QEMU guest agent plugs root escalation in SSH key commands

QEMU's guest agent has been patched for a local privilege escalation in its guest SSH key helpers.

CVE-2026-12080 affected the commands that add authorized keys for an existing local user. The agent runs as root. When it decided whether to create the user's .ssh directory and then wrote and chowned authorized_keys, it followed symbolic links. A local unprivileged user who owned their home directory could pre-stage .ssh or authorized_keys as a symlink to a root-owned file or directory. When the host or operator triggered a key add for that user, the agent followed the link and transferred ownership to the unprivileged account, which could then rewrite the target and obtain root.

Kostiantyn Kostiuk fixed the flaw by dropping the agent's effective user and group IDs to the target account for those operations, rather than creating and chowning files as root. Daniel P. Berrangé reviewed the change. The issue was reported by Valentino Paulon.