CVE-2026-99170 is a Linux kernel vulnerability involving mount namespace handling where a mount that copy_tree() omits can take over a lock, allowing a root task in its own user namespace to remove a locked mount cover without a race or modifying the underlying filesystem. The issue arises from improper handling of locked mounts of mount namespace files during mount tree copying and moving operations. The vulnerability was addressed by refusing to leave out or move such locked mounts, preventing unshare() failures and unauthorized mount removals.