determine backup mode in src/uucore/src/lib/features/backup control.rs only checks --backup/-b and returns BackupMode::None when only --suffix is given. GNU enables backup mode when --suffix is used alone (defaulting to existing/numbered, or $VERSION CONTROL). Affects cp, install, mv, ln which share this code.

# uutils: no backup created
$ coreutils cp --suffix=.bak src dest   # dest.bak NOT created
# GNU: dest.bak created
$ cp --suffix=.bak src dest

Impact: users/scripts relying on --suffix to back up a file before overwrite get silent data loss; breaks GNU compatibility across four utilities. Recommendation: enable backup mode when --suffix is present.

Note: this is primarily a GNU-compatibility/data-safety divergence rather than a classic exploitable vulnerability — review whether it warrants a CVE.

Remediation: Acknowledged by Canonical; fixed in PR #9741 (uucore: use --suffix to enable backup mode), commit 939ab037a, merged 2025-12-21. determine backup mode now has a --suffix-alone branch that resolves the mode from $VERSION CONTROL (defaulting to existing). Released in uucore 0.6.0 and later (vulnerable: < 0.6.0). Regression tests added in the same file: test backup mode suffix without backup option and test backup mode suffix without backup option with env var.

Reported by Zellic in the uutils coreutils Program Security Assessment (prepared for Canonical, Jan 20 2026), audited commit 3a07ffc5a9bd4c283e75afa548ba1f1957bad242. Finding 3.7. Credit: Zellic.