CVE-2011-0492 in Tor
Summary
by MITRE
Tor before 0.2.1.29 and 0.2.2.x before 0.2.2.21-alpha allows remote attackers to cause a denial of service (assertion failure and daemon exit) via blobs that trigger a certain file size, as demonstrated by the cached-descriptors.new file.
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Analysis
by VulDB Data Team • 10/12/2021
The vulnerability described in CVE-2011-0492 represents a critical denial of service flaw affecting the Tor anonymization network software. This issue specifically impacts Tor versions prior to 0.2.1.29 and 0.2.2.21-alpha, where remote attackers can exploit a weakness in the software's handling of certain file structures to cause daemon termination. The vulnerability manifests when maliciously crafted data blobs are processed, particularly within the cached-descriptors.new file which is crucial for maintaining the network's directory information. The attack leverages an assertion failure that results in the complete termination of the Tor daemon process, effectively disrupting the anonymity service for all connected clients.
The technical root cause of this vulnerability lies in inadequate input validation and error handling within Tor's descriptor processing subsystem. When the software encounters file size parameters that fall outside expected ranges, it triggers an assertion failure that is not properly caught or handled within the code execution flow. This assertion failure causes the program to terminate abruptly rather than gracefully handling the malformed data or rejecting the input. The vulnerability specifically targets the cached-descriptors.new file which serves as a temporary storage location for directory information that Tor nodes use to maintain network topology data. This file processing mechanism lacks proper bounds checking for file size parameters, allowing attackers to craft inputs that cause integer overflow conditions or memory allocation failures.
The operational impact of this vulnerability extends beyond simple service disruption as it can severely compromise the integrity and availability of the Tor network infrastructure. When the Tor daemon exits due to this assertion failure, it affects not only the local node but also creates cascading effects throughout the network as other nodes lose connectivity to the affected system. The attack requires minimal resources from the attacker while potentially causing significant disruption to network operations, making it particularly dangerous in the context of anonymity networks where service availability is paramount. Network administrators and users who rely on Tor for privacy and security purposes face the risk of having their connections interrupted or the entire service rendered unavailable for extended periods.
Mitigation strategies for this vulnerability require immediate software updates to versions 0.2.1.29 or 0.2.2.21-alpha and later, which contain the necessary patches to address the assertion failure. System administrators should implement monitoring solutions to detect unusual file size patterns in cached-descriptors.new files and other temporary storage areas. The fix typically involves adding proper input validation checks and robust error handling mechanisms that prevent assertion failures from causing daemon termination. Organizations should also consider implementing network segmentation and access controls to limit exposure to potentially malicious inputs. This vulnerability aligns with CWE-248, which addresses "Uncaught Exception" conditions, and represents a classic example of how improper error handling can lead to denial of service attacks. From an ATT&CK perspective, this vulnerability maps to T1499.004, which covers "Toggle Execution Flag" and "Resource Hijacking," as it effectively hijacks system resources through service disruption.
The broader implications of this vulnerability highlight the importance of robust input validation in network security software, particularly in anonymity systems where service availability directly impacts user privacy and security. The attack vector demonstrates how seemingly minor flaws in file processing logic can have major consequences in distributed systems, emphasizing the need for comprehensive security testing and code review processes. Organizations maintaining Tor infrastructure should implement regular security audits and establish incident response procedures specifically designed to handle denial of service attacks targeting anonymity networks. The vulnerability also underscores the critical nature of keeping security software updated, as the fix for this issue was provided through version updates that addressed the underlying assertion handling problems in the Tor codebase.