Ubuntu 20.04 — shim-signed — multiple vulnerabilities (11 CVEs) — patch and remediation guide

📖 ~4 min read  •  Source: Ubuntu Security Notice USN-6355-1

Related CVEs: CVE-2022-28737 CVE-2021-3695 CVE-2021-3696 CVE-2021-3697 CVE-2021-3981 CVE-2022-28733 CVE-2022-28734 CVE-2022-28735  +3 more

Upstream summary: Daniel Axtens discovered that specially crafted images could cause a
heap-based out-of-bonds write. A local attacker could possibly use
this to circumvent secure boot protections. (CVE-2021-3695)

Daniel Axtens discovered that specially crafted images could cause
out-of-bonds read and write. A local attacker could possibly use this
to circumvent secure boot protections. (CVE-2021-3696)

Daniel Axtens discovered that specially crafted images could cause
buffer underwrite which

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 hosts running shim-signed, administrators report behaviour consistent with Ubuntu Security Notice USN-6355-1: 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 shim-signed sits on the serving path.

Environment & Reproduction

Reproduction targets Ubuntu 20.04. Confirm release with lsb_release -a and cat /etc/os-release, and the currently installed package with dpkg -l shim-signed and apt-cache policy shim-signed. Capture system state with sudo ubuntu-bug shim-signed or sudo apport-collect for an evidence bundle. Trigger the workflow that exposes shim-signed — multiple vulnerabilities (11 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-6355-1. Canonical security maintainers shipped fixes in the corresponding shim-signed update for Ubuntu 20.04; running an outdated build leaves the host exposed to the failure modes described in the advisory.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 shim-signed, journalctl -u shim-signed -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 shim-signed — multiple vulnerabilities (11 CVEs) — patch and remediation guide.

Step-by-Step Diagnosis

1) Confirm symptom with systemctl --failed. 2) Inspect logs: journalctl -xe and journalctl -u shim-signed. 3) Validate firewall: sudo ufw status numbered. 4) Check AppArmor: sudo aa-status and journalctl -k | grep apparmor. 5) Verify package integrity: dpkg -V shim-signed and sudo apt install --reinstall shim-signed. 6) Correlate findings with apt list --installed shim-signed, /var/log/apt/history.log, and Ubuntu Security Notice USN-6355-1 to pin the change that introduced shim-signed — multiple vulnerabilities (11 CVEs) — patch and remediation guide.

Solution – Primary Fix

Primary fix for shim-signed — multiple vulnerabilities (11 CVEs) — patch and remediation guide: apply the corrective apt transaction described in Ubuntu Security Notice USN-6355-1, reload the affected systemd unit, and reconcile UFW / AppArmor state. Typical commands: sudo apt update, sudo apt -y install --only-upgrade shim-signed or sudo unattended-upgrade -v, sudo systemctl daemon-reload, sudo systemctl restart shim-signed, then dpkg -l shim-signed 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.

Solution – Alternative Approaches

Alternatives include pinning a known-good version via /etc/apt/preferences.d/shim-signed.pref with apt-mark hold shim-signed, rolling back with sudo apt install shim-signed=<old-version>, rotating UFW rules with sudo ufw reload, switching AppArmor profiles to complain mode (sudo aa-complain /etc/apparmor.d/usr.bin.shim-signed) 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 shim-signed shows the expected fixed version, systemctl is-active shim-signed returns active, journalctl -u shim-signed --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 shim-signed — multiple vulnerabilities (11 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-shim-signed on ZFS-on-root installs. To revert, run sudo apt install --allow-downgrades shim-signed=<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 shim-signed — multiple vulnerabilities (11 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 20.04 series for adjacent failure modes.

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Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Primary reference: Ubuntu Security Notice USN-6355-1. 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/shim-signed/ for component-level notes implicated in shim-signed — multiple vulnerabilities (11 CVEs) — patch and remediation guide.