📖 ~4 min read • Source: FreeBSD VuXML
VuXML topic: py-yaml — FullLoader (still) exploitable for arbitrary command execution
Related CVEs: CVE-2017-18342 CVE-2020-1747
Upstream summary: Riccardo Schirone (https://github.com/ret2libc) reports: In FullLoader python/object/new constructor, implemented by construct_python_object_apply, has support for setting the state of a deserialized instance through the set_python_instance_state method. After setting the state, some operations are performed on the instance to complete its initialization, however it is possible for an attacker to set the instance' state in such a way that arbitrary code is executed by the Ful
Table of contents
Symptom & Impact
On FreeBSD 15 hosts running py27-yaml, 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 py27-yaml.
Environment & Reproduction
Reproduction targets FreeBSD 15. Confirm with freebsd-version -kru, uname -a, and the installed package via pkg info py27-yaml and pkg query "%n-%v" py27-yaml. Capture system state with pkg audit -F and service -e. Trigger the workflow that exposes py27-yaml — multiple vulnerabilities (2 CVEs) — 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 The FreeBSD ports security team shipped a corrective py27-yaml port revision; hosts on an outdated build remain exposed.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 py27-yaml status, tail -100 /var/log/messages, pkg audit -F, pkg version -v py27-yaml, 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 py27-yaml for integrity. 5) pkg install -fy py27-yaml to reinstall if tampered. 6) Correlate findings with /var/log/pkg.log and FreeBSD VuXML to pin the commit that introduced py27-yaml — multiple vulnerabilities (2 CVEs) — patch and remediation guide.
Solution – Primary Fix
Primary fix: install the corrective py27-yaml port revision referenced by FreeBSD VuXML. Typical commands: sudo pkg update, sudo pkg upgrade py27-yaml (or sudo pkg upgrade -y for the whole system), then sudo service py27-yaml restart, and pkg audit to confirm no remaining advisories. For ports tree builders: sudo portsnap fetch update + cd /usr/ports/<cat>/py27-yaml && sudo make deinstall reinstall clean. Reboot if the kernel module is involved.
Need help rolling this patch across a FreeBSD fleet? Our IT Solutions & Services team manages FreeBSD jail/bhyve patch windows. Get in touch for a free consultation.
Solution – Alternative Approaches
Alternatives include locking the package with sudo pkg lock py27-yaml 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 py27-yaml shows the expected fixed version, service py27-yaml status is running, pkg audit returns no advisory for py27-yaml, tail -50 /var/log/messages shows no errors after restart, and the original reproduction for py27-yaml — multiple vulnerabilities (2 CVEs) — 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-py27-yaml-patch. To revert, run sudo pkg install -f <previous-version> or boot the previous BE via bectl activate pre-py27-yaml-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 py27-yaml — multiple vulnerabilities (2 CVEs) — 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).
View all freebsd-15 tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
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 py27-yaml — multiple vulnerabilities (2 CVEs) — patch and remediation guide.
