CVE-2021-20233
📋 TL;DR
This GRUB2 vulnerability allows attackers to corrupt memory by one byte for each quote in menu input due to an incorrect length calculation. It affects systems using GRUB2 versions prior to 2.06, potentially compromising boot security. The flaw can lead to data confidentiality/integrity loss and system availability issues.
💻 Affected Systems
- GRUB2
📦 What is this software?
Fedora by Fedoraproject
Fedora by Fedoraproject
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise via bootkit installation, allowing persistent attacker control over the entire system including pre-OS execution.
Likely Case
System instability or crash during boot process, potentially requiring physical intervention to recover.
If Mitigated
Limited impact if secure boot is enabled and properly configured, though some configurations may still be vulnerable.
🎯 Exploit Status
Exploitation requires ability to modify GRUB configuration or boot parameters, typically needing administrative access. Memory corruption could lead to arbitrary code execution during boot.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: GRUB2 2.06 or later
Vendor Advisory: https://bugzilla.redhat.com/show_bug.cgi?id=1926263
Restart Required: Yes
Instructions:
1. Update GRUB2 package to version 2.06 or later using your distribution's package manager. 2. Regenerate GRUB configuration with 'grub2-mkconfig -o /boot/grub2/grub.cfg'. 3. Reboot the system to load the patched GRUB.
🔧 Temporary Workarounds
Secure Boot Enforcement
linuxEnable and enforce UEFI Secure Boot to prevent unauthorized boot components
Check Secure Boot status: 'mokutil --sb-state'
Enable via UEFI/BIOS settings
GRUB Configuration Hardening
linuxRestrict access to GRUB configuration files and boot parameters
chmod 600 /boot/grub2/grub.cfg
chmod 700 /etc/grub.d/
set superusers in GRUB configuration
🧯 If You Can't Patch
- Implement strict access controls on boot configuration files and directories
- Enable full disk encryption to protect against boot-time attacks
🔍 How to Verify
Check if Vulnerable:
Check GRUB2 version: 'grub2-install --version' or 'rpm -q grub2' on RPM systems, 'dpkg -l grub2' on Debian systemsCheck Version:
grub2-install --version | head -1Verify Fix Applied:
Verify installed GRUB2 version is 2.06 or higher and check that Secure Boot is properly configured📡 Detection & Monitoring
Log Indicators:
- Unexpected GRUB configuration changes
- Boot failures or unusual boot messages
- Secure Boot validation failures
Network Indicators:
- Not applicable - local boot-time vulnerability
SIEM Query:
Search for: (EventID:4 OR source:"grub") AND (message:"error" OR "failure" OR "corrupt")🔗 References
- https://bugzilla.redhat.com/show_bug.cgi?id=1926263
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZWZ36QK4IKU6MWDWNOOWKPH3WXZBHT2R/
- https://security.gentoo.org/glsa/202104-05
- https://security.netapp.com/advisory/ntap-20220325-0001/
- https://bugzilla.redhat.com/show_bug.cgi?id=1926263
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZWZ36QK4IKU6MWDWNOOWKPH3WXZBHT2R/
- https://security.gentoo.org/glsa/202104-05
- https://security.netapp.com/advisory/ntap-20220325-0001/
