CVE-2026-48491

Traefik is an HTTP reverse proxy and load balancer. From 3.7.0 until 3.7.3, 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. This vulnerability is fixed in 3.7.3.

Traefik is an HTTP reverse proxy and load balancer. From 3.7.0 until 3.7.3, 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. This vulnerability is fixed in 3.7.3.

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.

<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:

domains, err := httpmuxer.ParseDomains(routerHTTPConfig.Rule)
// ...
tlsOptionsForHost[domain] = tlsOptionsName

The HTTPS forwarder then installs SNI routes:

rule := fmt.Sprintf(`HostSNI(%q)`, sniHost)

HostSNI matching is wildcard-aware:

return muxer.DomainMatchHostExpression(meta.serverName, hostExpr)

But pkg/middlewares/snicheck/snicheck.go resolves the host's TLS option name with exact lookups only:

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…

Traefik is an HTTP reverse proxy and load balancer. From 3.7.0 until 3.7.3, 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. This vulnerability is fixed in 3.7.3.

Traefik is an HTTP reverse proxy and load balancer. From 3.7.0 until 3.7.3, 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. This vulnerability is fixed in 3.7.3.

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.

<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:

domains, err := httpmuxer.ParseDomains(routerHTTPConfig.Rule)
// ...
tlsOptionsForHost[domain] = tlsOptionsName

The HTTPS forwarder then installs SNI routes:

rule := fmt.Sprintf(`HostSNI(%q)`, sniHost)

HostSNI matching is wildcard-aware:

return muxer.DomainMatchHostExpression(meta.serverName, hostExpr)

But pkg/middlewares/snicheck/snicheck.go resolves the host's TLS option name with exact lookups only:

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…