QEMU: use-after-free in the VNC WebSocket handshake crashes the VM process before client authentication

Impact

If the QIOChannelWebsock object is freed while a handshake is still pending, its GSource is leaked and the callback fires later against freed memory. A client that can open the VNC WebSocket port can trigger this during the handshake, before VNC authentication runs, and take down the QEMU process. On a virtualized GPU node that is the whole guest: a VM with passthrough or vGPU devices dies with its training or inference job, and device reset and reattach on restart is not always clean. The exposure is denial of service - the record does not claim code execution.

Who can reach it

Anyone with network reach to the VNC WebSocket port of a running guest. No authentication needed - the crash happens before the VNC client authenticates, so any exposed or management-VLAN-reachable VNC WebSocket listener is enough.

What to do

Install the patched qemu-kvm from the Red Hat errata for your release (RHSA-2025:23228, RHSA-2026:0326, RHSA-2026:0332, RHSA-2026:0702, RHSA-2026:1831 across RHEL 8/9/10 and OCP 4.16). Running guests keep the old binary, so each VM must be stopped and restarted (or live-migrated where the GPU topology allows it) - budget a drain of the affected hypervisors. Mitigation in the meantime: disable the VNC websocket option or firewall the VNC WebSocket port to the management network only.

References