CVE-2011-4896 in Tor
Summary
by MITRE
Tor before 0.2.2.24-alpha continues to use a reachable bridge that was previously configured but is not currently configured, which might allow remote attackers to obtain sensitive information about clients in opportunistic circumstances by monitoring network traffic to the bridge port.
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Analysis
by VulDB Data Team • 02/13/2019
The vulnerability identified as CVE-2011-4896 affects the Tor anonymity network version 0.2.2.24-alpha and earlier, presenting a significant security risk through improper bridge management. Tor operates as a distributed network of relays that obscure user identities by routing traffic through multiple nodes, with bridges serving as entry points that help users bypass censorship and surveillance. This specific flaw occurs when the system continues to maintain a bridge configuration that was previously active but has since been removed from the current configuration, creating a persistent exposure that adversaries can exploit.
The technical nature of this vulnerability stems from the improper handling of bridge configurations within the Tor client software. When a bridge is removed from the active configuration, the system fails to completely terminate the associated network connections or clean up the bridge's operational state. This creates a scenario where network traffic destined for the bridge port continues to be processed by the system even though the bridge is no longer intended to be active. The flaw essentially maintains a "ghost" bridge that remains accessible to network monitoring tools, allowing attackers to observe traffic patterns and potentially correlate them with user activities.
From an operational perspective, this vulnerability creates opportunities for remote attackers to perform traffic analysis and correlation attacks against Tor users. The monitoring of network traffic to the bridge port enables adversaries to gather information about client behavior, connection patterns, and potentially even identify specific users based on timing characteristics or traffic signatures. This represents a significant threat to user anonymity since the bridge serves as the initial point of contact between the user and the Tor network, making it a critical target for surveillance operations. The vulnerability is particularly concerning because it operates in opportunistic circumstances, meaning that attackers can exploit it when they have the capability to monitor network traffic without requiring active participation in the network.
The impact of this vulnerability extends beyond simple information disclosure, as it fundamentally undermines the core security assumptions of the Tor network's design. The ability for attackers to observe bridge traffic creates a pathway for de-anonymization attacks that could compromise user privacy and safety, especially in environments where surveillance capabilities are advanced. This flaw represents a failure in proper resource cleanup and state management within the Tor client software, creating persistent network exposure that persists even after configuration changes have been applied.
Organizations and individuals using Tor should immediately update to version 0.2.2.24-alpha or later, which contains the necessary patches to address this bridge configuration management issue. The fix involves implementing proper cleanup procedures that ensure all network connections associated with previously configured bridges are terminated when those bridges are removed from the active configuration. This vulnerability aligns with CWE-254, which addresses security weaknesses in resource management and improper cleanup of system resources. From an ATT&CK framework perspective, this represents a technique for reconnaissance and information gathering that could be used to compromise user anonymity and potentially lead to more severe attacks. The vulnerability also demonstrates the importance of proper access control and resource management in maintaining network security, particularly in systems designed to protect user privacy and anonymity.