CVE-2011-2768 in Tor
Summary
by MITRE
Tor before 0.2.2.34, when configured as a client or bridge, sends a TLS certificate chain as part of an outgoing OR connection, which allows remote relays to bypass intended anonymity properties by reading this chain and then determining the set of entry guards that the client or bridge had selected.
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Analysis
by VulDB Data Team • 11/28/2021
The vulnerability described in CVE-2011-2768 represents a significant weakness in the Tor anonymity network's cryptographic handshake process that undermines the fundamental privacy guarantees users expect. This flaw affects Tor versions prior to 0.2.2.34 and specifically impacts clients and bridge nodes that establish outgoing OR (Onion Router) connections to other Tor relays. The vulnerability stems from the improper handling of TLS certificate chains during the connection establishment phase, where the client or bridge inadvertently exposes information about its network topology and entry guard selection to remote relays.
The technical flaw manifests when Tor nodes send their complete TLS certificate chain as part of the outgoing OR connection handshake process. This certificate chain contains metadata that reveals information about the client's or bridge's network behavior, particularly the selection of entry guards that serve as the initial points of entry into the Tor network. Entry guards are critical components of Tor's security model, as they are supposed to be selected by users based on their trust and should remain stable for extended periods to prevent traffic analysis attacks. When this information is exposed through the certificate chain, remote relays can correlate the certificate information with their own knowledge of the Tor network to identify which entry guards a particular client or bridge has chosen.
This vulnerability directly impacts the anonymity properties that Tor is designed to provide, as it allows adversaries to perform linkability analysis between different network connections and potentially de-anonymize users. The exposure of entry guard information creates a pathway for traffic correlation attacks, where attackers can monitor multiple points in the network and correlate traffic patterns to determine the identity of users. This represents a violation of the principle of forward secrecy and breaks the fundamental assumption that Tor users should be able to select entry guards without revealing this selection to other network participants. The impact extends beyond individual users to potentially compromise the entire Tor network's security model, as it undermines the trust relationships that are essential for maintaining anonymity.
The operational implications of this vulnerability are severe for Tor network participants, as it provides attackers with a method to track user behavior across the network and potentially identify individuals who may be subject to surveillance or harassment. Security researchers and network administrators should prioritize updating to Tor version 0.2.2.34 or later, which addresses this issue by properly handling certificate chains during the connection establishment process. The fix implemented in this version ensures that sensitive information about entry guard selection is not exposed through the TLS handshake, thereby restoring the intended anonymity properties of the Tor network. Organizations relying on Tor for privacy-sensitive communications should conduct immediate security assessments to ensure their Tor implementations are not vulnerable to this attack vector and consider additional network monitoring to detect potential exploitation attempts.
This vulnerability aligns with CWE-200, which addresses improper exposure of sensitive information, and represents a clear violation of the principle of least privilege in network security. The attack vector described in this vulnerability maps to techniques found in the ATT&CK framework under the T1566 category, which covers credential access and network infiltration methods that exploit information disclosure vulnerabilities. The security implications extend beyond the immediate privacy concerns to include potential compromise of operational security for individuals operating in hostile environments where anonymity is critical for safety and freedom of expression.