CVE-2011-0493 in Tor
Summary
by MITRE
Tor before 0.2.1.29 and 0.2.2.x before 0.2.2.21-alpha might allow remote attackers to cause a denial of service (assertion failure and daemon exit) via vectors related to malformed router caches and improper handling of integer values.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 10/12/2021
The vulnerability identified as CVE-2011-0493 represents a critical denial of service weakness affecting the Tor anonymity network software. This issue specifically impacts versions prior to 0.2.1.29 and 0.2.2.x versions before 0.2.2.21-alpha, creating a scenario where remote attackers can exploit malformed router cache data to trigger assertion failures within the Tor daemon. The flaw stems from inadequate validation of integer values during the processing of network topology information, which forms the foundation of Tor's distributed network structure.
The technical implementation of this vulnerability involves the improper handling of integer values within the router cache processing module of Tor's codebase. When malicious actors supply specially crafted router cache data containing malformed integer values, the software's assertion mechanisms fail to properly validate these inputs, leading to assertion failures that cause the Tor daemon to terminate unexpectedly. This behavior aligns with CWE-682, which describes improper integer arithmetic, and represents a classic example of how malformed input can lead to application instability. The vulnerability operates at the network protocol level, where Tor's directory servers and client nodes must process information about network participants, making it particularly dangerous for the anonymity network's operational integrity.
The operational impact of this vulnerability extends beyond simple service disruption, as it can compromise the reliability of the entire Tor network infrastructure. When the Tor daemon exits due to assertion failures, it creates gaps in the network's routing capabilities, potentially disrupting communication for users relying on Tor for anonymity. This denial of service condition affects not only individual user connections but also the broader network topology, as the daemon's termination can cause cascading failures in the distributed routing system. Attackers can exploit this vulnerability by sending maliciously crafted router descriptors to Tor directory servers, which then propagate these malformed entries throughout the network, creating a self-replicating denial of service condition that can persist until the affected nodes are manually restarted.
Mitigation strategies for CVE-2011-0493 primarily focus on upgrading to patched versions of Tor software, specifically versions 0.2.1.29 or 0.2.2.21-alpha and later. Network administrators should implement regular update schedules to ensure all Tor instances remain current with security patches. Additional protective measures include implementing network monitoring to detect unusual daemon termination patterns and establishing automated restart procedures for Tor services. From a defensive perspective, this vulnerability demonstrates the importance of input validation and robust error handling in distributed systems, particularly those designed to operate in adversarial environments where malicious input is expected. The issue also highlights the necessity of proper assertion handling in production systems, as outlined in ATT&CK technique T1489, which covers denial of service through system resource consumption and process termination. Organizations utilizing Tor for network anonymity should also consider implementing additional layers of network security to detect and prevent the injection of malformed router cache data into their Tor infrastructure.