VSA-2026-052: Unauthenticated ServerHello can downgrade later connections to classical key exchange
| Published | Updated | Severity | CVSS 4.0 | Affected products |
|---|---|---|---|---|
| 2026-10-29 | 2026-10-29 | 🟢 Low | Not available yet | - Xen Orchestra / Xen Orchestra Appliance |
A vulnerability was discovered in rustls affecting vectura in Xen Orchestra that is part of the Xen Orchestra Appliance. We classify the impact as Low. For details on how we assign severity levels, see our Severity Levels Explained page.
Summary​
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.
Impact​
There is a loss of post-quantum confidentiality for affected connections ("harvest now, decrypt later") between ESXi and vectura. There is no key compromise and no authentication bypass.
Status​
- Released
Affected Versions​
- Xen Orchestra before 6.10.0: Affected.
- Xen Orchestra Appliance: Affected.
Mitigations​
To reduce the risk of exploitation, ensure traffic between Xen Orchestra and ESXi flows only over a trusted network dedicated to administration.
Resolution​
Security fixes are delivered to the latest channel before the stable channel. Switching to latest therefore allows earlier remediation, although this channel may be less stable.
As of 2026-10-29, Xen Orchestra version 6.10.0 has been released and includes the fix for this issue. It is currently available on the latest channel and should become available on stable with the next version in about a month.
List of packages fixing this issue:
- @xen-orchestra/vmware-explorer: 1.1.0