{"id":"RUSTSEC-2026-0349","summary":"Unauthenticated ServerHello can downgrade later connections to classical key exchange","details":"The client recorded the server-selected key exchange group for each server\nname when processing the ServerHello, before the server was authenticated.\nThis record is used to predict which key share to send in future ClientHellos\nto that server name.\n\nA network attacker can answer one connection attempt with a ServerHello\nselecting X25519, and then close the connection. Later ClientHellos for that\nname carry only an X25519 key share, while `supported_groups` still lists\nX25519MLKEM768 first. If the genuine server accepts that share rather than\nsending a HelloRetryRequest for its preferred hybrid group, the handshake\ncompletes with classical-only key exchange and no error. This persists for as\nlong as the entry stays in the client's session store.\n\nThe impact is loss of post-quantum confidentiality for affected connections\n(\"harvest now, decrypt later\"). There is no key compromise and no\nauthentication bypass.\n\nThe following are not affected:\n\n- connections to servers which send a HelloRetryRequest for their preferred group,\n- clients with resumption disabled (as nothing is stored), and\n- other server names.\n\nThis behaviour is discussed in\n[draft-ietf-tls-key-share-prediction](https://www.ietf.org/archive/id/draft-ietf-tls-key-share-prediction-01.html#name-security-considerations),\nso the practical impact depends largely on server policy.","aliases":["GHSA-w44r-93j7-qv28"],"modified":"2026-10-14T13:30:02.475705129Z","published":"2026-10-14T12:00:00Z","database_specific":{"license":"CC0-1.0"},"references":[{"type":"PACKAGE","url":"https://crates.io/crates/rustls"},{"type":"ADVISORY","url":"https://rustsec.org/advisories/RUSTSEC-2026-0349.html"},{"type":"WEB","url":"https://github.com/rustls/rustls/pull/3355"}],"affected":[{"package":{"name":"rustls","ecosystem":"crates.io","purl":"pkg:cargo/rustls"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0.0.0-0"},{"fixed":"0.23.46"}]}],"ecosystem_specific":{"affected_functions":null,"affects":{"arch":[],"os":[],"functions":[]}},"database_specific":{"informational":null,"source":"https://github.com/rustsec/advisory-db/blob/osv/crates/RUSTSEC-2026-0349.json","categories":["crypto-failure"],"cvss":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N"}]}