FreeBSD 15 — elm — security advisory — patch and remediation guide

📖 ~4 min read  •  Source: FreeBSD VuXML

VuXML topic: elm — remote buffer overflow in Expires header

Upstream summary: Ulf Harnhammar has discovered a remotely exploitable buffer overflow in Elm e-mail client when parsing the Expires header of an e-mail message: The attacker only needs to send the victim an e-mail message. When the victim with that message in his or her inbox starts Elm or simply views the inbox in an already started copy of Elm, the buffer overflow will happen immediately. The overflow is stack-based, and it gives full control over EIP, EBP and EBX. It is caused by a bad ssc

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

On FreeBSD 15 hosts running elm, 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 elm.

Environment & Reproduction

Reproduction targets FreeBSD 15. Confirm with freebsd-version -kru, uname -a, and the installed package via pkg info elm and pkg query "%n-%v" elm. Capture system state with pkg audit -F and service -e. Trigger the workflow that exposes elm — security advisory — 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 The FreeBSD ports security team shipped a corrective elm port revision; hosts on an outdated build remain exposed.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 elm status, tail -100 /var/log/messages, pkg audit -F, pkg version -v elm, 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 elm for integrity. 5) pkg install -fy elm to reinstall if tampered. 6) Correlate findings with /var/log/pkg.log and FreeBSD VuXML to pin the commit that introduced elm — security advisory — patch and remediation guide.

Solution – Primary Fix

Primary fix: install the corrective elm port revision referenced by FreeBSD VuXML. Typical commands: sudo pkg update, sudo pkg upgrade elm (or sudo pkg upgrade -y for the whole system), then sudo service elm restart, and pkg audit to confirm no remaining advisories. For ports tree builders: sudo portsnap fetch update + cd /usr/ports/<cat>/elm && sudo make deinstall reinstall clean. Reboot if the kernel module is involved.

Solution – Alternative Approaches

Alternatives include locking the package with sudo pkg lock elm 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 elm shows the expected fixed version, service elm status is running, pkg audit returns no advisory for elm, tail -50 /var/log/messages shows no errors after restart, and the original reproduction for elm — security advisory — 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-elm-patch. To revert, run sudo pkg install -f <previous-version> or boot the previous BE via bectl activate pre-elm-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 elm — security advisory — 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 elm — security advisory — patch and remediation guide.