GHSA-fqf6-gxhh-2xhw uutils coreutils: cp/install/mv/ln --suffix alone does not enable backup mode (silent data loss vs GNU)

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.