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

What to do

1. See which version you have: cargo tree -i rustls (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p rustls. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example rustls = "0.23.46") or update the dependency that pulls it in.
3. Check again with cargo audit, or paste your Cargo.lock into our Cargo.toml checker.

Details from the advisory

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.

References

• Package: crates.io/crates/rustls
• Advisory: rustsec.org/advisories/RUSTSEC-2026-0349.html
• Web: github.com/rustls/rustls/pull/3355

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.

Other advisories for rustls

• Vulnerability HighRUSTSEC-2024-0336 · 2024-04-19`rustls::ConnectionCommon::complete_io` could fall into an infinite loop based on network input
• VulnerabilityRUSTSEC-2024-0399 · 2024-11-22rustls network-reachable panic in `Acceptor::accept`
• Vulnerability MediumRUSTSEC-2026-0285 · 2026-09-14TLS 1.3 handshake messages incorrectly accepted across encryption level boundaries
• Vulnerability MediumRUSTSEC-2026-0346 · 2026-10-14`Debug` impls for client session values expose TLS secrets
• Vulnerability LowRUSTSEC-2026-0347 · 2026-10-14Buffered application data sent to outer server after ECH rejection
• Vulnerability LowRUSTSEC-2026-0348 · 2026-10-14Brotli certificate decompression can allocate up to 1 GiB before authentication

Full security history of rustls