CVE-2021-3739
📋 TL;DR
A NULL pointer dereference vulnerability in the Linux kernel's Btrfs filesystem allows local attackers with CAP_SYS_ADMIN privileges to crash the system or leak kernel memory information. This affects Linux systems using Btrfs filesystem with vulnerable kernel versions. The primary impact is denial of service through system crashes.
💻 Affected Systems
- Linux Kernel
📦 What is this software?
Fedora by Fedoraproject
Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →⚠️ Risk & Real-World Impact
Worst Case
Complete system crash leading to denial of service and potential kernel memory information disclosure
Likely Case
System crash requiring reboot, causing service disruption
If Mitigated
Minimal impact if CAP_SYS_ADMIN is properly restricted and systems are patched
🎯 Exploit Status
Requires local access and CAP_SYS_ADMIN privileges; exploit involves specific Btrfs device removal operations
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Linux kernel 5.13.4 and later
Vendor Advisory: https://ubuntu.com/security/CVE-2021-3739
Restart Required: Yes
Instructions:
1. Update Linux kernel to version 5.13.4 or later. 2. For distributions with backported fixes, apply security updates. 3. Reboot system after kernel update.
🔧 Temporary Workarounds
Restrict CAP_SYS_ADMIN
linuxLimit CAP_SYS_ADMIN capabilities to essential users only
# Use Linux capabilities framework to restrict CAP_SYS_ADMIN
# Example: setcap -r /path/to/binary
# Or use namespaces and containers to isolate privileges
Avoid Btrfs device removal
linuxAvoid using Btrfs device removal operations on vulnerable systems
# Monitor and restrict btrfs device remove operations
# Consider using alternative filesystems if possible
🧯 If You Can't Patch
- Implement strict privilege separation and limit CAP_SYS_ADMIN to essential users only
- Monitor system logs for Btrfs device removal operations and investigate suspicious activity
🔍 How to Verify
Check if Vulnerable:
Check kernel version: uname -r and compare with vulnerable versions (<5.13.4). Also verify Btrfs usage with: lsblk -f | grep btrfsCheck Version:
uname -rVerify Fix Applied:
Verify kernel version is 5.13.4 or later: uname -r. Check that security updates have been applied via package manager.📡 Detection & Monitoring
Log Indicators:
- Kernel panic logs
- System crash dumps
- Btrfs device removal operation logs in system logs
Network Indicators:
- None - local vulnerability only
SIEM Query:
source="kernel" AND ("panic" OR "Oops" OR "NULL pointer dereference") AND "btrfs"🔗 References
- https://bugzilla.redhat.com/show_bug.cgi?id=1997958
- https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e4571b8c5e9ffa1e85c0c671995bd4dcc5c75091
- https://github.com/torvalds/linux/commit/e4571b8c5e9ffa1e85c0c671995bd4dcc5c75091
- https://security.netapp.com/advisory/ntap-20220407-0006/
- https://ubuntu.com/security/CVE-2021-3739
- https://www.openwall.com/lists/oss-security/2021/08/25/3
- https://bugzilla.redhat.com/show_bug.cgi?id=1997958
- https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=e4571b8c5e9ffa1e85c0c671995bd4dcc5c75091
- https://github.com/torvalds/linux/commit/e4571b8c5e9ffa1e85c0c671995bd4dcc5c75091
- https://security.netapp.com/advisory/ntap-20220407-0006/
- https://ubuntu.com/security/CVE-2021-3739
- https://www.openwall.com/lists/oss-security/2021/08/25/3
