CVE-2011-2769 in Tor
Summary
by MITRE
Tor before 0.2.2.34, when configured as a bridge, accepts the CREATE and CREATE_FAST values in the Command field of a cell within an OR connection that it initiated, which allows remote relays to enumerate bridges by using these values.
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Analysis
by VulDB Data Team • 11/28/2021
The vulnerability described in CVE-2011-2769 represents a significant security flaw in the Tor anonymity network that affected versions prior to 0.2.2.34. This issue specifically impacts Tor bridges, which serve as entry points into the Tor network for users who cannot access the public directory of relays due to censorship or other restrictions. The vulnerability stems from improper validation of cell commands within the OR (Onion Router) connection protocol, creating a mechanism through which remote adversaries can perform reconnaissance activities against bridge infrastructure. When a Tor bridge receives a cell with specific command values, it processes these commands in a manner that inadvertently reveals information about the bridge's existence to remote attackers.
The technical flaw manifests in the bridge's handling of CREATE and CREATE_FAST command values within the cell protocol of OR connections. These commands are normally used for establishing and managing connections between Tor relays, but the vulnerability allows remote relays to send these commands to a bridge that initiated the connection. The bridge's insufficient validation of these commands enables an attacker to determine whether the bridge is accepting specific connection types, effectively allowing enumeration of bridge addresses. This behavior violates fundamental security principles of the Tor network, where bridges should remain hidden from potential adversaries who might attempt to target them for removal from the network or to identify users connecting through them.
The operational impact of this vulnerability extends beyond simple information disclosure, as it fundamentally undermines the anonymity and security model that Tor provides to its users. Remote relays can systematically test various bridge addresses to determine which ones are operational, thereby compromising the stealth required for bridges to function effectively. This enumeration capability enables adversaries to identify bridges that may be used by censored users, potentially leading to targeted attacks against these infrastructure components. The vulnerability creates a reconnaissance vector that directly conflicts with the core design principle of Tor bridges, which are intended to be invisible to network observers and resistant to automated discovery techniques. This weakness particularly affects users in heavily censored environments who rely on bridges as their primary means of accessing the Tor network.
The implications of this vulnerability align with several cybersecurity frameworks, including CWE-20, which addresses "Improper Input Validation," and the ATT&CK framework's reconnaissance phase where adversaries gather information about targets before launching attacks. The flaw demonstrates how insufficient protocol validation can create information leakage channels that compromise the entire security architecture. Mitigation strategies for this vulnerability involve updating to Tor version 0.2.2.34 or later, which implements proper validation of cell commands and prevents remote relays from enumerating bridge addresses through malformed CREATE and CREATE_FAST commands. Additionally, network administrators should ensure that bridge configurations properly validate all incoming commands and implement monitoring to detect anomalous traffic patterns that might indicate enumeration attempts. The fix addresses the root cause by ensuring that bridges reject invalid commands rather than processing them in ways that reveal operational information to potential adversaries.