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RUSTSEC-2026-0347: Buffered application data sent to outer server after ECH rejection

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#21 0 comentarios 0 reacciones 0 asignados Ver en GitHub

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Evaluación

Dificultad
2/5
Tiempo estimado
1-3 horas
Aptitud para principiantes
55/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Activo
Stack tecnológico
rust
Área
networking, security

Línea de trabajo

The fix is a dependency bump: the rustls version in Cargo.toml and Cargo.lock must resolve to 0.23.46 or later. Start with the rustls entries in Cargo.toml and Cargo.lock, then run cargo update -p rustls and cargo test. Done when the lockfile pins a patched rustls and the build and tests pass. Check the advisory page for any API changes that affect ECH handling in this crate.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

Buffered application data sent to outer server after ECH rejection

Details
Package rustls
Version 0.23.45
URL https://github.com/rustls/rustls/pull/3300
Date 2026-10-14
Patched versions >=0.23.46
Unaffected versions <0.23.44

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.

See advisory page for additional details.

Lenguaje dominante
Rust
Estrellas
2
Forks
0
Merge medio
1 d 15 h
PR fusionados (30 d)
3

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