GHSA-239g-2685-54x3: install: TOCTOU symlink race (unlink-then-create without O_EXCL) allows arbitrary file overwrite

copy_file in install/src/install.rs removes the destination then recreates it by pathname via File::create / fs::copy without O_EXCL/create_new. Between the unlink and the recreate, a local attacker with write access to the destination directory can drop in a symlink and redirect the write. Impact: when install runs privileged into an attacker-writable directory (staging/build paths), the race allows redirecting writes to arbitrary files and overwriting sensitive system files (/etc/passwd, /etc/shadow). Recommendation: create atomically with create_new/O_EXCL and copy via the opened fd rather than reopening by path. Remediation: Acknowledged by Canonical; fixed in commit b5bbabc1. _Reported by Zellic in the *uutils coreutils Program Security Assessment* (prepared for Canonical, Jan 20 2026), audited commit 3a07ffc5a9bd4c283e75afa548ba1f1957bad242. Finding 3.50. Credit: Zellic._