FreeBSD 12 — ksh — security advisory — patch and remediation guide

📖 ~4 min read  •  Source: FreeBSD VuXML

VuXML topic: ksh93 — certain environment variables interpreted as arithmetic expressions on startup, leading to code injection

Upstream summary: Upstream ksh93 maintainer Siteshwar Vashisht reports: A flaw was found in the way ksh evaluates certain environment variables. An attacker could use this flaw to override or bypass environment restrictions to execute shell commands. Services and applications that allow remote unauthenticated attackers to provide one of those environment variables could allow them to exploit this issue remotely.

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 12 hosts that have ksh installed, operators see behaviour consistent with the FreeBSD VuXML entry: pkg audit flags the installed version; any daemon, CLI tool, or application linked against ksh may misbehave or fail to start after upgrade; and — for security-rated advisories — the host is exposed to the vulnerabilities above. Impact ranges from a single restart cycle to full availability incidents on jails, bhyve guests, or downstream consumers that depend on ksh.

Environment & Reproduction

Reproduction targets FreeBSD 12. Confirm release, installed package, and capture baseline state:

freebsd-version -kru
uname -a
pkg info ksh
pkg query "%n-%v" ksh
pkg audit -F
service -e

Trigger the workflow that exposes ksh — security advisory — patch and remediation guide while collecting:

tail -200 /var/log/messages
dmesg -a | tail -200
tail -200 /var/log/pkg.log

Root Cause Analysis

Root cause is tracked at FreeBSD VuXML. The FreeBSD ports security team shipped a corrective ksh port revision; hosts on an outdated build remain exposed. Correlate package logs with system logs and kernel state to isolate the change that triggered the failure mode:

tail -500 /var/log/pkg.log
tail -500 /var/log/messages
sysctl kern.lastpid
sysctl kern.osreldate     # numeric __FreeBSD_version, e.g. 1400097

Quick Triage

Run these checks on FreeBSD 12 to confirm the failure mode and current state of ksh:

pkg version -v ksh                # installed vs available version
pkg audit ksh                     # advisory match for this package
tail -100 /var/log/messages
dmesg -a | tail -100
kldstat                              # kernel module state (for kernel/driver pkgs)
pfctl -sr 2>/dev/null || ipfw list   # only if pf/ipfw is enabled
# If ksh ships an rc.d service (script name may differ from the pkg name,
# e.g. bind918→named, php83→php-fpm), check it:
service -e | grep -i ksh && service <rc-script-name> status

Step-by-Step Diagnosis

1. List enabled services (only relevant if the package provides one).

   service -e

2. Follow live logs.

   tail -F /var/log/messages
   dmesg

3. Validate firewall rules (skip if neither pf nor ipfw is enabled).

   pfctl -sr -v 2>/dev/null || ipfw show

4. Check package integrity for ksh.

   pkg check -B ksh
   pkg check -d ksh    # verify shared-library deps

5. Reinstall ksh if integrity check fails.

   pkg install -fy ksh

6. Correlate findings with /var/log/pkg.log and FreeBSD VuXML to pin the commit that introduced ksh — security advisory — patch and remediation guide.

Solution – Primary Fix

Install the corrective ksh port revision referenced by FreeBSD VuXML:

sudo pkg update
sudo pkg upgrade ksh              # or: sudo pkg upgrade -y for the whole system
# If ksh provides an rc.d service, restart it (script name may differ from pkg name):
# sudo service <rc-script-name> restart
pkg audit ksh                    # confirm no remaining advisory for this package

For ports-tree builders (FreeBSD 13.x and earlier used portsnap; on FreeBSD 14+ the ports tree is fetched with Git):

# FreeBSD 14+ (portsnap was removed):
sudo pkg install -y git-lite
sudo git clone --depth 1 https://git.FreeBSD.org/ports.git /usr/ports
# FreeBSD 13.x and earlier:
# sudo portsnap fetch update
cd /usr/ports/<category>/ksh
sudo make deinstall reinstall clean

Reboot only if the package ships a kernel module or replaces a shared library used by long-running daemons.

Solution – Alternative Approaches

If the primary fix is not viable, choose from these alternatives:

• Lock the package until the fix is vetted:

  sudo pkg lock ksh

• Downgrade to a known-good revision. pkg install pkgname-VERSION is not a real downgrade syntax — fetch a specific build instead:

  # 1. Discover available versions across configured repos:
  pkg search -e ksh
  pkg rquery -r FreeBSD-quarterly '%n-%v' ksh
  # 2. Install from a specific saved .pkg file:
  sudo pkg add -f /path/to/ksh-<older-version>.pkg
  # 3. Or switch the host repo to the quarterly branch (see snippet below) and:
  sudo pkg upgrade -fr FreeBSD-quarterly ksh

• Switch the pkg repository between quarterly and latest by editing /usr/local/etc/pkg/repos/FreeBSD.conf:

  FreeBSD: {
    url: "pkg+https://pkg.FreeBSD.org/${ABI}/quarterly",
    mirror_type: "srv",
    signature_type: "fingerprints",
    fingerprints: "/usr/share/keys/pkg",
    enabled: yes
  }

• Isolate the affected service in a jail with stricter firewall rules:

  iocage create -n ksh-jail -r 12.4-RELEASE
  iocage set allow_raw_sockets=0 ksh-jail
  # or with Bastille:
  bastille create ksh-jail 12.4-RELEASE 10.0.0.10

• Replace the service with a vendored static build for the period between exposure detection and full rollout.

Verification & Acceptance Criteria

All of these should pass after the fix:

pkg info ksh                # shows the expected fixed version
pkg audit ksh               # no advisory for this package (exit code 0)
tail -50 /var/log/messages   # no new errors after upgrade
# If ksh ships a service, confirm it is running under its rc.d name:
# service <rc-script-name> status

The original reproduction for ksh — security advisory — patch and remediation guide must not trigger across two consecutive runs.

Rollback Plan

Capture state before any change (only ZFS root has boot environments — UFS hosts skip bectl):

pkg query "%n-%v" > /root/pkg-pre.txt
# ZFS-on-root only:
sudo bectl create pre-ksh-patch

To revert if the upgrade is bad, reinstall the previously saved .pkg file:

sudo pkg add -f /var/cache/pkg/ksh-<previous-version>.pkg
# Or activate the pre-patch boot environment and reboot (ZFS-on-root only):
sudo bectl activate pre-ksh-patch
sudo shutdown -r now

For kernel/loader changes on a UFS host, boot the previous kernel from the loader prompt (press 3 at the menu, then boot kernel.old).

Prevention & Hardening

Prevent recurrence on FreeBSD 12 hosts running ksh:

• Enable the daily security pkg audit in /etc/periodic.conf:

  daily_status_security_pkgaudit_enable="YES"

• Subscribe to freebsd-security-notifications at lists.freebsd.org.

• Mirror through a local pkg repository managed by poudriere:

  poudriere jail -c -j 12amd64 -v 12.4-RELEASE
  poudriere ports -c -p default
  poudriere bulk -j 12amd64 -p default <category>/ksh

• Version-pin sensitive packages:

  sudo pkg lock ksh

• Take an automatic ZFS boot-environment snapshot before every upgrade (ZFS root only):

  sudo bectl create pre-upgrade-$(date +%Y%m%d)

• Monitor file integrity (create a baseline, verify against it later):

  # Create a baseline (use -c; target /usr/local/etc, /etc, /boot — NOT /):
  sudo mtree -c -K sha256digest -p /usr/local/etc > /var/db/usr-local-etc.mtree
  sudo mtree -c -K sha256digest -p /etc          > /var/db/etc.mtree
  # Verify later:
  sudo mtree -p /usr/local/etc < /var/db/usr-local-etc.mtree
  # Or with AIDE for a richer ruleset:
  sudo pkg install -y aide && sudo aide --init && sudo aide --check

• Harden jails with allow.* tunables in /etc/jail.conf:

  ksh_jail {
    allow.raw_sockets = 0;
    allow.sysvipc    = 0;
    allow.mount      = 0;
    allow.chflags    = 0;
  }

Related Errors & Cross-Refs

Issues that commonly surface alongside ksh — security advisory — patch and remediation guide: pkg lock contention, mismatched ABI after kernel/userland skew, pf rule drift, and stale shared-library references after upgrade. Triage with:

freebsd-version -kru
uname -K
pkg check -d
pfctl -sr

View all freebsd-12 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Primary reference: FreeBSD VuXML. Useful manual pages on FreeBSD 12:

man pkg
man freebsd-update
man pfctl
man ipfw
man bectl
man periodic.conf

Other resources: the FreeBSD Handbook, the FreeBSD Security Advisories at security.freebsd.org, and the /usr/ports/UPDATING file for port-specific notes implicated in ksh — security advisory — patch and remediation guide.