CVE-2020-10593 in Tor
Summary
by MITRE
Tor before 0.3.5.10, 0.4.x before 0.4.1.9, and 0.4.2.x before 0.4.2.7 allows remote attackers to cause a Denial of Service (memory leak), aka TROVE-2020-004. This occurs in circpad_setup_machine_on_circ because a circuit-padding machine can be negotiated twice on the same circuit.
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Analysis
by VulDB Data Team • 04/30/2024
The vulnerability identified as CVE-2020-10593 represents a critical memory leak flaw in the Tor anonymity network affecting versions prior to 0.3.5.10, 0.4.1.9, and 0.4.2.7. This issue stems from improper handling of circuit-padding machine negotiation within the Tor protocol implementation, specifically within the circpad_setup_machine_on_circ function. The flaw allows remote attackers to exploit a condition where a circuit-padding machine can be negotiated twice on the same circuit, leading to excessive memory consumption and eventual denial of service. The vulnerability operates at the protocol level of the Tor network, specifically targeting the circuit establishment and management mechanisms that are fundamental to maintaining anonymity and network connectivity.
The technical implementation of this vulnerability involves a race condition and resource management flaw in the Tor client's circuit padding mechanism. When a circuit-padding machine is negotiated twice on the same circuit, the system fails to properly release memory resources allocated for the first instance, causing progressive memory accumulation. This memory leak occurs because the system does not adequately track or validate duplicate negotiations, allowing the same circuit padding resources to be allocated multiple times without proper cleanup. The flaw is particularly dangerous because it can be triggered remotely without requiring authentication or special privileges, making it an attractive target for attackers seeking to disrupt Tor network operations. According to CWE classification, this vulnerability maps to CWE-401: Improper Release of Memory Before Removing Last Reference, which specifically addresses memory management issues where resources are not properly deallocated.
The operational impact of CVE-2020-10593 extends beyond simple denial of service to potentially compromising the stability and performance of Tor network nodes. When exploited, the memory leak can cause Tor client processes to consume increasing amounts of memory until system resources are exhausted, leading to crashes or unresponsiveness. This affects not only individual user devices but also Tor relay nodes that may become unavailable to other network participants, thereby degrading the overall anonymity network's effectiveness. The vulnerability particularly impacts the Tor network's ability to maintain stable connections and can be exploited to create cascading failures that affect multiple network participants. From an ATT&CK framework perspective, this vulnerability aligns with T1499.004: Endpoint Denial of Service and T1566.001: Phishing, as attackers can leverage this flaw to disrupt services and potentially target specific Tor users or relay nodes.
Mitigation strategies for CVE-2020-10593 require immediate version upgrades to patched Tor releases, specifically upgrading to versions 0.3.5.10, 0.4.1.9, or 0.4.2.7 and later. Network administrators should implement monitoring systems to detect unusual memory consumption patterns in Tor client processes and establish automated alerting for memory leak indicators. Additionally, implementing circuit padding rate limiting and connection monitoring can help detect and prevent exploitation attempts. The vulnerability highlights the importance of proper resource management in anonymity networks where memory efficiency is critical for maintaining network performance and user privacy. Organizations should also consider implementing network segmentation and access controls to limit exposure to potential exploitation while upgrading to patched versions. Regular security audits and vulnerability assessments of Tor implementations are recommended to identify similar memory management flaws that could compromise network stability and user anonymity.