📖 ~4 min read • Source: FreeBSD VuXML
VuXML topic: rxvt-unicode is vulnerable to a remote code execution
Related CVEs: CVE-2022-4170
Upstream summary: Marc Lehmann reports: The biggest issue is resolving CVE-2022-4170, which allows command execution inside urxvt from within the terminal (that means anything that can output text in the terminal can start commands in the context of the urxvt process, even remotely).
Table of contents
Symptom & Impact
Exploitation of rxvt-unicode is user-driven: the danger arrives when someone opens a hostile page, message or attachment, so exposure sits on each FreeBSD 15 workstation rather than on any serving path. Desktop sessions run for weeks, and the package manager only replaces files on disk — a rxvt-unicode process started before the update keeps the old code mapped until the user quits and relaunches it. Consequences are local: theft of saved credentials and session cookies, access to the user’s profile data under $HOME, and code execution as the logged-in account. rxvt-unicode is a graphical application launched by a user from their desktop session, not a system service: there is no service to restart, and service(8) or /var/log/messages will report nothing for it.
Environment & Reproduction
Reproduction targets FreeBSD 15. Confirm with freebsd-version -kru, uname -a, and the installed package via pkg info rxvt-unicode and pkg query "%n-%v" rxvt-unicode. Capture system state with pkg audit -F and service -e. Trigger the workflow that exposes rxvt-unicode 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 The FreeBSD ports security team shipped a corrective rxvt-unicode port revision; hosts on an outdated build remain exposed.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: tail -100 /var/log/messages, pkg audit -F, pkg version -v rxvt-unicode, 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 rxvt-unicode for integrity. 5) pkg install -fy rxvt-unicode to reinstall if tampered. 6) Correlate findings with /var/log/pkg.log and FreeBSD VuXML to pin the commit that introduced the regression.
Solution – Primary Fix
Primary fix: install the corrective rxvt-unicode port revision referenced by FreeBSD VuXML. Typical commands: sudo pkg update, sudo pkg upgrade rxvt-unicode (or sudo pkg upgrade -y for the whole system), then and pkg audit to confirm no remaining advisories. For ports tree builders: sudo portsnap fetch update + cd /usr/ports/<cat>/rxvt-unicode && 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 rxvt-unicode 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 rxvt-unicode shows the expected fixed version is running, pkg audit returns no advisory for rxvt-unicode, tail -50 /var/log/messages shows no errors after restart, and the original reproduction for rxvt-unicode 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-rxvt-unicode-patch. To revert, run sudo pkg install -f <previous-version> or boot the previous BE via bectl activate pre-rxvt-unicode-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 rxvt-unicode: 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 this advisory.
