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

When

2026-06-16 19:02 UTC

Where

Global (internet)

Category

cyber_advisory · go

## 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 demonstrates the same wildcard resolution gap as an availability/operational problem. This report demonstrates a security-impacting false-negative variant that can bypass router-specific mTLS when a permissive SNI exists on the same entrypoint. When the attacker chooses a permissive/default SNI and sends a protected wildcard host in the HTTP `Host` header, both sides can be classified as `default`, so `SNICheck` does not return `421`. The later HTTP router then matches the wildcard-protected backend and the request is forwarded without enforcing the wildcard route's mTLS policy. Related wildcard `SNICheck` behavior has also been observed in Kubernetes Ingress setups, as described in public issue #12349. The PoC below uses the file provider and v3 rule syntax to keep the reproduction minimal and self-contained. Minimal dynamic configuration: ```yaml http: routers: protected: rule: Host(`*.example.com`) service: protected tls: options: mtls public: rule: Host(`public.example.net`) service: public tls: {} services: protected: loadBalancer: servers: - url: http://protected:80 public: loadBalancer: servers: - url: http://public:80 tls: certificates: - certFile: /certs/server.crt keyFile: /certs/server.key options: mtls: clientAuth: caFiles: - /certs/ca.crt clientAuthType: RequireAndVerifyClientCert ``` Minimal Docker Compose: ```yaml services: traefik: image: traefik:v3.7.1 command: - --log.level=DEBUG - --entrypoints.websecure.address=:8443 - --providers.file.filename=/etc/traefik/dynamic.yml - --providers.file.watch=false ports: - "8443:8443" volumes: - ./dynamic.yml:/etc/traefik/dynamic.yml:ro - ./certs:/certs:ro depends_on: - protected - public protected: image: traefik/whoami:v1.11 command: - --name=PROTECTED public: image: traefik/whoami:v1.11 command: - --name=PUBLIC ``` Certificate generation: ```bash rm -rf certs mkdir -p certs openssl req -x509 -newkey rsa:2048 -nodes -days 7 \ -keyout certs/ca.key \ -out certs/ca.crt \ -subj "/CN=traefik-poc-ca" openssl req -newkey rsa:2048 -nodes \ -keyout certs/server.key \ -out certs/server.csr \ -subj "/CN=public.example.net" \ -addext "subjectAltName=DNS:public.example.net,DNS:api.example.com,DNS:*.example.com" openssl x509 -req \ -in certs/server.csr \ -CA certs/ca.crt \ -CAkey certs/ca.key \ -CAcreateserial \ -out certs/server.crt \ -days 7 \ -sha256 \ -copy_extensions copyall ``` ### PoC Start Traefik with the configuration above. Test environment: - Traefik images tested: `v3.7.0`, `v3.7.1` - Backend image: `traefik/whoami:v1.11` - Client: `curl` with HTTPS/HTTP2 support - EntryPoint: TCP port `8443` exposed locally - Provider: file provider Control 1: the permissive public route works norm

Sources

• GitHub Advisory Database ↗ · first seen 2026-06-16 19:02 UTC

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