CVE-2017-0376 in Tor
Summary
by MITRE
The hidden-service feature in Tor before 0.3.0.8 allows a denial of service (assertion failure and daemon exit) in the connection_edge_process_relay_cell function via a BEGIN_DIR cell on a rendezvous circuit.
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Analysis
by VulDB Data Team • 12/08/2022
The vulnerability identified as CVE-2017-0376 represents a critical denial of service weakness within the Tor anonymization network that affected versions prior to 0.3.0.8. This flaw specifically targets the hidden-service functionality of Tor, which enables users to host and access services anonymously on the network. The vulnerability manifests through a carefully crafted BEGIN_DIR cell being transmitted on a rendezvous circuit, which triggers an assertion failure within the connection_edge_process_relay_cell function. This assertion failure ultimately causes the Tor daemon to terminate unexpectedly, effectively disrupting the service for legitimate users and creating a denial of service condition that undermines the network's reliability and availability.
The technical root cause of this vulnerability lies in inadequate input validation within the Tor protocol implementation. When processing a BEGIN_DIR cell on a rendezvous circuit, the connection_edge_process_relay_cell function fails to properly validate the cell data, leading to an assertion failure that terminates the daemon process. This represents a classic software defect pattern where insufficient boundary checking and input sanitization allows malicious or malformed data to trigger unexpected program behavior. The vulnerability operates at the network protocol level, specifically within the edge connection handling mechanism that manages data flow between the Tor client and the network, making it particularly dangerous as it can be exploited without requiring elevated privileges or specialized access to the system.
The operational impact of CVE-2017-0376 extends beyond simple service disruption to potentially compromise the integrity of the Tor network itself. When the Tor daemon crashes due to this assertion failure, it creates gaps in the network's connectivity that can affect not only the targeted node but also disrupt the broader routing infrastructure. This vulnerability particularly impacts hidden services that rely on rendezvous circuits for communication, as it can prevent legitimate users from establishing connections to these services. The attack vector is relatively simple to execute, requiring only the ability to send a specific type of cell to a target Tor node, making it a potential tool for adversaries seeking to degrade Tor network performance or target specific hidden services. From an attacker perspective, this vulnerability aligns with tactics described in the MITRE ATT&CK framework under the service stoppage and denial of service categories, representing a method to disrupt network availability through software-level exploitation.
Mitigation of CVE-2017-0376 requires immediate deployment of Tor version 0.3.0.8 or later, which includes the necessary patches to address the assertion failure in the connection_edge_process_relay_cell function. Organizations and individuals utilizing Tor should implement comprehensive patch management procedures to ensure all Tor instances are updated to versions that contain the fix. Network administrators should also consider implementing monitoring solutions that can detect unusual daemon termination patterns or assertion failures that might indicate exploitation attempts. The fix implemented in version 0.3.0.8 involves strengthening input validation within the cell processing functions and adding proper error handling to prevent assertion failures from causing daemon termination. This vulnerability highlights the importance of robust software testing and validation procedures, particularly for network protocols where malformed inputs could lead to system-wide failures. The fix demonstrates the principle of defensive programming where developers must anticipate and handle all possible input conditions rather than relying on assertions to validate program state, which aligns with common security best practices and standards such as those outlined in the CWE database for assertion failure prevention.