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

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Valutazione

Difficoltà
4/5
Tempo stimato
3-5 giorni
Idoneità per principianti
35/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Attiva
Stack tecnologico
rust
Ambito
security

Direzione di ricerca

The report identifies the affected dependency as rustls 0.23.45 and links rustls pull request 3355; it does not name files or tests in sentry-rust. Start by checking how this SDK pins and configures rustls, then review the linked fix and advisory details. Done means the affected dependency behavior is addressed and relevant checks confirm the fix.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Rust

Unauthenticated ServerHello can downgrade later connections to classical key exchange

Details
Package rustls
Version 0.23.45
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.

Lingua principale
Rust
Stelle
752
Fork
192
Merge medio
16m
PR unite (30g)
4

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  4. Apri una pull request che faccia riferimento al numero della issue.

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