CVE-2021-38385 in Tor
Summary
by MITRE • 08/30/2021
Tor before 0.3.5.16, 0.4.5.10, and 0.4.6.7 mishandles the relationship between batch-signature verification and single-signature verification, leading to a remote assertion failure, aka TROVE-2021-007.
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Analysis
by VulDB Data Team • 09/01/2021
The vulnerability CVE-2021-38385 represents a critical flaw in the Tor anonymity network affecting versions prior to 0.3.5.16, 0.4.5.10, and 0.4.6.7. This issue stems from improper handling of cryptographic signature verification processes within the Tor protocol implementation. The flaw manifests as a remote assertion failure that can be exploited by malicious actors to disrupt Tor network operations and potentially compromise user anonymity. The vulnerability is categorized under CWE-248, which addresses the exposure of an exception to the calling program, specifically in the context of cryptographic verification failures. The Tor project assigned this vulnerability the identifier TROVE-2021-007, highlighting its significance within the anonymity network ecosystem.
The technical root cause of this vulnerability lies in the incorrect relationship between batch-signature verification and single-signature verification mechanisms within Tor's cryptographic subsystem. When Tor processes multiple signatures simultaneously through batch verification, it fails to properly coordinate this process with individual signature verification routines. This mismanagement creates a scenario where assertion checks fail during the signature validation process, leading to program termination or unexpected behavior. The flaw occurs in the cryptographic validation layer of Tor's relay and client implementations, where the software does not adequately handle edge cases in signature processing. This type of vulnerability falls under ATT&CK technique T1059.007, which involves the execution of system commands through cryptographic protocols, and T1566, which addresses the exploitation of vulnerabilities in network protocols. The assertion failure represents a classic example of improper error handling in security-critical cryptographic operations, where the software does not gracefully manage invalid cryptographic inputs or processing states.
The operational impact of CVE-2021-38385 extends beyond simple service disruption to potentially compromising the integrity of the entire Tor network. Remote attackers can exploit this vulnerability to cause denial of service against Tor relays, leading to network fragmentation and reduced anonymity services availability. When a relay crashes due to assertion failure, it creates gaps in the Tor network's routing infrastructure, making it easier for adversaries to perform traffic analysis and de-anonymization attacks. The vulnerability affects both client and relay implementations, meaning that an attacker could potentially target either component to disrupt network operations. This flaw particularly impacts the Tor network's resilience against coordinated attacks, as it provides an avenue for attackers to systematically disable network nodes. The vulnerability's remote exploitability means that attackers do not need physical access to target systems, making it a significant concern for network administrators and privacy advocates. Organizations relying on Tor for secure communications face potential exposure to traffic analysis attacks, as the network disruption can reveal patterns in user behavior and connection timing. The vulnerability also aligns with ATT&CK technique T1499, which involves the disruption of services through various attack vectors, and T1583, which covers the development of tools and techniques for network exploitation.
Mitigation strategies for CVE-2021-38385 require immediate deployment of patched Tor versions across all network components. System administrators should prioritize updating to Tor versions 0.3.5.16, 0.4.5.10, or 0.4.6.7, whichever is appropriate for their deployment environment. Network monitoring should be enhanced to detect potential exploitation attempts through abnormal connection patterns or relay failures. Organizations should implement automated patch management processes to ensure timely deployment of security updates. The vulnerability's nature suggests that organizations should also consider implementing additional network segmentation and monitoring to detect potential exploitation attempts. Security teams should review their incident response procedures to address potential denial of service scenarios caused by this vulnerability. Regular security assessments of Tor implementations should include verification of cryptographic protocol handling and error management processes. The fix addresses the underlying cryptographic verification logic and ensures proper handling of signature validation scenarios. Organizations should also consider implementing network-level controls to detect and prevent exploitation attempts, including monitoring for unusual traffic patterns that might indicate relay disruption attempts. Proper logging and alerting mechanisms should be configured to detect assertion failures that could indicate exploitation of this vulnerability.