CVE-2011-4895 in Tor
Summary
by MITRE
Tor before 0.2.2.34, when configured as a bridge, sets up circuits through a process different from the process used by a client, which makes it easier for remote attackers to enumerate bridges by observing circuit building.
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Analysis
by VulDB Data Team • 02/13/2019
The vulnerability described in CVE-2011-4895 affects the Tor anonymity network version 0.2.2.33 and earlier, specifically when the Tor software is configured as a bridge node. This represents a significant privacy and security weakness within the Tor ecosystem that directly impacts the network's ability to maintain user anonymity and protect against targeted attacks. The issue stems from the fundamental design inconsistency in how Tor bridges handle network connections compared to regular Tor clients, creating a distinguishable pattern that adversaries can exploit to identify and enumerate bridge nodes.
The technical flaw manifests in the distinct circuit building processes employed by Tor bridges versus regular clients. When Tor operates as a bridge, it follows a different protocol for establishing network connections and building circuits through the network. This divergence in behavior creates observable differences in traffic patterns, timing characteristics, and connection establishment methods that can be detected and analyzed by remote attackers. The inconsistency in circuit building processes essentially creates a fingerprint that identifies bridge nodes from regular Tor clients, making it possible for adversaries to distinguish between different types of Tor nodes in the network.
The operational impact of this vulnerability is severe for the Tor network's integrity and user privacy. Attackers who can enumerate bridge nodes gain significant advantages in targeting and monitoring Tor users who rely on bridges to avoid detection by network censors or adversaries. The ability to enumerate bridges undermines the core purpose of using bridges, which is to provide anonymity by making it difficult for adversaries to identify which nodes are part of the Tor network. This enumeration capability allows threat actors to focus their efforts on specific bridge nodes, potentially leading to targeted attacks against bridge operators or users who depend on those bridges.
From a cybersecurity perspective, this vulnerability aligns with CWE-254, which addresses security weaknesses in the implementation of access control mechanisms, and relates to ATT&CK technique T1566, which involves the use of social engineering and reconnaissance to identify network targets. The flaw represents a design weakness in the Tor protocol implementation that affects the network's resistance to traffic analysis and deanonymization attacks. The vulnerability essentially provides an information disclosure mechanism that reveals bridge node characteristics to remote observers, compromising the anonymity set that Tor aims to maintain.
The mitigation for this vulnerability required a complete redesign of the circuit building process for Tor bridges to ensure consistency with client behavior. The fix involved modifying the Tor software to use identical circuit establishment methods for both bridge and client configurations, eliminating the distinguishable patterns that attackers could exploit. This change ensures that bridge nodes operate with the same circuit building characteristics as regular Tor clients, making it significantly more difficult for remote attackers to enumerate bridges through traffic analysis. The update to Tor version 0.2.2.34 and later addresses the fundamental design inconsistency and restores the intended anonymity properties of the Tor network.