CVE-2017-0377 in Tor
Summary
by MITRE
Tor 0.3.x before 0.3.0.9 has a guard-selection algorithm that only considers the exit relay (not the exit relay's family), which might allow remote attackers to defeat intended anonymity properties by leveraging the existence of large families.
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Analysis
by VulDB Data Team • 12/09/2022
The vulnerability described in CVE-2017-0377 affects the Tor anonymity network version 0.3.x prior to 0.3.0.9, specifically targeting the guard selection algorithm that governs how clients choose their initial relay connections in the Tor circuit. This flaw represents a significant weakness in the network's defensive mechanisms against coordinated attacks that could compromise user anonymity. The issue stems from the algorithm's failure to account for the familial relationships between relays, which creates predictable patterns in circuit construction that adversaries can exploit to undermine the fundamental anonymity properties that Tor is designed to provide.
The technical flaw manifests in the guard selection process where Tor clients evaluate potential guard relays based solely on individual relay characteristics without considering whether these relays belong to the same family or organization. This oversight allows malicious actors to create large relay families and strategically place them within the Tor network, enabling them to monitor and correlate traffic patterns across multiple circuits. The vulnerability is particularly concerning because it directly violates the principle of dispersion that Tor relies on for security, where users should be protected from adversaries who can observe traffic patterns across multiple relays.
From an operational perspective, this vulnerability creates a pathway for sophisticated adversaries to perform traffic correlation attacks by leveraging the existence of large relay families. When attackers control multiple relays within the same family, they can more effectively track user activities across the network by observing traffic patterns that would normally be dispersed across many independent relays. This compromise undermines the core anonymity guarantees that Tor users rely upon, potentially exposing user identities and their online activities to surveillance. The impact extends beyond individual user privacy to threaten the overall integrity and security of the Tor network as a whole.
The vulnerability aligns with CWE-310, which addresses cryptographic weaknesses in the design of systems, and relates to ATT&CK technique T1018 which covers "Remote System Discovery" and T1071 which covers "Application Layer Protocol". The flaw represents a design weakness in the Tor network's consensus mechanism and relay selection algorithms that should have considered the potential for coordinated attacks through family-based relay clustering. The mitigation strategy involves updating to Tor version 0.3.0.9 or later, which implements proper family-aware guard selection algorithms that consider the relationships between relays when constructing circuits. Additionally, network operators should monitor for unusual relay clustering patterns and implement proper network monitoring to detect potential family-based attacks. This vulnerability demonstrates the critical importance of considering all potential attack vectors, including those involving coordinated adversary behavior, when designing security systems that rely on distributed networks and anonymity properties.