CVE-2021-3492
📋 TL;DR
CVE-2021-3492 is a kernel vulnerability in Ubuntu's Shiftfs filesystem where improper error handling during copy_from_user() operations can cause memory corruption. This allows attackers to potentially execute arbitrary code with kernel privileges or cause denial of service through kernel memory exhaustion. Only Ubuntu Linux systems using the Shiftfs filesystem are affected.
💻 Affected Systems
- Ubuntu Linux kernel with Shiftfs module
📦 What is this software?
Ubuntu Linux by Canonical
Ubuntu Linux by Canonical
Ubuntu Linux by Canonical
⚠️ Risk & Real-World Impact
Worst Case
Full kernel compromise leading to arbitrary code execution with root privileges, complete system takeover, and persistent backdoor installation.
Likely Case
Local privilege escalation from unprivileged user to root, enabling lateral movement and data exfiltration.
If Mitigated
Denial of service through kernel panic or system crash if exploit fails or is detected.
🎯 Exploit Status
Exploit requires local access and knowledge of kernel exploitation techniques. Proof-of-concept code has been published in security advisories.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Linux kernel 5.11.0-17.18 for Ubuntu 20.04 LTS and later versions
Vendor Advisory: https://ubuntu.com/security/notices/USN-4917-1
Restart Required: Yes
Instructions:
1. Run 'sudo apt update' 2. Run 'sudo apt upgrade linux-image-$(uname -r)' 3. Reboot the system 4. Verify kernel version with 'uname -r'
🔧 Temporary Workarounds
Disable Shiftfs module
linuxPrevent loading of the vulnerable Shiftfs kernel module
echo 'blacklist shiftfs' | sudo tee /etc/modprobe.d/blacklist-shiftfs.conf
sudo rmmod shiftfs 2>/dev/null || true
sudo update-initramfs -u
Restrict user access
allLimit local user accounts and implement strict access controls
🧯 If You Can't Patch
- Disable Shiftfs module if not required for system functionality
- Implement strict user access controls and monitor for privilege escalation attempts
🔍 How to Verify
Check if Vulnerable:
Check if Shiftfs module is loaded: 'lsmod | grep shiftfs' and check kernel version: 'uname -r'Check Version:
uname -rVerify Fix Applied:
Verify kernel version is patched: 'uname -r' should be 5.11.0-17.18 or later for Ubuntu 20.04📡 Detection & Monitoring
Log Indicators:
- Kernel panic logs
- Unexpected privilege escalation in audit logs
- Failed module loading attempts for shiftfs
Network Indicators:
- None - local exploit only
SIEM Query:
EventID=4688 AND (ProcessName LIKE '%shiftfs%' OR CommandLine LIKE '%shiftfs%') OR EventID=4624 AND LogonType=2 AND NewElevatedToken=1🔗 References
- http://packetstormsecurity.com/files/162614/Kernel-Live-Patch-Security-Notice-LSN-0077-1.html
- https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal/commit/?id=25c891a949bf918b59cbc6e4932015ba4c35c333
- https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal/commit/?id=8fee52ab9da87d82bc6de9ebb3480fff9b4d53e6
- https://ubuntu.com/security/notices/USN-4917-1
- https://www.openwall.com/lists/oss-security/2021/04/16/2
- https://www.zerodayinitiative.com/advisories/ZDI-21-422/
- http://packetstormsecurity.com/files/162614/Kernel-Live-Patch-Security-Notice-LSN-0077-1.html
- https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal/commit/?id=25c891a949bf918b59cbc6e4932015ba4c35c333
- https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal/commit/?id=8fee52ab9da87d82bc6de9ebb3480fff9b4d53e6
- https://ubuntu.com/security/notices/USN-4917-1
- https://www.openwall.com/lists/oss-security/2021/04/16/2
- https://www.zerodayinitiative.com/advisories/ZDI-21-422/
