CVE-2017-0375 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 relay_send_end_cell_from_edge_ function via a malformed BEGIN cell.
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Analysis
by VulDB Data Team • 12/08/2022
The vulnerability identified as CVE-2017-0375 resides within the Tor anonymity network's hidden-service functionality, specifically affecting versions prior to 0.3.0.8. This issue manifests as a denial of service condition that can cause the Tor daemon to terminate unexpectedly due to an assertion failure. The flaw occurs within the relay_send_end_cell_from_edge_ function, which processes communication between Tor relays and hidden service clients. The vulnerability is triggered by sending a malformed BEGIN cell to the Tor relay, exploiting a weakness in the input validation mechanisms that govern how the network handles connection requests for hidden services.
The technical exploitation of this vulnerability demonstrates a classic buffer over-read or improper input validation flaw that falls under CWE-129, which represents an insufficient input validation scenario. When a malicious actor sends a crafted BEGIN cell with malformed data, the relay's processing logic fails to properly validate the incoming data structure before attempting to process it. This leads to an assertion failure within the relay_send_end_cell_from_edge_ function, causing the daemon to crash and exit. The assertion failure represents a fundamental breakdown in the software's error handling capabilities, where the system fails to gracefully handle unexpected input rather than rejecting it safely. This behavior directly violates the principle of robust error handling that is essential for maintaining service availability in network infrastructure.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromising the anonymity network's reliability and integrity. When a Tor relay becomes unavailable due to this assertion failure, it affects the entire network topology, as relays serve as critical nodes in the path between clients and hidden services. The daemon exit can result in the loss of ongoing connections, forcing users to re-establish their anonymity circuits and potentially exposing them to traffic analysis attacks. Network operators may experience increased maintenance overhead as they must restart affected services and monitor for further exploitation attempts. The vulnerability also creates opportunities for attackers to perform coordinated denial of service attacks against multiple relays simultaneously, potentially degrading the overall quality of service for the entire Tor network.
Mitigation strategies for CVE-2017-0375 should prioritize immediate patching of affected Tor installations to version 0.3.0.8 or later, which contains the necessary code modifications to properly validate BEGIN cell data before processing. Network administrators should implement monitoring solutions that can detect anomalous traffic patterns indicative of malformed cell transmissions, enabling rapid response to potential exploitation attempts. The fix implemented in the patched versions involves strengthening input validation within the relay_send_end_cell_from_edge_ function to ensure that all BEGIN cell data conforms to expected formats before any processing occurs. Security teams should also consider implementing network-level filtering to block suspicious traffic patterns and establish incident response procedures that account for potential service disruption scenarios. Organizations relying on Tor for anonymity services should conduct thorough testing of patched versions to ensure that legitimate network functionality remains intact while the vulnerability is addressed. This vulnerability also highlights the importance of proper software testing and validation processes, particularly for network infrastructure components that must handle untrusted input from diverse sources. The ATT&CK framework categorizes this as a denial of service attack pattern, where the adversary leverages software flaws to disrupt service availability, and the mitigation approaches align with defensive techniques focused on input validation and system hardening.