CVE-2026-43046 is a vulnerability in the Linux kernel's btrfs filesystem where corrupted root items with a non-zero drop_progress and zero drop_level can cause a kernel BUG during relocation recovery at mount time. This occurs because the kernel did not validate this invariant when reading root items from disk, allowing malformed metadata to trigger a kernel panic. The issue has been fixed by adding validation in the tree-checker to reject such malformed metadata before it causes a crash.