Buffered application data sent to outer server after ECH rejection

Overview

When a server rejects Encrypted Client Hello (ECH), the client completes the handshake using the outer ClientHello and authenticates the server against the ECH config's public_name. At the end of that handshake, rustls encrypted and queued any application data buffered via ClientConnection::writer() before queueing the ech_required alert.

The application correctly receives Error::PeerIncompatible(ServerRejectedEncryptedClientHello), but the next write_tls() call (needed to send the alert) also sends the buffered application data. Plaintext meant for the inner server therefore goes to the client-facing server named by public_name.

That server is authenticated, so this is not exploitable by an unauthenticated party, but it violates RFC 9849 section 6.1.6, which requires the client to abort with ech_required "before sending any application data to the server".

This affects only clients that:

• use EchMode::Enable (GREASE ECH is not affected),
• write application data before the handshake completes, and
• connect to a server that rejects their ECH config (for example, because it is stale).

The underlying ordering bug is older, but before 0.23.44 the client checked the server certificate against the inner server name rather than public_name, so certificate verification failed first and no data was sent.

Affected packages

• rustls >= 0.23.44, < 0.23.46

Remediation

Upgrade to a patched release:

• rustls 0.23.46