CVE-2023-40546 Overview
A flaw was discovered in Shim, the bootloader used to enable UEFI Secure Boot on Linux systems. The vulnerability occurs when an error happens while creating a new ESL (EFI Signature List) variable. If Shim fails to create the new variable, it attempts to print an error message to the user; however, the number of parameters used by the logging function doesn't match the format string used by it, leading to a crash under certain circumstances.
Critical Impact
This Null Pointer Dereference vulnerability (CWE-476) can cause system crashes during the boot process, resulting in a denial of service condition that prevents affected systems from booting properly.
Affected Products
- Red Hat Shim (all versions prior to patched release)
- Fedora Project Fedora 39
- Red Hat Enterprise Linux 8.0 and 9.0
Discovery Timeline
- 2024-01-29 - CVE-2023-40546 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2023-40546
Vulnerability Analysis
This vulnerability is classified as a Null Pointer Dereference (CWE-476) that occurs within Shim's error handling routines during ESL variable creation. Shim is a critical component in the UEFI Secure Boot chain, acting as the first-stage bootloader that verifies and loads the main Linux bootloader (typically GRUB).
The flaw exists in the error logging mechanism. When Shim attempts to create a new ESL variable and fails, it invokes a logging function to display an error message. However, there is a mismatch between the format string specifiers and the actual parameters passed to this function. This type of format string vulnerability can result in the function attempting to access memory locations that don't contain valid data, ultimately causing a null pointer dereference and system crash.
The local attack vector requires an attacker to have some level of access to the system, potentially through physical access or a compromised local account with privileges to manipulate UEFI variables.
Root Cause
The root cause is a programming error in the Shim bootloader's error handling code path. When variable creation fails, the logging function is called with a format string that expects a certain number of arguments, but fewer arguments are actually provided. This results in the function reading arbitrary stack data or dereferencing invalid pointers when trying to fulfill the format string requirements.
Attack Vector
The vulnerability requires local access to exploit. An attacker with the ability to trigger the ESL variable creation failure condition can cause the logging function to crash the bootloader. This could be achieved by manipulating UEFI variables or triggering specific error conditions during the Secure Boot validation process. The attack results in denial of service by preventing the system from completing the boot process.
The format string mismatch occurs in Shim's error handling path during ESL variable operations. When the variable creation fails and error logging is triggered, the logging function receives fewer parameters than the format string expects, causing it to read invalid memory and crash. This manifests as a null pointer dereference when the function attempts to access a missing parameter.
Detection Methods for CVE-2023-40546
Indicators of Compromise
- Unexpected system crashes or boot failures during UEFI Secure Boot initialization
- Boot loop conditions where the system fails to progress past the Shim bootloader
- Error messages in UEFI firmware logs related to ESL variable creation failures
- Repeated kernel panic or crash dumps occurring at early boot stages
Detection Strategies
- Monitor for unusual boot failures across fleet systems using endpoint telemetry
- Implement boot integrity monitoring to detect crashes in the Shim bootloader stage
- Check Shim version inventory across systems using mokutil --sb-state and package manager queries
- Review UEFI firmware event logs for ESL variable creation errors
Monitoring Recommendations
- Deploy boot-time monitoring solutions that capture pre-OS crash events
- Establish baseline boot timing metrics to detect anomalous boot failures
- Implement automated alerting for systems that fail to complete the boot process
- Use SentinelOne endpoint agents to monitor for post-boot indicators of boot-time attacks
How to Mitigate CVE-2023-40546
Immediate Actions Required
- Update Shim to the latest patched version from your distribution's package repositories
- Apply security updates referenced in Red Hat Security Advisories (RHSA-2024:1834, RHSA-2024:1835, RHSA-2024:1873, and related advisories)
- Prioritize patching on systems where boot integrity is critical to operations
- For Debian-based systems, refer to the Debian LTS Announcement for applicable updates
Patch Information
Red Hat has released multiple security advisories addressing this vulnerability:
- RHSA-2024:1834 - Shim security update
- RHSA-2024:1835 - Shim security update
- RHSA-2024:1873 - Shim security update
- RHSA-2024:1876 - Shim security update
Additional technical details are available in Red Hat Bug Report #2241796 and the Red Hat CVE Analysis.
Workarounds
- No direct workaround is available; applying vendor patches is the recommended remediation
- Ensure physical security of affected systems to limit local attack surface
- Implement boot monitoring to quickly detect and respond to boot failures
- Consider temporary disabling of Secure Boot only if absolutely necessary and in controlled environments (not recommended for production)
# Check current Shim version on Red Hat/Fedora systems
rpm -q shim-x64 shim-ia32
# Update Shim package to patched version
sudo dnf update shim-x64 shim-ia32
# Verify Secure Boot status
mokutil --sb-state
# On Debian/Ubuntu systems
dpkg -l | grep shim
sudo apt update && sudo apt upgrade shim-signed
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
