CVE-2012-5573 in Tor
Summary
by MITRE
The connection_edge_process_relay_cell function in or/relay.c in Tor before 0.2.3.25 maintains circuits even if an unexpected SENDME cell arrives, which might allow remote attackers to cause a denial of service (memory consumption or excessive cell reception rate) or bypass intended flow-control restrictions via a RELAY_COMMAND_SENDME command.
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Analysis
by VulDB Data Team • 04/19/2021
The vulnerability described in CVE-2012-5573 resides within the Tor anonymization network's relay implementation, specifically in the connection_edge_process_relay_cell function located in or/relay.c. This flaw represents a critical security issue that affects Tor versions prior to 0.2.3.25, where the system fails to properly validate incoming relay cells and maintains circuit connections even when unexpected SENDME cells are received. The improper handling of these cells creates a pathway for malicious actors to exploit the network's flow control mechanisms, potentially leading to significant operational disruptions.
The technical nature of this vulnerability stems from the absence of proper validation checks for SENDME cells within the relay processing pipeline. When an unexpected RELAY_COMMAND_SENDME command is received, the Tor relay continues to maintain the circuit connection rather than properly terminating or rejecting the invalid cell. This behavior violates fundamental principles of network protocol implementation and flow control as defined by cybersecurity standards. The flaw allows attackers to manipulate the relay's circuit management system, creating a scenario where memory resources can be consumed excessively or where the relay becomes susceptible to abnormal cell reception rates that could overwhelm system resources.
From an operational perspective, this vulnerability presents multiple attack vectors that could severely impact Tor network integrity and availability. Remote attackers can leverage this weakness to cause denial of service conditions by consuming excessive memory resources through the maintenance of invalid circuits or by overwhelming the relay with abnormal cell processing rates. Additionally, the vulnerability enables bypassing intended flow-control restrictions, which undermines the fundamental security model of Tor's relay architecture. This compromise of flow control mechanisms could potentially allow attackers to disrupt network operations or create conditions where legitimate traffic is affected by resource exhaustion.
The impact of this vulnerability aligns with several cybersecurity frameworks and threat modeling approaches, particularly those addressing protocol implementation flaws and resource exhaustion attacks. This issue can be categorized under CWE-691, which covers insufficient control flow management, and relates to ATT&CK technique T1499.004 for network disruption attacks. The vulnerability demonstrates a classic case of inadequate input validation and improper error handling within network protocol implementations. Organizations using Tor relay services must implement immediate mitigation strategies including updating to patched versions, implementing circuit monitoring, and establishing rate limiting controls to prevent exploitation of this flow control bypass mechanism.
The remediation approach for CVE-2012-5573 requires deployment of Tor version 0.2.3.25 or later, which includes proper validation of SENDME cells and appropriate circuit management behavior. System administrators should also implement monitoring solutions to detect anomalous cell reception patterns and establish network-level controls to limit the impact of potential exploitation attempts. This vulnerability underscores the critical importance of proper flow control implementation in network security systems and demonstrates how seemingly minor protocol handling issues can create significant operational risks within anonymity networks.