Impact

There is a potential vulnerability in Traefik ACME TLS certificates' automatic generation: the ACME TLS-ALPN fast path can allow unauthenticated clients to tie up goroutines and file descriptors indefinitely when the ACME TLS challenge is enabled.

A malicious client can open many connections, send a minimal ClientHello with acme-tls/1, then stop responding, leading to denial of service of the entrypoint.

Patches

• https://github.com/traefik/traefik/releases/tag/v2.11.35
• https://github.com/traefik/traefik/releases/tag/v3.6.7

For more information

If you have any questions or comments about this advisory, please open an issue.

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

[Security] ACME TLS-ALPN fast path lacks timeouts and close on handshake stall

Dear Traefik security team,

We believe we have identified a resource-exhaustion issue in the ACME TLS-ALPN fast path that can allow unauthenticated clients to tie up goroutines and file descriptors indefinitely when the ACME TLS challenge is enabled.

Summary

• Affected code: pkg/server/router/tcp/router.go (ACME TLS-ALPN handling).
• When a ClientHello advertises acme-tls/1, Traefik intercepts it and calls tls.Server(...).Handshake() without any read/write deadlines and without closing the connection afterward.
• Immediately before this branch, existing deadlines set by the entrypoint are cleared.
• A client that sends the ALPN marker and then stops responding can keep the goroutine and socket open indefinitely, potentially exhausting the entrypoint under load.
• Exposure is limited to entrypoints where the ACME TLS-ALPN challenge is enabled and ACME bypass is not allowed.

Relevant snippets

// Deadlines are cleared before protocol dispatch
if err := conn.SetDeadline(time.Time{}); err != nil {
    log.Error().Err(err).Msg("Error while setting deadline")
}

// ACME TLS-ALPN fast path
if !r.acmeTLSPassthrough && slices.Contains(hello.protos, tlsalpn01.ACMETLS1Protocol) {
    r.acmeTLSALPNHandler().ServeTCP(r.GetConn(conn, hello.peeked))
    return
}

// Handler invoked by the branch above
return tcp.HandlerFunc(func(conn tcp.WriteCloser) {
    _ = tls.Server(conn, r.httpsTLSConfig).Handshake()
})

</details>