CVE-2026-32305: Traefik has a Potential mTLS Bypass via Fragmented TLS ClientHello Causing Pre-SNI Sniff Fallback to Default Non-mTLS TLS Config

There is a potential vulnerability in Traefik’s TLS SNI pre-sniffing logic related to fragmented ClientHello packets.

When a TLS ClientHello is fragmented across multiple records, Traefik’s SNI extraction may fail with an EOF and return an empty SNI. The TCP router then falls back to the default TLS configuration, which does not require client certificates by default. This allows an attacker to bypass route-level mTLS enforcement and access services that should require mutual TLS authentication.

I found a behavior in Traefik’s latest version where fragmented ClientHello packets can cause pre-sniff SNI extraction to not find the sni (EOF during sniff), which makes the TCP router fall back to default routing TLS config.

If the default TLS config does not require client certificates (which is NoClientCert by default), the handshake succeeds without client auth, and the request is later routed to the HTTP Host which should be the protected with client certificate authentication (RequireAndVerifyClientCert tls config).