CVE-2026-63300: Cross-project instance move bypasses all project restrictions allowing host command execution

An improper validation vulnerability in the instancePostMigration function in lxd/instancepost.go of LXD allows an authenticated attacker with cancreateinstances permissions on a restricted project to bypass project-level security restrictions. When migrating an instance between projects, LXD fails to validate the instance's configuration against the target project's enforced restrictions (such as restricted.containers.lowlevel, restricted.devices., and restricted.networks.access). An attacker can exploit this by creating a disallowed or high-privilege instance in an unrestricted project and subsequently moving it into the restricted project.