CVE-2011-0490 in Tor
Summary
by MITRE
Tor before 0.2.1.29 and 0.2.2.x before 0.2.2.21-alpha makes calls to Libevent within Libevent log handlers, which might allow remote attackers to cause a denial of service (daemon crash) via vectors that trigger certain log messages.
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Analysis
by VulDB Data Team • 10/12/2021
The vulnerability identified as CVE-2011-0490 represents a critical denial of service flaw within the Tor anonymity network software. This issue affects versions prior to 0.2.1.29 and 0.2.2.21-alpha, where the Tor daemon exhibits problematic behavior when handling certain log messages. The flaw stems from the improper interaction between Tor's core functionality and the Libevent library, creating a dangerous recursive call scenario that can lead to complete daemon crashes.
The technical root cause of this vulnerability lies in the improper handling of logging mechanisms within the Libevent library. When Tor processes certain network conditions or malicious inputs, it triggers specific log messages that are handled by Libevent's logging subsystem. However, the Libevent log handlers themselves make calls back into Libevent functions, creating a recursive loop that can exhaust system resources and ultimately cause the Tor daemon to crash. This recursive behavior violates fundamental principles of event-driven programming and demonstrates poor resource management practices within the software architecture.
From an operational perspective, this vulnerability presents significant risks to the Tor network's stability and availability. Remote attackers can exploit this flaw by crafting specific network traffic or input conditions that trigger the problematic logging sequence. The resulting daemon crashes can disrupt anonymity services, potentially exposing users to deanonymization risks while also affecting the overall network infrastructure. The vulnerability's impact extends beyond individual daemon failures to potentially compromise the broader Tor network's reliability and trustworthiness.
This vulnerability aligns with CWE-121, which addresses buffer overflow conditions, and specifically relates to CWE-362, concerning concurrent execution using shared resources. The recursive nature of the flaw also connects to ATT&CK technique T1499.004, which involves network disruption through resource exhaustion. The issue demonstrates how seemingly minor logging implementation flaws can create severe operational impacts in security-critical software environments.
Mitigation strategies for CVE-2011-0490 primarily involve immediate upgrading to patched versions of Tor software, specifically versions 0.2.1.29 or 0.2.2.21-alpha and later. Organizations should also implement monitoring systems to detect unusual daemon behavior and establish automated recovery procedures. Additionally, network administrators should consider implementing rate limiting and input validation mechanisms to reduce the likelihood of triggering the vulnerable logging paths. The fix implemented in the patched versions addresses the recursive call issue by restructuring how logging handlers interact with Libevent functions, preventing the dangerous feedback loop that previously caused daemon crashes.