Ubuntu 20.04 — gnome-control-center — vulnerability — patch and remediation guide

📖 ~4 min read  •  Source: Ubuntu Security Notice USN-6554-1

Related CVEs: CVE-2023-5616

Upstream summary: Zygmunt Krynicki discovered that GNOME Settings did not accurately reflect
the SSH remote login status when the system was configured to use systemd
socket activation for OpenSSH. Remote SSH access may be unknowingly
enabled, contrary to expectation.

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 gnome-control-center installed, administrators report behaviour consistent with Ubuntu Security Notice USN-6554-1: apt reports pending security updates, services backed by gnome-control-center 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 gnome-control-center 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 gnome-control-center | tail -2
apt-cache policy gnome-control-center
uname -r

Trigger the workflow that exposes gnome-control-center — vulnerability — patch and remediation guide while collecting:

sudo journalctl -u gnome-control-center -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-6554-1. Canonical security maintainers shipped fixes in the corresponding gnome-control-center update for Ubuntu 20.04; running an outdated build leaves the host exposed to the failure modes described in the advisory.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 gnome-control-center /var/log/apt/history.log
zgrep -A2 -B2 gnome-control-center /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 gnome-control-center:

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

Step-by-Step Diagnosis

1. List failing services.

   systemctl --failed --no-pager

2. Tail the journal / syslog for gnome-control-center.

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

5. Verify gnome-control-center integrity and reinstall if anything is altered.

   sudo dpkg -V gnome-control-center
   sudo debsums -c gnome-control-center 2>/dev/null
   sudo apt install --reinstall -y gnome-control-center

6. Correlate findings with /var/log/apt/history.log, /var/log/dpkg.log, and Ubuntu Security Notice USN-6554-1 to pin the change that introduced gnome-control-center — vulnerability — patch and remediation guide.

Solution – Primary Fix

Apply the corrective apt transaction referenced by Ubuntu Security Notice USN-6554-1, then reload the affected service:

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

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 gnome-control-center
  apt-mark showhold | grep gnome-control-center
  # Release the hold later with:
  sudo apt-mark unhold gnome-control-center

• Pin a known-good version via apt preferences:

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

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

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

Verification & Acceptance Criteria

All of these should pass after the fix is applied:

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

To revert:

sudo apt install --allow-downgrades -y gnome-control-center=<old-version>
sudo systemctl daemon-reload
sudo systemctl restart gnome-control-center
# 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-gnome-control-center

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 gnome-control-center — 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-6554-1. 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/gnome-control-center/ for components implicated in gnome-control-center — vulnerability — patch and remediation guide.