CVE-2012-2249 in Tor
Summary
by MITRE
Tor before 0.2.3.23-rc allows remote attackers to cause a denial of service (assertion failure and daemon exit) via a renegotiation attempt that occurs after the initiation of the V3 link protocol.
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Analysis
by VulDB Data Team • 03/18/2019
The vulnerability identified as CVE-2012-2249 represents a critical denial of service weakness within the Tor anonymization network affecting versions prior to 0.2.3.23-rc. This flaw specifically targets the V3 link protocol implementation within Tor's architecture, creating a scenario where remote attackers can deliberately trigger system failures through carefully crafted network interactions. The vulnerability exploits a fundamental timing and state management issue within the protocol negotiation process, allowing malicious actors to disrupt the normal operation of Tor daemon processes.
The technical root cause of this vulnerability lies in the improper handling of SSL/TLS renegotiation attempts that occur during the V3 link protocol establishment phase. When a Tor client or relay receives a renegotiation request after the initial protocol negotiation has begun but before it is fully completed, the system fails to properly validate or handle this unexpected state transition. This assertion failure occurs within the core protocol handling code, leading to an immediate daemon termination that effectively removes the compromised node from the Tor network. The flaw demonstrates poor error handling and state machine management that violates fundamental security principles for network services.
From an operational impact perspective, this vulnerability enables remote attackers to systematically disrupt Tor network infrastructure by targeting individual relays or bridges. The denial of service condition results in immediate daemon exit, causing network partitions and reducing the overall availability of the anonymization network. This attack vector is particularly concerning because it can be executed without requiring authentication or specialized privileges, making it accessible to any network entity capable of establishing communication with the target Tor node. The cascading effects of such attacks can degrade network performance and compromise user anonymity by reducing the number of available network paths.
The vulnerability aligns with CWE-248, which addresses "Uncaught Exception" conditions in software implementations, and demonstrates how improper exception handling can lead to complete service disruption. From an ATT&CK framework perspective, this represents a denial of service technique that leverages protocol-level weaknesses to achieve system compromise. The attack pattern follows T1499.004, which covers "Unsuccessful Attack" scenarios where adversaries attempt to disrupt services through protocol manipulation. The vulnerability also relates to T1566, covering "Phishing" techniques, as attackers might use this weakness to target specific Tor nodes during network reconnaissance.
Mitigation strategies for CVE-2012-2249 require immediate deployment of Tor version 0.2.3.23-rc or later, which includes proper state management and assertion validation for SSL/TLS renegotiation scenarios. Network administrators should implement monitoring solutions to detect unusual daemon exit patterns and establish automated restart procedures for Tor services. Additionally, organizations using Tor infrastructure should consider implementing network segmentation and access controls to limit exposure to potential attackers. The fix addresses the underlying protocol implementation by strengthening the state machine transitions and ensuring proper error handling during renegotiation attempts, thereby preventing the assertion failure that previously caused daemon termination.
This vulnerability highlights the critical importance of robust protocol handling in anonymity networks where service availability directly impacts user privacy and security. The flaw demonstrates how seemingly minor implementation details in cryptographic protocol handling can lead to significant operational consequences, particularly in distributed systems where reliability is paramount. The remediation process requires careful consideration of backward compatibility while ensuring that the fix does not introduce new vulnerabilities or performance degradation in the Tor network infrastructure.