📖 ~4 min read • Source: AlmaLinux/RHEL advisory ALSA-2024:1903
Related CVEs: CVE-2023-40546 CVE-2023-40547 CVE-2023-40548 CVE-2023-40549 CVE-2023-40550 CVE-2023-40551 CVE-2021-3695 CVE-2021-3696 +6 more
Upstream summary: The shim package contains a first-stage UEFI boot loader that handles chaining
to a trusted full boot loader under secure boot environments.
Security Fix(es):
* shim: RCE in http boot support may lead to Secure Boot bypass (CVE-2023-40547)
* shim: Interger overflow leads to heap buffer overflow in verify_sbat_section
on 32-bits systems (CVE-2023-40548)
* shim: Out-of-bounds read printing error messages (CVE-2023-40546)
* shim: Out-of-bounds read in verify_buffer_authenticod
Table of contents
Symptom & Impact
On CentOS Stream 9 hosts running shim, operators report behaviour consistent with AlmaLinux/RHEL advisory ALSA-2024:1903: dnf refusing to install or restart affected services, SELinux AVC denials in /var/log/audit/audit.log, 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 sits on the serving path.
Environment & Reproduction
Reproduction targets CentOS Stream 9. Confirm release with cat /etc/centos-release and cat /etc/os-release, and the currently installed package with rpm -q shim. Capture system state with sosreport --batch -k crio.all=on if you need an evidence bundle for a vendor support case. Trigger the workflow that exposes shim — multiple vulnerabilities (14 CVEs) — patch and remediation guide while collecting journalctl -b, dnf history, and rpm -qa output.
Root Cause Analysis
Root cause is documented in AlmaLinux/RHEL advisory ALSA-2024:1903. Upstream maintainers shipped fixes in the corresponding shim update for CentOS Stream 9; running an outdated build leaves the host exposed to the failure modes described in the advisory. Correlate journalctl --since timestamps with dnf history entries and any SELinux denials in /var/log/audit/audit.log (or ausearch -m AVC,USER_AVC -ts recent) to isolate the originating change.
Quick Triage
Quick triage: run systemctl status shim, journalctl -u shim -n 200, dnf check-update --security, dnf updateinfo list cves, firewall-cmd --list-all, and getenforce. If SELinux is enforcing, capture ausearch -m AVC -ts recent to surface denials linked to shim — multiple vulnerabilities (14 CVEs) — patch and remediation guide.
Step-by-Step Diagnosis
1) Confirm symptom with systemctl --failed. 2) Inspect logs: journalctl -xe and journalctl -u shim. 3) Validate firewall: firewall-cmd --list-all-zones. 4) Check SELinux: getenforce, sestatus, ausearch -m AVC,USER_AVC -ts recent. 5) Verify package integrity: rpm -V shim and dnf reinstall shim. 6) Correlate findings with dnf history, /var/log/dnf.log, and AlmaLinux/RHEL advisory ALSA-2024:1903 to pin the change that introduced shim — multiple vulnerabilities (14 CVEs) — patch and remediation guide.
Solution – Primary Fix
Primary fix for shim — multiple vulnerabilities (14 CVEs) — patch and remediation guide: apply the corrective dnf transaction described in AlmaLinux/RHEL advisory ALSA-2024:1903, reload the affected systemd unit, and reconcile firewalld and SELinux state. Typical commands: sudo dnf -y makecache, sudo dnf -y upgrade --security or sudo dnf -y upgrade shim, sudo systemctl daemon-reload, sudo systemctl restart shim, then rpm -q shim to validate the new build is installed. For kernel advisories add sudo systemctl reboot or apply kpatch-dnf live patches where covered by your subscription.
Need help rolling this patch across a CentOS Stream fleet? Our IT Solutions & Services team manages CentOS / RHEL patch windows with zero-downtime change controls. Get in touch for a free consultation.
Solution – Alternative Approaches
Alternatives include rolling back the offending transaction with sudo dnf history undo <id>, version-locking the package via sudo dnf install python3-dnf-plugin-versionlock then sudo dnf versionlock add shim, switching firewalld backends between nftables and iptables in /etc/firewalld/firewalld.conf, switching SELinux to permissive temporarily with sudo setenforce 0 to confirm policy is the cause before authoring a custom policy module via audit2allow. Where kpatch is licensed, kpatch list and kpatch load apply kernel fixes without reboot.
Verification & Acceptance Criteria
Acceptance: rpm -q shim shows the expected fixed version, systemctl is-active shim returns active, journalctl -u shim --since "5 minutes ago" shows no errors, dnf updateinfo list cves --installed no longer lists the CVEs above, firewall-cmd --list-services includes the required services, getenforce reports the intended mode, and the original reproduction steps for shim — multiple vulnerabilities (14 CVEs) — patch and remediation guide no longer trigger the failure across two consecutive runs.
Rollback Plan
Capture state with dnf history list, rpm -qa > /root/rpm-pre.txt, and where available lvcreate -s -n preupgrade -L 4G /dev/<vg>/<lv> for an LVM snapshot. To revert, run sudo dnf history undo <id> (or sudo dnf install --allowerasing shim-<old-version>) and reload systemctl daemon-reload. Remove custom SELinux modules with sudo semodule -r <module>. Reboot if the kernel or initramfs was changed and re-verify symptoms.
Prevention & Hardening
Prevent recurrence by enabling dnf-automatic with upgrade_type = security in /etc/dnf/automatic.conf, subscribing to centos-announce / rhsa-announce, mirroring through a local Pulp / Foreman / Spacewalk-style repo for controlled rollouts, version-locking sensitive packages, and monitoring file integrity with aide --check. Apply CIS CentOS Stream 9 hardening, keep SELinux enforcing, and where supported enable kpatch live patching so future advisories like this can be remediated without reboot.
Related Errors & Cross-Refs
Related issues that commonly surface alongside shim — multiple vulnerabilities (14 CVEs) — patch and remediation guide: dnf transaction lock contention, systemd unit ordering cycles, SELinux AVC denials in journalctl -k, firewalld zone drift, and kernel taint flags shown by cat /proc/sys/kernel/tainted. See sibling common-problem articles in this CentOS Stream 9 series for adjacent failure modes.
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Browse all common problems & solutions on the Tutorials Hub.
References & Further Reading
Primary reference: AlmaLinux/RHEL advisory ALSA-2024:1903. Supporting docs: Red Hat / CentOS Stream Administration Guide, man dnf, man systemctl, man firewall-cmd, man semanage, man audit2allow, man journalctl, the Red Hat CVE database at access.redhat.com/security/cve/, and the kpatch documentation. Review /usr/share/doc/shim/ for component-level notes implicated in shim — multiple vulnerabilities (14 CVEs) — patch and remediation guide.
