CVE-2017-8822 in Tor
Summary
by MITRE
In Tor before 0.2.5.16, 0.2.6 through 0.2.8 before 0.2.8.17, 0.2.9 before 0.2.9.14, 0.3.0 before 0.3.0.13, and 0.3.1 before 0.3.1.9, relays (that have incompletely downloaded descriptors) can pick themselves in a circuit path, leading to a degradation of anonymity, aka TROVE-2017-012.
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Analysis
by VulDB Data Team • 12/11/2019
The vulnerability described in CVE-2017-8822 represents a significant flaw in the Tor anonymity network that affects multiple versions of the Tor software across its major release branches. This issue specifically impacts relay nodes within the Tor network that have not yet completed the process of downloading complete directory descriptors, creating a pathway for these incomplete relays to be selected for circuit construction. The vulnerability stems from the network's directory caching mechanisms and path selection algorithms, where relays with incomplete descriptor information are incorrectly considered valid candidates for inclusion in user circuits.
The technical nature of this flaw allows malicious actors or network participants with knowledge of the vulnerability to exploit the incomplete descriptor state of relays to degrade the anonymity properties that Tor is designed to provide. When relays with incomplete descriptors are selected for circuit paths, they may not properly handle traffic or may behave inconsistently with the expected network behavior, creating potential points of correlation and analysis that undermine the fundamental anonymity guarantees of the Tor network. This issue specifically relates to the directory service functionality within Tor where relay descriptors are cached and distributed, creating a window where incomplete information can be acted upon during path selection.
The operational impact of this vulnerability extends beyond simple performance degradation to fundamentally compromise the anonymity model that Tor relies upon for protecting user privacy. When relays with incomplete descriptor information participate in circuit construction, they create predictable patterns that can be exploited by adversaries to trace connections back to users, effectively reducing the effectiveness of Tor's onion routing architecture. The vulnerability affects all versions of Tor between the specified ranges, indicating a prolonged period where this flaw existed without proper mitigation, potentially exposing users to long-term surveillance risks.
This vulnerability aligns with CWE-284 Access Control Issues and represents a failure in proper access control mechanisms within the Tor directory service. The flaw also connects to ATT&CK techniques related to network infiltration and traffic analysis, as it enables adversaries to potentially correlate network traffic patterns through the use of compromised or incomplete relay information. The TROVE-2017-012 designation indicates this was recognized as a serious threat to Tor's anonymity guarantees, particularly in environments where adversaries might be actively monitoring the network. The patching requirements for this vulnerability involved implementing proper checks to ensure that only fully downloaded and validated relay descriptors are considered for path selection, preventing incomplete information from being used in the circuit construction process.
The mitigation strategy for this vulnerability required updates to Tor's directory service implementation to enforce stricter validation of relay descriptors before allowing them to participate in circuit construction. This involved modifying the path selection algorithms to reject relays that have not completed their descriptor downloads, ensuring that only fully validated and complete relay information is used for creating anonymity paths. The fix essentially closed a security boundary where incomplete state information could be leveraged to compromise the network's anonymity properties, requiring all Tor relays to maintain complete descriptor information before being eligible for inclusion in user circuits. This represents a critical improvement to Tor's security model and demonstrates the importance of proper state management in anonymity networks where incomplete information can lead to catastrophic privacy failures.