Ubuntu 20.04 — lz4 — vulnerability — patch and remediation guide

📖 ~4 min read  •  Source: Ubuntu Security Notice USN-4968-1

Related CVEs: CVE-2021-3520

Upstream summary: It was discovered that LZ4 incorrectly handled certain memory operations.
If a user or automated system were tricked into uncompressing a specially-
crafted LZ4 file, a remote attacker could use this issue to cause LZ4 to
crash, resulting in a denial of service, or possibly execute arbitrary
code.

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 Ubuntu 20.04 (focal) hosts that have lz4 installed, administrators report behaviour consistent with Ubuntu Security Notice USN-4968-1: apt reports pending security updates, services backed by lz4 fail or restart unexpectedly, AppArmor denials appear in the kernel log, and — for security-rated advisories — the host is exposed to the vulnerability set above. Impact ranges from a single service-restart loop to wider availability incidents whenever lz4 sits on the serving path.

Environment & Reproduction

Reproduction targets Ubuntu 20.04 (focal). Confirm release and installed package:

lsb_release -a
cat /etc/os-release
dpkg -l lz4 | tail -2
apt-cache policy lz4
uname -r

Trigger the workflow that exposes lz4 — vulnerability — patch and remediation guide while collecting:

sudo journalctl -u lz4 -b --no-pager | tail -200
sudo journalctl -xe --no-pager | tail -200
sudo tail -200 /var/log/apt/history.log
sudo tail -200 /var/log/kern.log | grep -i apparmor

Root Cause Analysis

Root cause is documented in Ubuntu Security Notice USN-4968-1. Canonical security maintainers shipped fixes in the corresponding lz4 update for Ubuntu 20.04; running an outdated build leaves the host exposed to the failure modes described in the advisory. Fixes land in the focal-security pocket of the main archive. Correlate apt history with the journal:

grep -A2 -B2 lz4 /var/log/apt/history.log
zgrep -A2 -B2 lz4 /var/log/apt/history.log.*.gz 2>/dev/null
cat /proc/sys/kernel/tainted   # non-zero = tainted kernel / out-of-tree modules

Quick Triage

Run these on Ubuntu 20.04 to capture the current state of lz4:

dpkg -l lz4 | tail -1                  # installed version
dpkg -V lz4                             # verify shipped files
sudo apt update && apt list --upgradable 2>/dev/null | grep -i security
systemctl is-active lz4
sudo ufw status verbose 2>/dev/null | head -20
sudo aa-status 2>/dev/null | head -20
# If lz4 ships a service unit (unit/job name often differs from pkg name, e.g.
# bind9→named, apache2→apache2, postgresql-NN→postgresql@NN-main):
systemctl list-unit-files | grep -i lz4 | head

Step-by-Step Diagnosis

1. List failing services.

   systemctl --failed --no-pager

2. Tail the journal / syslog for lz4.

   sudo journalctl -u lz4 -f --no-pager
   sudo journalctl -xe -f --no-pager

3. Inspect UFW (Uncomplicated Firewall) state.

   sudo ufw status numbered
   sudo ufw show added
   sudo iptables -L -n -v | head -30

4. Surface AppArmor denials and switch the profile to complain mode if needed.

   sudo journalctl -k 2>/dev/null | grep -i 'apparmor="DENIED"' | tail -30
   sudo aa-status
   # /etc/apparmor.d/usr.bin.lz4 or usr.sbin.lz4 — inspect first
   sudo aa-complain /etc/apparmor.d/usr.bin.lz4 2>/dev/null || true

5. Verify lz4 integrity and reinstall if anything is altered.

   sudo dpkg -V lz4
   sudo debsums -c lz4 2>/dev/null
   sudo apt install --reinstall -y lz4

6. Correlate findings with /var/log/apt/history.log, /var/log/dpkg.log, and Ubuntu Security Notice USN-4968-1 to pin the change that introduced lz4 — vulnerability — patch and remediation guide.

Solution – Primary Fix

Apply the corrective apt transaction referenced by Ubuntu Security Notice USN-4968-1, then reload the affected service:

sudo apt update
sudo apt -y install --only-upgrade lz4
sudo systemctl daemon-reload
# Service name may differ from pkg name; check first:
systemctl list-unit-files | grep -i lz4 | head
sudo systemctl restart lz4
dpkg -l lz4 | tail -1            # confirm new version
systemctl is-active lz4

For kernel / glibc / systemd / openssl advisories a reboot (or Livepatch) is required:

sudo apt install -y needrestart
sudo needrestart -r l       # list units that need restart
sudo systemctl reboot       # or: sudo shutdown -r now
# Livepatch (Ubuntu Pro) avoids reboot for many kernel CVEs:
sudo canonical-livepatch status
sudo canonical-livepatch refresh

Solution – Alternative Approaches

If the primary upgrade is not viable, pick from these:

• Hold the package so apt cannot upgrade it:

  sudo apt-mark hold lz4
  apt-mark showhold | grep lz4
  # Release the hold later with:
  sudo apt-mark unhold lz4

• Pin a known-good version via apt preferences:

  # /etc/apt/preferences.d/lz4.pref
  Package: lz4
  Pin: version <good-version>
  Pin-Priority: 1001

• Downgrade to an older version if a regression is suspected:

  apt-cache madison lz4
  sudo apt install --allow-downgrades -y lz4=<older-version>

• Investigate AppArmor blocking the new binary; switch to complain briefly, capture denials, then re-enforce:

  sudo aa-complain /etc/apparmor.d/usr.bin.lz4 2>/dev/null
  # reproduce the failure
  sudo journalctl -k | grep apparmor | tail
  sudo aa-enforce /etc/apparmor.d/usr.bin.lz4 2>/dev/null

• Apply Canonical Livepatch (Ubuntu Pro) to land kernel fixes without reboot:

  sudo canonical-livepatch status
  sudo canonical-livepatch refresh

• Take only the security pocket update and defer the full point-release upgrade:

  sudo apt -y install --only-upgrade -t focal-security lz4

Verification & Acceptance Criteria

All of these should pass after the fix is applied:

dpkg -l lz4 | tail -1                                  # expected fixed version
apt list --upgradable 2>/dev/null | grep -i security || echo OK
systemctl is-active lz4
sudo journalctl -u lz4 --since "5 minutes ago" --no-pager | grep -iE "error|fail" || echo OK
sudo ufw status numbered | head
sudo aa-status 2>/dev/null | head -5

The original reproduction for lz4 — vulnerability — patch and remediation guide must not trigger across two consecutive runs.

Rollback Plan

Capture state before any change:

apt list --installed 2>/dev/null > /root/apt-pre.txt
dpkg --get-selections > /root/dpkg-pre.txt
# ZFS-on-root (Ubuntu 20.04+ default installer option):
sudo zfs snapshot rpool/ROOT/ubuntu@pre-lz4
# LVM-on-root:
sudo lvcreate -L 4G -s -n root_pre_patch /dev/<vg>/<root-lv>

To revert:

sudo apt install --allow-downgrades -y lz4=<old-version>
sudo systemctl daemon-reload
sudo systemctl restart lz4
# Kernel rollback: pick the prior kernel from the GRUB menu, then:
sudo systemctl reboot
# ZFS rollback (rolls the whole root dataset):
sudo zfs rollback -r rpool/ROOT/ubuntu@pre-lz4

Prevention & Hardening

Reduce the chance of this recurring on Ubuntu 20.04 (focal):

• Enable scheduled security updates via unattended-upgrades:

  sudo apt install -y unattended-upgrades update-notifier-common
  sudo dpkg-reconfigure -plow unattended-upgrades
  # /etc/apt/apt.conf.d/50unattended-upgrades:
  Unattended-Upgrade::Allowed-Origins { "${distro_id}:${distro_codename}-security"; };

• Install needrestart so services restart automatically after library upgrades:

  sudo apt install -y needrestart
  # /etc/needrestart/needrestart.conf -> $nrconf{restart} = 'a';

• Attach Ubuntu Pro to enable Livepatch and extended security coverage:

  sudo pro attach <token>
  sudo pro enable livepatch

• Subscribe to ubuntu-security-announce and watch ubuntu.com/security/cves.

• Monitor file integrity with debsums and AIDE:

  sudo apt install -y debsums aide
  sudo debsums -ca
  sudo aideinit && sudo mv /var/lib/aide/aide.db.new /var/lib/aide/aide.db
  sudo aide --check

• For estate-wide patching, manage with Canonical Landscape:

  sudo apt install -y landscape-client
  sudo landscape-config

• Keep AppArmor profiles in enforce mode and apply CIS Ubuntu Linux Benchmark hardening.

Related Errors & Cross-Refs

Issues that commonly surface alongside lz4 — vulnerability — patch and remediation guide: apt lock contention, broken dpkg state, systemd ordering cycles, AppArmor denials, and UFW rule drift. Useful triage:

sudo dpkg --configure -a
sudo apt --fix-broken install
systemd-analyze critical-chain
sudo journalctl -k 2>/dev/null | grep -i apparmor | tail
cat /proc/sys/kernel/tainted

View all ubuntu-20-04 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Primary reference: Ubuntu Security Notice USN-4968-1. Manual pages useful on Ubuntu 20.04:

man apt
man apt-get
man apt-mark
man dpkg
man systemctl
man journalctl
man ufw
man apparmor
man aa-status
man unattended-upgrades
man canonical-livepatch
man pro

Other resources: Ubuntu Security Notices, Ubuntu CVE Tracker, Ubuntu upgrade notes, and per-package notes in /usr/share/doc/lz4/ for components implicated in lz4 — vulnerability — patch and remediation guide.