📖 ~4 min read • Source: FreeBSD VuXML
VuXML topic: zsh — Arbitrary command execution vulnerability
Related CVEs: CVE-2021-45444
Upstream summary: Marc Cornellà reports: Some prompt expansion sequences, such as %F, support 'arguments' which are themselves expanded in case they contain colour values, etc. This additional expansion would trigger PROMPT_SUBST evaluation, if enabled. This could be abused to execute code the user didn't expect. e.g., given a certain prompt configuration, an attacker could trick a user into executing arbitrary code by having them check out a Git branch with a specially crafted name.
Table of contents
Symptom & Impact
On FreeBSD 15 hosts running zsh, operators see behaviour consistent with the FreeBSD VuXML entry: pkg audit flags the installed version, services may refuse to start after upgrade or restart, and — for security-rated advisories — the host is exposed to the vulnerabilities above. Impact spans isolated service restart cycles to full availability incidents on jails or bhyve guests that depend on zsh.
Environment & Reproduction
Reproduction targets FreeBSD 15. Confirm with freebsd-version -kru, uname -a, and the installed package via pkg info zsh and pkg query "%n-%v" zsh. Capture system state with pkg audit -F and service -e. Trigger the workflow that exposes zsh — vulnerability — patch and remediation guide while collecting tail -200 /var/log/messages, dmesg -a, and /var/log/pkg.log.
Root Cause Analysis
Root cause is tracked at FreeBSD VuXML. The FreeBSD ports security team shipped a corrective zsh port revision; hosts on an outdated build remain exposed. Correlate /var/log/pkg.log with /var/log/messages and kernel state in sysctl kern.lastpid + sysctl kern.osreldate to isolate the change that triggered the failure mode.
Quick Triage
Quick triage: service zsh status, tail -100 /var/log/messages, pkg audit -F, pkg version -v zsh, and pfctl -sr (or ipfw list) to confirm firewall posture. For kernel issues: dmesg -a | tail -100 and kldstat.
Step-by-Step Diagnosis
1) service -e to enumerate failed services. 2) tail -F /var/log/messages and dmesg. 3) Validate firewall via pfctl -sr -v or ipfw show. 4) pkg check -B zsh for integrity. 5) pkg install -fy zsh to reinstall if tampered. 6) Correlate findings with /var/log/pkg.log and FreeBSD VuXML to pin the commit that introduced zsh — vulnerability — patch and remediation guide.
Solution – Primary Fix
Primary fix: install the corrective zsh port revision referenced by FreeBSD VuXML. Typical commands: sudo pkg update, sudo pkg upgrade zsh (or sudo pkg upgrade -y for the whole system), then sudo service zsh restart, and pkg audit to confirm no remaining advisories. For ports tree builders: sudo portsnap fetch update + cd /usr/ports/<cat>/zsh && sudo make deinstall reinstall clean. Reboot if the kernel module is involved.
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Solution – Alternative Approaches
Alternatives include locking the package with sudo pkg lock zsh until vetted, downgrading via pkg install <older-version> from a pinned repo, switching the FreeBSD pkg repository between quarterly and latest in /usr/local/etc/pkg/repos/FreeBSD.conf, isolating the affected service in a jail (iocage/bastille) with stricter firewall rules, or replacing the service with a vendored static build for the period between exposure detection and full rollout.
Verification & Acceptance Criteria
Acceptance: pkg info zsh shows the expected fixed version, service zsh status is running, pkg audit returns no advisory for zsh, tail -50 /var/log/messages shows no errors after restart, and the original reproduction for zsh — vulnerability — patch and remediation guide no longer triggers across two consecutive runs.
Rollback Plan
Capture state with pkg query "%n-%v" > /root/pkg-pre.txt and a ZFS boot-environment snapshot: bectl create pre-zsh-patch. To revert, run sudo pkg install -f <previous-version> or boot the previous BE via bectl activate pre-zsh-patch && reboot. For kernel/loader changes, drop to the loader prompt and select the previous boot environment.
Prevention & Hardening
Prevent recurrence by scheduling pkg audit -F via periodic.conf (daily_status_security_pkgaudit_enable="YES"), subscribing to freebsd-security-notifications, mirroring through a local pkg repo managed by poudriere, version-pinning sensitive packages with pkg lock, enabling automatic ZFS BE snapshots before upgrades, and monitoring file integrity via mtree or aide. Apply the CIS FreeBSD hardening checklist where applicable and harden jails with allow.* tunables in /etc/jail.conf.
Related Errors & Cross-Refs
Related issues that commonly surface alongside zsh — vulnerability — patch and remediation guide: pkg lock contention, mismatched ABI after kernel/userland skew (freebsd-version vs uname -K), pf rule drift, and stale shared-library references after upgrade (pkg check -d).
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References & Further Reading
Primary reference: FreeBSD VuXML. Supporting docs: FreeBSD Handbook, man pkg, man freebsd-update, man pfctl, man ipfw, man bectl, man periodic.conf, the FreeBSD Security Advisories at security.freebsd.org, and /usr/ports/UPDATING for port-specific notes implicated in zsh — vulnerability — patch and remediation guide.
