mv: symlinks expanded during cross-device move (resource exhaustion / data duplication)

When moving directories across filesystems, uutils mv dereferences symlinks inside the tree, copying their targets as real files/dirs instead of preserving the symlinks. GNU preserves symlinks by default. E.g. a etc_link -> /etc inside the source becomes a full copy of /etc at the destination.

Impact: (1) resource exhaustion — a small tree can expand into a huge copy (time/disk DoS); (2) unintended duplication of sensitive paths referenced by symlink; (3) symlink-loop amplification causing deep recursion. Recommendation: in cross-device fallback, detect symlinks via symlink_metadata() and recreate with read_link()/symlink(); add loop detection.

Remediation: Acknowledged by Canonical; fixed in commit 9654e4ab.

Reported by Zellic in the uutils coreutils Program Security Assessment (prepared for Canonical, Jan 20 2026), audited commit 3a07ffc5a9bd4c283e75afa548ba1f1957bad242. Finding 3.63. Credit: Zellic.