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RUSTSEC-2026-0349: Unauthenticated ServerHello can downgrade later connections to classical key exchange

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Evaluación

Dificultad
1/5
Tiempo estimado
Menos de una hora
Aptitud para principiantes
55/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Activo
Stack tecnológico
rust
Área
security

Línea de trabajo

The vulnerable dependency is rustls at 0.23.40, so start with Cargo.toml and Cargo.lock to find where it is pinned. Raise it to at least 0.23.46 with cargo update -p rustls, then run cargo build and cargo test to confirm nothing breaks. Done when the lockfile resolves rustls to 0.23.46 or later and the build and tests pass.

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

Descripción

Unauthenticated ServerHello can downgrade later connections to classical key exchange

Details
Package rustls
Version 0.23.40
URL https://github.com/rustls/rustls/pull/3355
Date 2026-10-14
Patched versions >=0.23.46

The client recorded the server-selected key exchange group for each server
name when processing the ServerHello, before the server was authenticated.
This record is used to predict which key share to send in future ClientHellos
to that server name.

A network attacker can answer one connection attempt with a ServerHello
selecting X25519, and then close the connection. Later ClientHellos for that
name carry only an X25519 key share, while supported_groups still lists
X25519MLKEM768 first. If the genuine server accepts that share rather than
sending a HelloRetryRequest for its preferred hybrid group, the handshake
completes with classical-only key exchange and no error. This persists for as
long as the entry stays in the client's session store.

The impact is loss of post-quantum confidentiality for affected connections
("harvest now, decrypt later"). There is no key compromise and no
authentication bypass.

The following are not affected:

  • connections to servers which send a HelloRetryRequest for their preferred group,
  • clients with resumption disabled (as nothing is stored), and
  • other server names.

This behaviour is discussed in
draft-ietf-tls-key-share-prediction,
so the practical impact depends largely on server policy.

See advisory page for additional details.

Lenguaje dominante
Rust
Estrellas
0
Forks
2
Sin PR fusionados en 30 d

Preparar el entorno

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Primeros pasos

  1. Lee el issue completo y luego la guía de contribución del proyecto.
  2. Comenta en el issue que vas a ocuparte — evita que dos personas hagan lo mismo.
  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

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