CVE-2011-4894 in Tor
Summary
by MITRE
Tor before 0.2.2.34, when configured as a bridge, uses direct DirPort access instead of a Tor TLS connection for a directory fetch, which makes it easier for remote attackers to enumerate bridges by observing DirPort connections.
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Analysis
by VulDB Data Team • 02/13/2019
The vulnerability identified as CVE-2011-4894 affects the Tor anonymity network and specifically impacts versions prior to 0.2.2.34 when the network is configured as a bridge. This flaw represents a significant security weakness that undermines the fundamental privacy guarantees that Tor is designed to provide. The issue manifests when Tor bridges attempt to fetch directory information from other Tor nodes, creating a potential reconnaissance vector for adversaries seeking to identify and enumerate bridge relays within the network infrastructure.
The technical flaw stems from the improper implementation of directory fetching mechanisms within Tor bridge configurations. Normally, Tor employs encrypted TLS connections for all directory communications to ensure confidentiality and integrity of the network topology information. However, in vulnerable versions, when a Tor bridge operates in a configuration that requires direct access to the directory port, the system falls back to using unencrypted direct connections instead of establishing the proper TLS encrypted sessions. This deviation from standard security practices creates a clear communication channel that adversaries can monitor and analyze without the need for decryption or advanced interception techniques.
The operational impact of this vulnerability extends beyond simple information disclosure, as it directly compromises the anonymity and security of Tor users who rely on bridge relays to circumvent censorship and surveillance. Remote attackers who can observe network traffic patterns can now identify which Tor bridges are active and operational by monitoring the direct DirPort connections, effectively de-anonymizing the bridge infrastructure. This enumeration capability allows adversaries to target specific bridge relays for disruption or further attacks, fundamentally weakening the network's resistance to traffic analysis and correlation attacks.
From a cybersecurity perspective, this vulnerability aligns with CWE-310, which addresses cryptographic weaknesses in network communications, and represents a clear violation of secure communication protocols that should be enforced at all network layers. The attack surface expands significantly as threat actors can now perform passive reconnaissance without requiring active network manipulation or exploitation of other vulnerabilities. This weakness also connects to ATT&CK technique T1071.004, which covers application layer protocol usage for command and control communications, as the direct connections create observable patterns that attackers can leverage for network reconnaissance.
The mitigation for this vulnerability requires immediate deployment of Tor version 0.2.2.34 or later, which properly implements TLS encryption for all directory fetch operations regardless of the bridge configuration. Network administrators should also implement monitoring solutions to detect and alert on any direct DirPort connections that may still occur due to misconfigurations. Additionally, organizations using Tor bridges should consider implementing network-level controls that enforce encrypted connections and regularly audit their network configurations to ensure compliance with secure communication standards. The fix represents a critical security update that restores the expected cryptographic protections and maintains the integrity of the Tor network's anonymity infrastructure.