Executive brief
Canonical Multipass is a tool used to easily create and manage Ubuntu virtual machines on local workstations. A security flaw in its file-sharing component allows a user with administrative access inside a virtual machine to break out and access sensitive files on the host computer. This could lead to the theft of private keys, cloud credentials, and other personal data stored on the host system.
Technical details
A path containment bypass exists in the host-side SFTP server (sshfs_server) of Canonical Multipass. The 'validate_path' function in 'src/sshfs_mount/sftp_server.cpp' performs a simple string prefix comparison without validating path separators or performing dot-dot (..) normalization. An attacker with root privileges inside a guest VM can bypass the FUSE layer by injecting raw SFTP frames (e.g., SSH_FXP_OPEN) directly into the sshfs_server process pipes via procfs. By crafting a path that starts with the allowed mount prefix but uses directory traversal sequences, the attacker can force the host-side root process to access files outside the intended mount boundary, resulting in a full VM escape. This issue is patched in version 1.16.3.
Affected products
- Canonical Multipass (macOS) <= 1.16.2
Timeline
- 2026-05-28: advisory: Canonical published security advisory GHSA-rhp2-23c4-r34w
- 2026-05-28: disclosed