CVE-2021-3669 in Linux
Summary
by MITRE • 08/26/2022
A flaw was found in the Linux kernel. Measuring usage of the shared memory does not scale with large shared memory segment counts which could lead to resource exhaustion and DoS.
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Analysis
by VulDB Data Team • 11/05/2025
The vulnerability identified as CVE-2021-3669 represents a significant scalability issue within the Linux kernel's shared memory management subsystem that can lead to denial of service conditions. This flaw specifically impacts systems utilizing large shared memory segment counts where the kernel's measurement mechanisms for tracking shared memory usage fail to scale appropriately. The issue manifests when systems attempt to manage numerous shared memory segments simultaneously, creating a scenario where the kernel's resource accounting becomes inefficient and ultimately leads to resource exhaustion.
The technical root cause of this vulnerability lies in the kernel's implementation of shared memory segment accounting and measurement algorithms. When the number of shared memory segments exceeds certain thresholds, the kernel's internal data structures and measurement routines become overwhelmed, causing performance degradation and eventual resource exhaustion. This problem is particularly pronounced in high-performance computing environments, database servers, and applications that rely heavily on shared memory inter-process communication mechanisms. The flaw operates at the kernel level where shared memory segments are managed through the System V shared memory interface, making it a fundamental issue in the kernel's memory management subsystem.
From an operational impact perspective, systems affected by CVE-2021-3669 can experience complete denial of service conditions where legitimate processes are unable to allocate additional shared memory segments or access existing ones. This vulnerability particularly affects environments running applications such as Oracle databases, SAP systems, and other enterprise applications that utilize large shared memory segments for optimal performance. The resource exhaustion can cause system instability, application crashes, and in severe cases, complete system hangs requiring manual intervention and reboot cycles. Organizations running high-throughput systems or those with memory-intensive workloads are most vulnerable to this issue, as these environments typically maintain large numbers of shared memory segments.
The vulnerability maps to CWE-775 which describes the weakness of missing or insufficient resource cleanup, and can be categorized under the ATT&CK technique T1499.004 which involves network denial of service. Organizations should implement immediate mitigations including kernel updates from vendors such as Red Hat, SUSE, and Canonical that contain patches addressing this specific scalability issue. System administrators should also consider implementing monitoring solutions to detect abnormal shared memory usage patterns and establish resource limits on shared memory segments to prevent complete exhaustion. Additionally, workload optimization strategies such as reducing the number of shared memory segments or implementing alternative IPC mechanisms may provide temporary relief while permanent patches are deployed.
Security teams should prioritize this vulnerability for remediation due to its potential for causing widespread service disruption and its relatively straightforward exploitation path. The patching process requires careful planning and testing in production environments to ensure compatibility with existing applications that depend on shared memory functionality. Organizations should conduct thorough testing of shared memory intensive applications after applying the kernel updates to verify continued proper operation. Long-term mitigation strategies should include regular kernel updates, implementation of resource management policies, and monitoring systems that can detect and alert on unusual shared memory usage patterns before they escalate into denial of service conditions.