CVE-2005-2643 in Tor
Summary
by MITRE
Tor 0.1.0.13 and earlier, and experimental versions 0.1.1.4-alpha and earlier, does not reject certain weak keys when using ephemeral Diffie-Hellman (DH) handshakes, which allows malicious Tor servers to obtain the keys that a client uses for other systems in the circuit.
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Analysis
by VulDB Data Team • 06/09/2019
The vulnerability described in CVE-2005-2643 represents a critical weakness in the Tor anonymity network's cryptographic implementation that undermines the security guarantees expected from the system. This flaw exists in Tor versions 0.1.0.13 and earlier, as well as experimental versions 0.1.1.4-alpha and earlier, affecting the fundamental cryptographic handshake mechanism that protects communications within the Tor network. The vulnerability specifically targets the ephemeral Diffie-Hellman key exchange protocol, which is a cornerstone of secure communication in the Tor infrastructure.
The technical flaw stems from insufficient validation of Diffie-Hellman parameters during the handshake process. When Tor clients establish connections with relay nodes, they perform ephemeral Diffie-Hellman key exchanges to generate session keys that protect data transmission. However, the vulnerable versions fail to properly validate the strength of the generated keys, allowing malicious actors to exploit weak key generation. This weakness specifically affects the validation of the Diffie-Hellman parameters, particularly when the system uses small or predictable prime numbers that can be easily factored or attacked through computational methods. The vulnerability aligns with CWE-326, which addresses the weakness of inadequate encryption strength, and CWE-327, which covers the use of weak cryptographic algorithms.
The operational impact of this vulnerability is severe and directly compromises the anonymity and security of Tor users. Malicious Tor servers can exploit this weakness to recover the session keys that clients use for establishing circuits, potentially allowing them to correlate traffic and identify users. This attack vector undermines the core principle of Tor's design, which aims to prevent traffic analysis by ensuring that connections between relays cannot be easily linked to specific users or their activities. The vulnerability creates a scenario where attackers can perform man-in-the-middle attacks more effectively, potentially compromising the entire anonymity chain that users rely upon for privacy protection. This weakness specifically maps to attack techniques in the MITRE ATT&CK framework under T1041, which covers data from network shared modules, and T1566, which addresses credential access through malicious file execution.
The implications extend beyond simple key recovery to include broader network security risks that affect the integrity of the entire Tor ecosystem. When weak keys are used in the cryptographic handshake, they create backdoors that malicious actors can exploit to monitor and potentially manipulate communications. This vulnerability particularly affects the protection of user data that flows through the Tor network, as compromised session keys could allow attackers to decrypt traffic between relays and potentially link the origin of communications to specific users. The attack surface is further expanded because the vulnerability affects both stable and experimental versions of Tor, meaning that even users running the latest experimental builds were at risk. Organizations and individuals using Tor for legitimate privacy purposes would face significant security risks, as this vulnerability essentially provides adversaries with a method to break through the network's anonymity protections and potentially identify users based on their network behavior and communication patterns.