mtls: HTTP/2 requires a client certificate, but HTTP/3 with mixed-case SNI/Host such as API.EXAMPLE.COM reaches the protected backend without one.

Confirmed versions:

• wildcard HTTP/3 bypass: v3.7.0, v3.7.1
• exact-host mixed-case HTTP/3 bypass: v3.6.17, v3.7.0, v3.7.1