CVE-2026-49238: SFTP Server VM Escape in Canonical Multipass

An issue was discovered in Canonical Multipass before version 1.16.3. The host-side SFTP server component (sshfsserver), which executes with root privileges on the host, contains a path containment bypass vulnerability within its validatepath function in src/sshfsmount/sftpserver.cpp. The function performs a plain string prefix comparison on requested paths without path separator validation or dot-dot (..) normalization. A local attacker with root privileges inside a guest virtual machine can bypass the FUSE layer by injecting raw SFTP frames (such as an SSHFXPOPEN request) directly into the sshfsserver process stdin/stdout pipes via procfs. By supplying a path containing directory traversal sequences that match the allowed mount prefix, the attacker can force the host-side root process to resolve the traversal and open files outside the designated mount boundary. This allows a guest-side user to read arbitrary files on the host filesystem, resulting in a virtual machine escape.