Please report a security problem privately, not in a public issue:

• Report it on GitHub (private vulnerability reporting), or
• email kelly@theideaplace.net.

Include what you found, how to reproduce it, and what it could affect. You'll hear back within a few days.

Parallels Manager runs prlctl (Parallels Desktop's command-line tool) and, for New Ubuntu VM and New Windows VM, an install script in Terminal. It isn't sandboxed, because the sandbox forbids running other programs.

• Passwords. New Ubuntu VM's password goes to the install script in a file only you can read, in a private temporary folder; the script deletes it as soon as it has read it, and stores only a SHA-512 hash in the new VM.
• Downloads. The Ubuntu installer comes from Ubuntu's own servers over HTTPS and is checked against Ubuntu's published SHA-256 checksums before it's used.
• SSH. A new Ubuntu VM gets your Mac's SSH public keys (~/.ssh/*.pub) so you can sign in to it. Private keys never leave your Mac.
• Releases are signed with Developer ID and notarized by Apple, and each comes with a SHA-256 checksum.

Only the latest release gets fixes.