CVE-2017-8820 in Tor
Summary
by MITRE
In Tor before 0.2.5.16, 0.2.6 through 0.2.8 before 0.2.8.17, 0.2.9 before 0.2.9.14, 0.3.0 before 0.3.0.13, and 0.3.1 before 0.3.1.9, remote attackers can cause a denial of service (NULL pointer dereference and application crash) against directory authorities via a malformed descriptor, aka TROVE-2017-010.
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Analysis
by VulDB Data Team • 12/11/2019
The vulnerability identified as CVE-2017-8820 represents a critical denial of service weakness affecting the Tor anonymity network across multiple version ranges. This flaw manifests as a NULL pointer dereference that occurs when directory authorities process malformed descriptors, leading to application crashes and complete service disruption. The vulnerability specifically impacts Tor versions prior to the listed patched releases, creating a window of exposure where malicious actors can exploit the network's directory authority nodes. Directory authorities serve as crucial components in Tor's infrastructure, responsible for maintaining and distributing network topology information to all participating nodes. When these authorities become compromised through the denial of service attack, the entire network's functionality is severely degraded, as users lose access to the distributed directory services that enable anonymous communication.
The technical implementation of this vulnerability stems from inadequate input validation within the descriptor processing logic of Tor's directory authority implementations. When a malformed descriptor is received, the system attempts to dereference a NULL pointer without proper null checks, resulting in immediate application termination. This type of vulnerability falls under CWE-476, which specifically addresses NULL pointer dereference conditions in software implementations. The flaw demonstrates poor defensive programming practices where the software fails to validate incoming data before attempting operations on it, creating an exploitable condition that remote attackers can leverage through crafted malicious descriptors. The attack vector requires only that an attacker be able to send specially crafted descriptors to directory authority nodes, making the exploitation relatively simple and effective.
The operational impact of CVE-2017-8820 extends beyond simple service disruption to threaten the fundamental integrity of the Tor network's distributed architecture. Directory authority nodes form the backbone of Tor's directory services, and their compromise directly affects user anonymity and network availability. When these nodes crash, they become unavailable to distribute updated network information, causing other Tor nodes to lose connectivity and potentially forcing users to abandon the network entirely. The vulnerability's designation as TROVE-2017-010 indicates its recognition within the security community and highlights the potential for coordinated attacks that could systematically target multiple directory authorities. This attack pattern aligns with ATT&CK technique T1499.004, which covers network denial of service attacks, and demonstrates how attackers can leverage software flaws to create cascading failures in distributed systems.
Mitigation strategies for CVE-2017-8820 require immediate deployment of patched Tor versions across all directory authority nodes and user installations. System administrators should prioritize updating to versions 0.2.5.16, 0.2.8.17, 0.2.9.14, 0.3.0.13, or 0.3.1.9 respectively, depending on their current Tor version. Network monitoring should be enhanced to detect unusual descriptor traffic patterns that might indicate attempted exploitation, while implementing rate limiting and input validation measures at network boundaries. The vulnerability underscores the importance of robust input validation and defensive programming practices in distributed systems, particularly those handling sensitive network data. Organizations should also consider implementing redundant directory authority configurations and regular security audits to identify similar weaknesses in their network infrastructure. Additionally, the incident highlights the critical need for timely security patch management in open source projects where vulnerabilities can affect widespread deployments across diverse user bases.