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3.7
rustls clients may lose post‑quantum protection after downgrade
published 4 days ago
Summary
Clients that use the rustls library can be tricked into remembering a weaker encryption method after a brief failed connection. A network attacker can then cause later connections to use only the older encryption, removing the extra quantum‑resistant security. To stay safe, apply updates to rustls as soon as they are available, or disable session resumption if you cannot update right away.
What to do
- Update ctz rustls to version 0.23.46.
Affected software
| Ecosystem | Vendor | Product | Affected versions |
|---|---|---|---|
| crates.io | ctz | rustls |
>= 0.0.0-0, < 0.23.46 Fix: upgrade to 0.23.46
|
Original advisory text
Unauthenticated ServerHello can downgrade later connections to classical key exchange
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](https://www.ietf.org/archive/id/draft-ietf-tls-key-share-prediction-01.html#name-security-considerations),
so the practical impact depends largely on server policy.
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](https://www.ietf.org/archive/id/draft-ietf-tls-key-share-prediction-01.html#name-security-considerations),
so the practical impact depends largely on server policy.
References
Internet-facing
14 days
Internal
60 days
- Not known to be exploited
- Can be exploited automatically (estimated)
- Gives an attacker partial control (estimated)
Timeline
Published14 Oct 2026
Updated14 Oct 2026
First seen14 Oct 2026
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