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RUSTSEC-2026-0349
rustlsThe 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.
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The vulnerability can be exploited over the network without needing physical access. It is difficult for an attacker to exploit this vulnerability and may require special conditions. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a low impact on the confidentiality of the information.
Limited exploitation activity has been observed. Close monitoring and planned remediation are recommended.
Probability that this vulnerability will be exploited in the wild within the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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