CVE-2021-3679 Overview
A CPU resource exhaustion vulnerability exists in the Linux kernel tracing module functionality in versions prior to 5.14-rc3. The flaw was found in the way users interact with the trace ring buffer in a specific manner. Only privileged local users with the CAP_SYS_ADMIN capability could exploit this vulnerability to starve system resources, resulting in a denial of service condition.
Critical Impact
Privileged local attackers can cause complete system denial of service by exhausting CPU resources through manipulation of the kernel's trace ring buffer, affecting system availability.
Affected Products
- Linux Kernel versions prior to 5.14-rc3
- Red Hat Enterprise Linux 8.0
- Debian Linux 9.0 and 10.0
Discovery Timeline
- August 5, 2021 - CVE-2021-3679 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2021-3679
Vulnerability Analysis
This vulnerability is classified under CWE-400 (Uncontrolled Resource Consumption) and CWE-835 (Loop with Unreachable Exit Condition). The flaw resides in the Linux kernel's tracing subsystem, specifically in how the trace ring buffer handles certain operations. When a privileged user interacts with the trace ring buffer in a particular manner, the kernel enters a state that consumes excessive CPU resources, effectively starving other processes and causing system-wide denial of service.
The attack requires local access and elevated privileges (CAP_SYS_ADMIN), which limits the attack surface but makes it particularly dangerous in multi-tenant environments or systems where administrative access may be compromised. The vulnerability affects system availability without impacting confidentiality or integrity of data.
Root Cause
The root cause stems from improper resource management within the kernel's trace ring buffer implementation. When specific operations are performed on the trace ring buffer, the kernel fails to properly limit CPU consumption, allowing a loop condition that cannot be easily exited. This results in uncontrolled resource consumption that monopolizes CPU cycles and prevents other system processes from executing.
Attack Vector
The attack vector is local, requiring an attacker to have privileged access to the target system with CAP_SYS_ADMIN capability. The attacker must be able to interact with the kernel tracing subsystem, specifically manipulating the trace ring buffer through operations that trigger the resource exhaustion condition.
The exploitation involves accessing the kernel tracing functionality through the debugfs or tracefs interfaces, typically mounted at /sys/kernel/debug/tracing/ or /sys/kernel/tracing/. By performing specific sequences of operations on the trace ring buffer, the attacker can trigger the infinite loop condition that exhausts CPU resources.
Detection Methods for CVE-2021-3679
Indicators of Compromise
- Abnormally high CPU utilization by kernel processes without corresponding user-space activity
- System unresponsiveness or severe performance degradation affecting all running processes
- Unusual access patterns to /sys/kernel/debug/tracing/ or /sys/kernel/tracing/ directories
- Kernel logs indicating tracing subsystem operations from non-standard administrative processes
Detection Strategies
- Monitor system calls related to the tracing subsystem from processes with elevated privileges
- Implement real-time CPU utilization monitoring with alerts for kernel-space anomalies
- Use audit rules to track access to tracing interfaces: auditctl -w /sys/kernel/debug/tracing/ -p rwxa
- Deploy endpoint detection solutions capable of identifying kernel-level resource exhaustion patterns
Monitoring Recommendations
- Configure system monitoring tools to alert on sustained high kernel CPU usage (>90% for extended periods)
- Implement logging for all operations performed on the kernel tracing subsystem
- Monitor for processes with CAP_SYS_ADMIN capability accessing tracing interfaces
- Review system audit logs for suspicious privileged user activity related to debugging interfaces
How to Mitigate CVE-2021-3679
Immediate Actions Required
- Update the Linux kernel to version 5.14-rc3 or later where the vulnerability has been addressed
- Restrict CAP_SYS_ADMIN capability to only essential administrative accounts
- Limit access to the tracing subsystem interfaces through filesystem permissions
- Consider disabling kernel tracing functionality if not required for operations
Patch Information
The vulnerability has been addressed in the upstream Linux kernel. The fix is available in the Linux Kernel Commit. Distribution-specific patches are available:
- Red Hat: See Red Hat Bug Report for Enterprise Linux 8 updates
- Debian: Security updates available via Debian Security Advisory DSA-4978, Debian LTS Announcement October 2021, and Debian LTS Announcement December 2021
Workarounds
- Restrict access to /sys/kernel/debug/tracing/ and /sys/kernel/tracing/ directories using filesystem permissions
- Remove unnecessary users from groups with administrative capabilities
- Implement mandatory access control (SELinux/AppArmor) policies to restrict tracing subsystem access
- Monitor and audit all privileged account activity for suspicious tracing operations
# Configuration example
# Restrict access to tracing interfaces
chmod 700 /sys/kernel/debug/tracing/
chmod 700 /sys/kernel/tracing/
# Add audit rules for tracing access
auditctl -w /sys/kernel/debug/tracing/ -p rwxa -k kernel_tracing
auditctl -w /sys/kernel/tracing/ -p rwxa -k kernel_tracing
# Verify kernel version
uname -r
# Ensure version is 5.14-rc3 or later
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
