CVE-2015-2929 in Tor
Summary
by MITRE
The Hidden Service (HS) client implementation in Tor before 0.2.4.27, 0.2.5.x before 0.2.5.12, and 0.2.6.x before 0.2.6.7 allows remote servers to cause a denial of service (assertion failure and application exit) via a malformed HS descriptor.
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Analysis
by VulDB Data Team • 05/03/2022
The vulnerability identified as CVE-2015-2929 represents a critical denial of service weakness within the Tor network's Hidden Service client implementation. This flaw affects multiple versions of the Tor software including releases before 0.2.4.27, 0.2.5.x before 0.2.5.12, and 0.2.6.x before 0.2.6.7, creating a significant risk for users relying on Tor's anonymity services. The vulnerability specifically targets the client-side processing of Hidden Service descriptors, which are essential components for establishing connections to hidden services within the Tor network. These descriptors contain metadata about hidden services including cryptographic keys and routing information that clients must properly parse to maintain connectivity.
The technical nature of this vulnerability stems from inadequate input validation within the Hidden Service client implementation. When remote servers provide malformed HS descriptors, the client fails to properly handle these invalid inputs, leading to assertion failures that cause the application to terminate unexpectedly. This behavior represents a classic buffer overflow or input validation vulnerability where the software does not adequately sanitize or validate incoming data before processing it. The flaw operates at the protocol level within Tor's hidden service subsystem, specifically affecting how client software processes descriptor data structures that are critical for maintaining hidden service connections.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromising the reliability and availability of the entire Tor network infrastructure. When exploited, the vulnerability can cause legitimate Tor clients to crash and exit, forcing users to restart their applications and potentially disrupting ongoing communications. This denial of service condition affects not only individual users but also the broader Tor ecosystem since it can be leveraged to create cascading failures within the network. The vulnerability's exploitation requires minimal resources from attackers, making it particularly dangerous as it can be used to target large numbers of clients simultaneously without significant computational overhead.
From a cybersecurity perspective, this vulnerability aligns with CWE-129, which describes improper validation of input ranges, and represents a failure in defensive programming practices within the Tor codebase. The issue demonstrates poor error handling and input validation mechanisms that should have been implemented to prevent malformed data from causing application termination. The ATT&CK framework categorizes this as a denial of service attack pattern where adversaries can disrupt services by exploiting implementation flaws in software components. Organizations and individuals using Tor should prioritize immediate patching of affected versions, as the vulnerability represents a known weakness that has been documented and addressed through software updates.
Mitigation strategies for CVE-2015-2929 require immediate deployment of patched Tor versions that contain proper input validation and error handling mechanisms. System administrators should ensure all Tor clients are updated to versions 0.2.4.27, 0.2.5.12, or 0.2.6.7 respectively, depending on their current software version. Network monitoring should be enhanced to detect unusual client termination patterns that might indicate exploitation attempts. Additionally, implementing network segmentation and access controls can help limit the impact of potential exploitation attempts, while maintaining regular security audits of Tor implementations to identify similar vulnerabilities in other network components. The vulnerability serves as a reminder of the critical importance of robust input validation in security-sensitive applications, particularly those handling untrusted network data in anonymity networks where reliability is paramount.