CVE-2012-2250 in Tor
Summary
by MITRE
Tor before 0.2.3.24-rc allows remote attackers to cause a denial of service (assertion failure and daemon exit) by performing link protocol negotiation incorrectly.
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Analysis
by VulDB Data Team • 03/18/2019
The vulnerability identified as CVE-2012-2250 represents a critical denial of service weakness in the Tor anonymization network prior to version 0.2.3.24-rc. This flaw resides in the link protocol negotiation mechanism that governs how Tor relays communicate with each other to establish secure connections. The issue manifests when remote attackers manipulate the link protocol negotiation process in a manner that triggers assertion failures within the Tor daemon, ultimately causing the service to terminate unexpectedly. This vulnerability specifically targets the core operational integrity of the Tor network infrastructure, affecting the reliability and availability of anonymization services that millions of users depend upon for privacy protection.
The technical implementation of this vulnerability stems from inadequate validation mechanisms during the link protocol negotiation phase of Tor's relay communication. When malicious actors craft specially crafted negotiation messages, they can force the Tor daemon to encounter assertion failures that are not properly handled within the codebase. These assertion failures occur because the software fails to validate the legitimacy of incoming negotiation parameters before proceeding with connection establishment. The flaw operates at the protocol level, specifically targeting the lower-level communication protocols that enable Tor relays to form circuits and maintain network connectivity. According to CWE classification, this vulnerability maps to CWE-617: Reachable Assertion, which describes situations where assertions can be triggered by external inputs, leading to program termination. The issue demonstrates a classic example of insufficient input validation and error handling in security-critical network software.
The operational impact of CVE-2012-2250 extends beyond simple service disruption to potentially compromise the overall stability and anonymity guarantees of the Tor network. When Tor relays crash due to assertion failures, they temporarily remove themselves from the network, which can result in circuit breakage and reduced network resilience. This disruption affects not only the immediate target relay but also impacts the broader network topology as routing paths are recalculated and alternative routes must be established. The vulnerability creates a vector for attackers to systematically destabilize Tor infrastructure by targeting multiple relays across different geographical locations, potentially leading to network partitioning. From an adversarial perspective, this weakness aligns with ATT&CK technique T1499.004: Network Denial of Service, where attackers can leverage service weaknesses to compromise availability. The attack can be executed remotely without requiring authentication or privileged access, making it particularly dangerous for network infrastructure that relies on Tor's distributed architecture for anonymity protection.
Mitigation strategies for CVE-2012-2250 require immediate deployment of Tor version 0.2.3.24-rc or later, which includes proper assertion handling and enhanced validation of link protocol negotiation messages. Network administrators should implement monitoring solutions that can detect unusual daemon termination patterns and automatically trigger alerting mechanisms when assertion failures occur. The fix implemented in the patched version addresses the root cause by strengthening input validation procedures and ensuring that assertion failures do not result in daemon exit conditions. Organizations operating Tor infrastructure should also consider implementing redundant relay configurations to minimize the impact of single-point failures. Additionally, regular security audits of network protocols and careful code review processes should be implemented to identify similar assertion-related vulnerabilities in other network security applications. The remediation approach emphasizes the importance of robust error handling in security-critical systems and demonstrates the necessity of thorough testing of protocol negotiation mechanisms under adversarial conditions.