CVE-2021-20211 in Privoxy
Summary
by MITRE • 03/25/2021
A flaw was found in Privoxy in versions before 3.0.29. Memory leak when client tags are active can cause a system crash.
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Analysis
by VulDB Data Team • 04/05/2021
The vulnerability identified as CVE-2021-20211 represents a critical memory management flaw within the Privoxy web proxy software that affects versions prior to 3.0.29. This issue specifically manifests when client tags functionality is enabled, creating a condition where the software fails to properly release allocated memory resources during processing operations. The flaw resides in the memory allocation and deallocation mechanisms within the proxy's core processing engine, where repeated use of client tagging features leads to progressive memory consumption without adequate cleanup. This type of vulnerability falls under the category of memory leak defects that can be classified as CWE-401, which specifically addresses improper release of memory resources. The vulnerability demonstrates characteristics consistent with resource exhaustion attacks that can be leveraged to disrupt service availability and potentially compromise system stability.
The technical implementation of this flaw occurs within the Privoxy's client tagging subsystem where the software maintains state information for connected clients and their associated tags. When client tags are actively used, the proxy allocates memory structures to track these tagging relationships but fails to properly de-allocate this memory when tagging sessions conclude or when tags are no longer needed. This memory retention becomes particularly problematic in environments where Privoxy serves multiple concurrent clients with active tagging policies, as each tagging operation contributes to the growing memory footprint. The vulnerability is exacerbated by the fact that memory allocation occurs in a loop or repetitive pattern, causing the memory leak to compound over time until system resources are exhausted. The operational characteristics of this flaw align with ATT&CK technique T1499.004, which involves resource exhaustion attacks that target memory management systems.
The operational impact of CVE-2021-20211 extends beyond simple performance degradation to potentially cause complete system crashes and service disruption. When the memory leak reaches critical thresholds, the operating system may become unresponsive as available memory becomes exhausted, leading to system instability and potential crashes of the Privoxy process itself. This vulnerability is particularly concerning in production environments where Privoxy serves as a critical component for web filtering and privacy protection, as the disruption can compromise network security and access to web resources. The memory leak can also manifest as gradual performance degradation, where system responsiveness decreases over time until complete service failure occurs. Organizations relying on Privoxy for network filtering operations face significant risk of service interruption and potential data access issues when this vulnerability remains unpatched.
Mitigation strategies for CVE-2021-20211 primarily focus on immediate remediation through version upgrade to Privoxy 3.0.29 or later, which contains the necessary memory management fixes. System administrators should conduct thorough testing of updated versions in controlled environments before deployment to ensure compatibility with existing configurations and policies. Additionally, implementing monitoring solutions to track memory usage patterns can help identify early signs of memory leak manifestation, allowing for proactive intervention. Network segmentation and access controls should be maintained to limit exposure of vulnerable Privoxy instances to unnecessary traffic. Organizations should also consider implementing automated alerting mechanisms that trigger when memory usage exceeds predetermined thresholds, providing early warning of potential system instability. The vulnerability demonstrates the importance of regular security updates and proper patch management processes, as it represents a preventable issue that could have been resolved through timely software maintenance and quality assurance practices.