📖 ~4 min read • Source: Ubuntu Security Notice USN-7804-2
Related CVEs: CVE-2025-59362 CVE-2025-62168 CVE-2024-37894 CVE-2021-28651 CVE-2022-41318 CVE-2023-49285 CVE-2023-49286 CVE-2023-50269 +12 more
Upstream summary: USN-7804-1 fixed a vulnerability in Squid. This update provides the
corresponding update for Ubuntu 16.04 LTS, Ubuntu 18.04 LTS and Ubuntu
20.04 LTS.
Original advisory details:
It was discovered that Squid incorrectly handled certain long SNMP OIDs. A
remote attacker could possibly use this issue to cause Squid to crash,
resulting in a denial of service.
Table of contents
Symptom & Impact
On Ubuntu 16.04 hosts running squid3, administrators report behaviour consistent with Ubuntu Security Notice USN-7804-2: apt refusing to install or restart affected services, AppArmor denials in journalctl -k, and — for security-rated advisories — exposure to the vulnerability set above. In production estates the visible impact ranges from a single service restart to wider availability incidents whenever squid3 sits on the serving path.
Environment & Reproduction
Reproduction targets Ubuntu 16.04. Confirm release with lsb_release -a and cat /etc/os-release, and the currently installed package with dpkg -l squid3 and apt-cache policy squid3. Capture system state with sudo ubuntu-bug squid3 or sudo apport-collect for an evidence bundle. Trigger the workflow that exposes squid3 — multiple vulnerabilities (20 CVEs) — patch and remediation guide while collecting journalctl -b, /var/log/apt/history.log, and dpkg -l output.
Root Cause Analysis
Root cause is documented in Ubuntu Security Notice USN-7804-2. Canonical security maintainers shipped fixes in the corresponding squid3 update for Ubuntu 16.04; running an outdated build leaves the host exposed to the failure modes described in the advisory. Correlate journalctl --since timestamps with apt history (/var/log/apt/history.log) and any AppArmor denials in /var/log/syslog to isolate the originating change.
Quick Triage
Quick triage: run systemctl status squid3, journalctl -u squid3 -n 200, sudo apt update && apt list --upgradable, sudo ufw status verbose, and sudo aa-status. If AppArmor is in enforce mode, capture journalctl -k | grep apparmor to surface denials linked to squid3 — multiple vulnerabilities (20 CVEs) — patch and remediation guide.
Step-by-Step Diagnosis
1) Confirm symptom with systemctl --failed. 2) Inspect logs: journalctl -xe and journalctl -u squid3. 3) Validate firewall: sudo ufw status numbered. 4) Check AppArmor: sudo aa-status and journalctl -k | grep apparmor. 5) Verify package integrity: dpkg -V squid3 and sudo apt install --reinstall squid3. 6) Correlate findings with apt list --installed squid3, /var/log/apt/history.log, and Ubuntu Security Notice USN-7804-2 to pin the change that introduced squid3 — multiple vulnerabilities (20 CVEs) — patch and remediation guide.
Solution – Primary Fix
Primary fix for squid3 — multiple vulnerabilities (20 CVEs) — patch and remediation guide: apply the corrective apt transaction described in Ubuntu Security Notice USN-7804-2, reload the affected systemd unit, and reconcile UFW / AppArmor state. Typical commands: sudo apt update, sudo apt -y install --only-upgrade squid3 or sudo unattended-upgrade -v, sudo systemctl daemon-reload, sudo systemctl restart squid3, then dpkg -l squid3 to validate the new build is installed. For kernel advisories add sudo reboot or apply Ubuntu Livepatch (canonical-livepatch status) where covered by your Ubuntu Pro subscription.
Need help rolling this patch across an Ubuntu fleet? Our IT Solutions & Services team manages Ubuntu patch windows with zero-downtime change controls and Ubuntu Pro / Landscape integration. Get in touch for a free consultation.
Solution – Alternative Approaches
Alternatives include pinning a known-good version via /etc/apt/preferences.d/squid3.pref with apt-mark hold squid3, rolling back with sudo apt install squid3=<old-version>, rotating UFW rules with sudo ufw reload, switching AppArmor profiles to complain mode (sudo aa-complain /etc/apparmor.d/usr.bin.squid3) to confirm policy is the cause before authoring a custom profile, or applying Canonical Livepatch fixes via canonical-livepatch refresh where Ubuntu Pro is subscribed.
Verification & Acceptance Criteria
Acceptance: dpkg -l squid3 shows the expected fixed version, systemctl is-active squid3 returns active, journalctl -u squid3 --since "5 minutes ago" shows no errors, apt list --upgradable no longer lists the advisory, sudo ufw status shows required services, sudo aa-status reports the intended profile mode, and the original reproduction steps for squid3 — multiple vulnerabilities (20 CVEs) — patch and remediation guide no longer trigger the failure across two consecutive runs.
Rollback Plan
Capture state with apt list --installed > /root/apt-pre.txt, dpkg --get-selections > /root/dpkg-pre.txt, and where available sudo zfs snapshot rpool/ROOT/ubuntu@pre-squid3 on ZFS-on-root installs. To revert, run sudo apt install --allow-downgrades squid3=<old-version> and reload systemctl daemon-reload. Remove custom AppArmor profiles with sudo apparmor_parser -R. Reboot if the kernel or initramfs changed and re-verify symptoms.
Prevention & Hardening
Prevent recurrence by enabling unattended-upgrades with Unattended-Upgrade::Allowed-Origins tuned to ${distro_id}:${distro_codename}-security, subscribing to the ubuntu-security-announce mailing list, mirroring through Landscape / a local apt-mirror for controlled rollouts, version-locking sensitive packages, and monitoring file integrity with aide --check. Apply CIS Ubuntu hardening, keep AppArmor in enforce, and enable Canonical Livepatch under Ubuntu Pro so kernel advisories can be remediated without reboot.
Related Errors & Cross-Refs
Related issues that commonly surface alongside squid3 — multiple vulnerabilities (20 CVEs) — patch and remediation guide: apt lock contention (dpkg --configure -a), systemd unit ordering cycles, AppArmor denials in journalctl -k, UFW zone drift, and kernel taint flags in cat /proc/sys/kernel/tainted. See sibling common-problem articles in this Ubuntu 16.04 series for adjacent failure modes.
View all ubuntu-16-04 tutorials on the Tutorials Hub →
Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Primary reference: Ubuntu Security Notice USN-7804-2. Supporting docs: Ubuntu Server Guide, man apt, man systemctl, man ufw, man aa-status, man apparmor, man journalctl, the Ubuntu CVE Tracker at ubuntu.com/security/cves, and Canonical Livepatch docs. Review /usr/share/doc/squid3/ for component-level notes implicated in squid3 — multiple vulnerabilities (20 CVEs) — patch and remediation guide.
