Traefik HTTP/3 TLS Configuration Bypass via SNI Mismatch

Executive Summary

This vulnerability exists in Traefik's HTTP/3 (QUIC) TLS configuration selection process. When HTTP/3 is enabled, the TLS handshake uses an exact, case-sensitive lookup on the Server Name Indication (SNI) value to select the TLS configuration. However, this lookup fails to match wildcard host patterns (like *.example.com) or case variants of hostnames.

Because of this failure, the handshake falls back to the default TLS configuration, which may not require client certificates. This allows unauthenticated clients to complete the QUIC handshake without presenting a certificate, even though the HTTP routing layer still dispatches requests to backends protected by router-specific mutual TLS (mTLS) policies.

The vulnerability affects deployments where HTTP/3 is enabled, routers use wildcard Host rules or case-insensitive hostname matching, router-specific TLSOptions enforce client certificate authentication, and UDP access to the entrypoint is reachable by an attacker. It is fixed in Traefik version 3.7.3.

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Detection Guidance

Detection involves identifying if your Traefik deployment is using HTTP/3 (QUIC) with router-specific mTLS enforcement and if the TLS handshake is selecting TLS configurations based on exact, case-sensitive SNI lookups that fail to match wildcard host patterns or case variants.

You can check your Traefik configuration for enabled HTTP/3 entrypoints and verify if any routers use wildcard Host rules or case-insensitive hostname matching combined with TLSOptions enforcing client certificate authentication.

Network-level detection can include monitoring UDP traffic to the HTTP/3 entrypoint and checking for QUIC handshakes that complete without client certificates.

• Inspect Traefik configuration files for HTTP/3 enabled entrypoints and router-specific mTLS settings.
• Use network packet capture tools (e.g., tcpdump or Wireshark) to monitor UDP traffic on the HTTP/3 port and analyze QUIC handshakes for absence of client certificates.
• Example tcpdump command to capture QUIC traffic on UDP port 443: tcpdump -i <interface> udp port 443
• Check Traefik logs for any unusual client connections that bypass mTLS enforcement.

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Impact Analysis

This vulnerability can allow unauthenticated clients to bypass router-specific mutual TLS (mTLS) enforcement, meaning attackers can access backend services that are supposed to require client certificate authentication without actually presenting a valid certificate.

As a result, sensitive services protected by mTLS could be accessed by unauthorized users, potentially leading to data exposure, unauthorized actions, or other security breaches.

The issue specifically impacts environments where HTTP/3 is enabled, wildcard or case-insensitive host matching is used, and UDP access to the HTTP/3 entrypoint is possible for attackers.

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Compliance Impact

This vulnerability allows unauthenticated clients to bypass router-specific mutual TLS (mTLS) enforcement in Traefik when HTTP/3 is enabled and certain conditions are met. This bypass can lead to unauthorized access to backend services that are expected to be protected by mTLS.

Such unauthorized access could potentially result in exposure or unauthorized processing of sensitive data, which may impact compliance with data protection regulations like GDPR or HIPAA that require strict access controls and encryption for sensitive information.

Therefore, if exploited, this vulnerability could undermine the security controls necessary to meet compliance requirements related to data confidentiality and integrity.

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Mitigation Strategies

Immediate mitigation steps include disabling HTTP/3 on entrypoints that rely on router-specific mTLS enforcement to prevent the bypass.

Alternatively, enforce mutual TLS authentication in the default TLS options to ensure client certificates are always required regardless of SNI matching.

Blocking UDP access to the HTTP/3 entrypoint can also prevent exploitation since QUIC uses UDP.

Another mitigation is to enforce client authentication at an additional layer behind Traefik.

Ultimately, upgrading Traefik to version 3.7.3 or later, where the vulnerability is fixed, is the recommended solution.

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