Affected: 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.

Affected: crates.io: rustls

In rustls 0.23.44 and 0.23.45, application data that a client buffers before the handshake completes is sent to the server even when the server rejects Encrypted Client Hello (ECH).

When the server rejects ECH, the client completes the handshake with the outer ClientHello and authenticates the server against the ECH config's `public_name`. At the end of the handshake, `start_traffic()` encrypts and queues any plaintext buffered by `ClientConnection::writer()`. It does this *before* checking for `EchStatus::Rejected` and queueing the `ech_required` alert.

The application then receives `Error::PeerIncompatible(ServerRejectedEncryptedClientHello)`, as expected. However, the client is still expected to call `write_tls()` to send the fatal alert, and that call also sends the encrypted application data. The server decrypts the data and only then receives the alert.

The ordering bug is older than 0.23.44. Before 0.23.44, the client checked the server certificate against the inner server name rather than `public_name`, so a correctly configured server failed certificate verification first and no data was sent. 0.23.44 fixed that check, which makes the ordinary rejection path reachable.

# Impact

Plaintext meant for the inner server, the one named in the encrypted ClientHello, goes to the client-facing server identified by `public_name`. That server is properly authenticated, so this is not an attack by an unauthenticated party. However this violates [RFC 9849](https://www.rfc-ed…