CVE-2020-27779
📋 TL;DR
This vulnerability in GRUB2 allows privileged attackers to bypass Secure Boot protections by using the cutmem command to remove memory address ranges. This could enable loading of unauthorized code or kernel modifications. Systems using GRUB2 versions prior to 2.06 with Secure Boot enabled are affected.
💻 Affected Systems
- GRUB2
- Linux distributions using GRUB2
- UEFI systems with Secure Boot
📦 What is this software?
Fedora by Fedoraproject
Fedora by Fedoraproject
⚠️ Risk & Real-World Impact
Worst Case
Complete Secure Boot bypass allowing persistent malware installation, system compromise, and data theft/encryption.
Likely Case
Privileged attacker circumvents Secure Boot to load malicious kernel modules or bootloaders.
If Mitigated
Attack fails due to proper access controls, Secure Boot enforcement, or lack of privileged access.
🎯 Exploit Status
Exploit requires physical access or administrative privileges to access GRUB command line. Proof of concept exists in security advisories.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: GRUB2 2.06 and later
Vendor Advisory: https://bugzilla.redhat.com/show_bug.cgi?id=1900698
Restart Required: Yes
Instructions:
1. Update GRUB2 package to version 2.06 or later. 2. Update bootloader configuration. 3. Reboot system to apply changes.
🔧 Temporary Workarounds
Disable cutmem command
linuxRemove or restrict cutmem command from GRUB configuration to prevent exploitation.
Edit /etc/grub.d/40_custom or grub.cfg to remove cutmem references
Run grub2-mkconfig -o /boot/grub2/grub.cfg
Password protect GRUB
linuxSet GRUB password to prevent unauthorized access to GRUB command line.
grub2-mkpasswd-pbkdf2
Add password entry to /etc/grub.d/40_custom
grub2-mkconfig -o /boot/grub2/grub.cfg
🧯 If You Can't Patch
- Implement strict physical security controls to prevent unauthorized console access
- Use full disk encryption to protect against offline attacks even if Secure Boot is bypassed
🔍 How to Verify
Check if Vulnerable:
Check GRUB2 version: grub2-install --version or rpm -q grub2Check Version:
grub2-install --version | head -1Verify Fix Applied:
Verify GRUB2 version is 2.06 or later and check that cutmem command is restricted in grub.cfg📡 Detection & Monitoring
Log Indicators:
- GRUB command line access logs
- UEFI/Secure Boot violation logs
- Unexpected system reboots or boot process modifications
Network Indicators:
- No network indicators - local attack only
SIEM Query:
Search for: 'grub' AND ('command line' OR 'cutmem') in system logs OR monitor for Secure Boot status changes🔗 References
- https://bugzilla.redhat.com/show_bug.cgi?id=1900698
- 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=1900698
- 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/
