CVE-2021-3679 in Linux
Summary
by MITRE • 08/06/2021
A lack of CPU resource in the Linux kernel tracing module functionality in versions prior to 5.14-rc3 was found in the way user uses trace ring buffer in a specific way. Only privileged local users (with CAP_SYS_ADMIN capability) could use this flaw to starve the resources causing denial of service.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 08/09/2025
The vulnerability identified as CVE-2021-3679 represents a critical resource exhaustion flaw within the Linux kernel's tracing module functionality. This issue specifically affects systems running kernel versions prior to 5.14-rc3 where the trace ring buffer implementation contains a design flaw that allows for improper CPU resource management. The vulnerability resides in the kernel's tracing subsystem which is designed to collect and store execution traces for debugging and performance analysis purposes. When exploited, this flaw enables malicious actors to consume excessive CPU cycles and memory resources through improper utilization of the tracing infrastructure.
The technical implementation of this vulnerability stems from inadequate resource management within the trace ring buffer mechanism. The flaw occurs when privileged local users with CAP_SYS_ADMIN capability manipulate the tracing module in a specific pattern that causes the ring buffer to continuously consume CPU cycles without proper resource cleanup or bounds checking. This particular weakness falls under CWE-400 which categorizes improper resource management and specifically addresses resource exhaustion vulnerabilities. The vulnerability demonstrates how kernel subsystems can be manipulated to create denial of service conditions through subtle implementation flaws in resource handling mechanisms.
Operationally, this vulnerability presents a significant risk to system stability and availability as it allows a local attacker with administrative privileges to cause a denial of service condition that affects the entire system. The attack vector requires only local access with CAP_SYS_ADMIN capability, making it particularly dangerous in environments where privilege escalation is possible or where system administrators maintain elevated access for operational purposes. Once exploited, the vulnerability causes the system to become unresponsive or significantly degraded as CPU resources are consumed in an uncontrolled manner, potentially affecting critical system services and user processes. This type of vulnerability aligns with ATT&CK technique T1499 which covers resource exhaustion attacks and demonstrates how kernel-level flaws can be leveraged for system disruption.
The impact of this vulnerability extends beyond simple service disruption as it can affect system availability for legitimate users and potentially provide a foothold for further attacks. System administrators may find that normal operations become impossible as the kernel tracing subsystem consumes all available CPU resources, leading to cascading failures in dependent services. The vulnerability's exploitation requires minimal privileges but can cause maximum damage, making it particularly concerning for production environments where kernel tracing is enabled. Organizations should prioritize patching to kernel versions 5.14-rc3 or later where this vulnerability has been addressed through improved resource management and bounds checking within the trace ring buffer implementation.
Mitigation strategies should focus on immediate patching of affected kernel versions to ensure the proper resource management fixes are in place. System administrators should also consider disabling unnecessary tracing functionality on systems where it is not required for operational purposes. Monitoring for unusual CPU usage patterns and tracing module activity can help detect potential exploitation attempts. Additionally, implementing privilege separation and ensuring that only essential users maintain CAP_SYS_ADMIN capability can reduce the attack surface for this particular vulnerability. The fix implemented in kernel 5.14-rc3 includes enhanced bounds checking and resource management within the trace ring buffer subsystem to prevent the uncontrolled resource consumption that previously enabled this denial of service condition.