CVE-2021-28090 in Tor
Summary
by MITRE • 03/19/2021
Tor before 0.4.5.7 allows a remote attacker to cause Tor directory authorities to exit with an assertion failure, aka TROVE-2021-002.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 04/03/2021
The vulnerability identified as CVE-2021-28090 represents a critical assertion failure flaw in the Tor anonymization network that affects versions prior to 0.4.5.7. This issue specifically targets Tor directory authorities which serve as the cornerstone of the Tor network's distributed trust infrastructure. These authorities maintain the network's consensus documents and directory information that clients use to establish connections through the anonymization network. The flaw enables remote attackers to manipulate the network's operational integrity by triggering assertion failures that cause these critical directory authorities to terminate unexpectedly.
The technical implementation of this vulnerability stems from improper handling of malformed directory information within the Tor directory authority software. When processing certain crafted inputs or directory entries, the software encounters conditions that violate internal assertions, causing the process to crash and exit abruptly. This assertion failure represents a fundamental breakdown in the software's error handling mechanisms, where the system fails to gracefully manage unexpected input data rather than properly validating or rejecting malformed entries. The vulnerability specifically affects the directory authority's ability to process and validate network information, creating a potential denial-of-service vector that could compromise the network's stability and availability.
From an operational perspective, this vulnerability poses significant risks to the Tor network's reliability and security posture. Directory authorities are essential components that maintain the network's consensus and ensure that clients can properly route their traffic through the anonymization network. When these authorities crash due to assertion failures, it creates gaps in the network's directory information, potentially disrupting client connectivity and compromising the anonymity guarantees that Tor provides. The remote nature of this attack means that adversaries can exploit this vulnerability from anywhere on the internet without requiring local access or authentication, making it particularly dangerous for maintaining network integrity.
The impact extends beyond simple service disruption as this vulnerability could enable attackers to systematically weaken the Tor network's infrastructure. If multiple directory authorities are compromised simultaneously, it could lead to a cascading failure that affects the entire network's ability to function properly. Network operators and security professionals must consider this vulnerability in their risk assessments as it represents a potential attack vector that could be used to degrade the anonymity services provided by Tor. The vulnerability also demonstrates poor adherence to secure coding practices and highlights the importance of proper input validation and error handling in critical infrastructure software.
Mitigation strategies for CVE-2021-28090 primarily focus on upgrading to Tor version 0.4.5.7 or later, which contains the necessary patches to address the assertion failure issue. Network administrators should implement comprehensive monitoring of directory authority processes to detect any unexpected terminations or assertion failures that could indicate exploitation attempts. The vulnerability aligns with CWE-617, which addresses reachable assertions, and represents a specific instance of software reliability failures that can be exploited for denial-of-service attacks. Security teams should also consider implementing network segmentation and access controls around directory authority systems to limit potential attack surfaces and reduce the impact of successful exploitation attempts. Organizations using Tor services should prioritize patch management and maintain awareness of the broader Tor security landscape to ensure continued protection against similar vulnerabilities.