## Summary 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. ## Patches - https://github.com/traefik/traefik/releases/tag/v3.7.3 ## For more information If you have any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues). <details> <summary>Original Description</summary> ### Summary 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. ### Details For the v3 rule-syntax / file-provider path used in this PoC, wildcard `Host` / `HostSNI` matching and TLSOptions association for wildcard domains were introduced in Traefik v3.7. The normal HTTPS/TCP router path uses wildcard-aware matching. The `SNICheck` middleware does not. The router build records TLS option names for host rules: ```go domains, err := httpmuxer.ParseDomains(routerHTTPConfig.Rule) // ... tlsOptionsForHost[domain] = tlsOptionsName ``` The HTTPS forwarder then installs SNI routes: ```go rule := fmt.Sprintf(`HostSNI(%q)`, sniHost) ``` `HostSNI` matching is wildcard-aware: ```go return muxer.DomainMatchHostExpression(meta.serverName, hostExpr) ``` But `pkg/middlewares/snicheck/snicheck.go` resolves the host's TLS option name with exact lookups only: ```go func findTLSOptionName(tlsOptionsForHost map[string]string, host string, fqdn bool) string { name := findTLSOptName(tlsOptionsForHost, host, fqdn) if name != "" { return name } name = findTLSOptName(tlsOptionsForHost, strings.ToLower(host), fqdn) if name != "" { return name } return traefiktls.DefaultTLSConfigName } func findTLSOptName(tlsOptionsForHost map[string]string, host string, fqdn bool) string { if tlsOptions, ok := tlsOptionsForHost[host]; ok { return tlsOptions } if !fqdn { return "" } if last := len(host) - 1; last >= 0 && host[last] == '.' { if tlsOptions, ok := tlsOptionsForHost[host[:last]]; ok { return tlsOptions } return "" } if tlsOptions, ok := tlsOptionsForHost[host+"."]; ok { return tlsOptions } return "" } ``` There is no wildcard matching step for entries such as `*.example.com`. As a result, `Host: api.example.com` can be classified as using default TLS options even though the router matched a wildcard host with stricter `TLSOptions`. Preconditions: - A protected router uses wildcard `Host` / `HostSNI` with router-specific `TLSOptions`. - The protected wildcard router uses stricter TLS options, such as `RequireAndVerifyClientCert`. - Another SNI/default TLS path on the same entrypoint allows a handshake without a client certificate. - The client can send an HTTP `Host` header different from the TLS SNI. Relationship to my previous HTTP/3 report: I previously submitted a related HTTP/3 mTLS bypass involving `Router.GetTLSGetClientInfo()` and exact/case-sensitive SNI lookup. This report is separate. It does not require HTTP/3 or QUIC. It affects the regular HTTPS/HTTP2 path and is caused by `SNICheck` resolving `tlsOptionsForHost` with exact lookups only, without wildcard matching. The exploit uses domain fronting: a permissive TLS SNI is used for the handshake, while the HTTP request authority / `Host` header targets a wildcard-protected backend. Relationship to public issue #12349: This is related to public issue #12349, where wildcard hosts were observed to be classified as `default` by `SNICheck`, causing unexpected `421 Misdirected Request` responses in some wildcard setups: ```text TLS options difference: SNI:https-ext@file, Header:default ``` The public issue demonstr