rustls 0.23.44 and 0.23.45 contain an information disclosure vulnerability that sends application data buffered before the handshake completes to the client-facing server when Encrypted Client Hello is rejected. Because start_traffic() queues plaintext from ClientConnection::writer() before sending the ech_required alert, the authenticated public_name server receives data intended for the inner server.

rustls 0.23.44 and 0.23.45 contain an information disclosure vulnerability that sends application data buffered before the handshake completes to the client-facing server when Encrypted Client Hello is rejected. Because start_traffic() queues plaintext from ClientConnection::writer() before sending the ech_required alert, the authenticated public_name server receives data intended for the inner server.

Affected: crates.io: rustls

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: Ubuntu:24.04:LTS: rust-rustls, Ubuntu:26.10: rust-rustls, Ubuntu:26.04:LTS: rust-rustls

rustls 0.23.44 and 0.23.45 contain an information disclosure vulnerability that sends application data buffered before the handshake completes to the client-facing server when Encrypted Client Hello is rejected. Because start_traffic() queues plaintext from ClientConnection::writer() before sending the ech_required alert, the authenticated public_name server receives data intended for the inner server.