Ubuntu 18.04 — libcommons-net-java — vulnerability — patch and remediation guide

📖 ~4 min read  •  Source: Ubuntu Security Notice USN-6037-1

Related CVEs: CVE-2021-37533

Upstream summary: ZeddYu Lu discovered that the FTP client from Apache Commons Net trusted
the host from PASV responses by default. A remote attacker with a
malicious FTP server could redirect the client to another server, which
could possibly result in leaked information about services running on the
private network of the client.

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 18.04 (bionic) hosts that have libcommons-net-java installed, administrators report behaviour consistent with Ubuntu Security Notice USN-6037-1: apt reports pending security updates, services backed by libcommons-net-java 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 libcommons-net-java sits on the serving path.

Environment & Reproduction

Reproduction targets Ubuntu 18.04 (bionic). Confirm release and installed package:

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

Trigger the workflow that exposes libcommons-net-java — vulnerability — patch and remediation guide while collecting:

sudo journalctl -u libcommons-net-java -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-6037-1. Canonical security maintainers shipped fixes in the corresponding libcommons-net-java update for Ubuntu 18.04; running an outdated build leaves the host exposed to the failure modes described in the advisory. On this release the fix typically arrives via the Ubuntu Pro ESM (esm-infra / esm-apps) channels rather than the standard archive. Correlate apt history with the journal:

grep -A2 -B2 libcommons-net-java /var/log/apt/history.log
zgrep -A2 -B2 libcommons-net-java /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 18.04 to capture the current state of libcommons-net-java:

dpkg -l libcommons-net-java | tail -1                  # installed version
dpkg -V libcommons-net-java                             # verify shipped files
sudo apt update && apt list --upgradable 2>/dev/null | grep -i security
systemctl is-active libcommons-net-java
sudo ufw status verbose 2>/dev/null | head -20
sudo aa-status 2>/dev/null | head -20
# If libcommons-net-java 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 libcommons | head

On bionic the standard archive no longer ships security fixes. Verify Ubuntu Pro ESM coverage:

# Ubuntu Pro CLI is the standard tool:
sudo pro status --format=json 2>/dev/null | head
apt-cache policy | grep -i esm

Step-by-Step Diagnosis

1. List failing services.

   systemctl --failed --no-pager

2. Tail the journal / syslog for libcommons-net-java.

   sudo journalctl -u libcommons-net-java -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.libcommons-net-java or usr.sbin.libcommons-net-java — inspect first
   sudo aa-complain /etc/apparmor.d/usr.bin.libcommons-net-java 2>/dev/null || true

5. Verify libcommons-net-java integrity and reinstall if anything is altered.

   sudo dpkg -V libcommons-net-java
   sudo debsums -c libcommons-net-java 2>/dev/null
   sudo apt install --reinstall -y libcommons-net-java

6. Correlate findings with /var/log/apt/history.log, /var/log/dpkg.log, and Ubuntu Security Notice USN-6037-1 to pin the change that introduced libcommons-net-java — vulnerability — patch and remediation guide.

Solution – Primary Fix

Apply the corrective apt transaction referenced by Ubuntu Security Notice USN-6037-1, then reload the affected service:

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

On bionic the standard archive is past EoL for security; enable Ubuntu Pro ESM to receive the fix:

# Standard pro CLI:
sudo pro attach <token>
sudo pro enable esm-infra
sudo pro enable esm-apps
sudo apt update
sudo apt -y install --only-upgrade libcommons-net-java

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 libcommons-net-java
  apt-mark showhold | grep libcommons-net-java
  # Release the hold later with:
  sudo apt-mark unhold libcommons-net-java

• Pin a known-good version via apt preferences:

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

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

  apt-cache madison libcommons-net-java
  sudo apt install --allow-downgrades -y libcommons-net-java=<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.libcommons-net-java 2>/dev/null
  # reproduce the failure
  sudo journalctl -k | grep apparmor | tail
  sudo aa-enforce /etc/apparmor.d/usr.bin.libcommons-net-java 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 bionic-security libcommons-net-java

Verification & Acceptance Criteria

All of these should pass after the fix is applied:

dpkg -l libcommons-net-java | tail -1                                  # expected fixed version
apt list --upgradable 2>/dev/null | grep -i security || echo OK
systemctl is-active libcommons-net-java
sudo journalctl -u libcommons-net-java --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 libcommons-net-java — 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-libcommons-net-java
# LVM-on-root:
sudo lvcreate -L 4G -s -n root_pre_patch /dev/<vg>/<root-lv>

To revert:

sudo apt install --allow-downgrades -y libcommons-net-java=<old-version>
sudo systemctl daemon-reload
sudo systemctl restart libcommons-net-java
# 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-libcommons-net-java

Prevention & Hardening

Reduce the chance of this recurring on Ubuntu 18.04 (bionic):

• 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 for ESM (mandatory on this past-EoL release) and Livepatch where supported:

  sudo pro attach <token>
  sudo pro enable esm-infra
  sudo pro enable esm-apps
  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 libcommons-net-java — 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-18-04 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Primary reference: Ubuntu Security Notice USN-6037-1. Manual pages useful on Ubuntu 18.04:

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/libcommons-net-java/ for components implicated in libcommons-net-java — vulnerability — patch and remediation guide.