Please report a vulnerability privately rather than in a public issue: through a private advisory on GitHub, or by mail to sachesi xsachesi@pm.me. Say what you found, how to reproduce it and which version you ran; a fix is worked out with you before anything is published.

Only the latest release gets fixes.

Machines manages virtual machines through libvirt, and a small part of it runs as root. The parts where a mistake matters most:

• The helper machines-hooks, which pkexec runs as root under the polkit action io.github.sachesi.machines.hooks once an administrator's password is given. It takes a UUID, a stage and a script, and writes only under /etc/machines/scripts and its one dispatcher (see docs/scripts.md). Reaching it without that password, or making it write any other file or follow a symbolic link, is a vulnerability.
• The dispatcher it installs in /etc/libvirt/hooks/qemu.d, which libvirt runs as root for every machine on the system connection. A machine's name or definition that makes it run anything but the script kept for that machine and stage is a vulnerability.
• The consoles, where what arrives over VNC, SPICE, Looking Glass and the serial console comes from the guest. A guest that makes Machines run code or read the host's files through them, or takes the microphone, the clipboard or a USB device that its settings do not give it, is a vulnerability.

A script an administrator saved doing what it says is not a vulnerability, since running it as root is its purpose. Neither is what a user can already do through libvirt: on the system connection, membership of the libvirt group is close to root by design. A flaw in libvirt, QEMU or a console protocol's library belongs with that project.

A machine that fails to start or a page that shows the wrong value is a bug; please file it as an ordinary issue.