CVE-2021-34548 in Tor
Summary
by MITRE • 06/29/2021
An issue was discovered in Tor before 0.4.6.5, aka TROVE-2021-003. An attacker can forge RELAY_END or RELAY_RESOLVED to bypass the intended access control for ending a stream.
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Analysis
by VulDB Data Team • 07/03/2021
The vulnerability identified as CVE-2021-34548 represents a critical security flaw in the Tor anonymity network affecting versions prior to 0.4.6.5. This issue falls under the category of access control bypass vulnerabilities and specifically targets the relay protocol implementation within Tor's network infrastructure. The vulnerability enables malicious actors to manipulate relay messages and potentially compromise the integrity of Tor's stream termination mechanisms.
The technical flaw resides in the improper validation of RELAY_END and RELAY_RESOLVED messages within Tor's relay protocol handling code. These messages are fundamental components of Tor's circuit establishment and termination process, where RELAY_END signals the conclusion of a data stream and RELAY_RESOLVED indicates the resolution of a hostname to an IP address. The vulnerability allows attackers to forge these messages with malicious intent, effectively bypassing the normal access control checks that should prevent unauthorized stream termination.
This vulnerability has significant operational impact on the Tor network and its users. Attackers who successfully exploit this flaw can potentially terminate streams prematurely, disrupt communication channels, or even perform man-in-the-middle attacks by manipulating the stream termination process. The ability to forge these specific relay messages undermines the fundamental security guarantees that Tor provides to its users, particularly those relying on the network for privacy and anonymity. The vulnerability affects the integrity of the Tor protocol's stream management system and can lead to denial of service conditions or data interception attacks.
The exploitation of this vulnerability aligns with several ATT&CK techniques including T1071.004 Application Layer Protocol and T1566.001 Phishing. From a CWE perspective, this issue maps to CWE-284 Improper Access Control, specifically in the context of network protocol implementation. The vulnerability demonstrates a classic case of insufficient input validation and message authentication within a security-critical protocol. Organizations and users should immediately upgrade to Tor version 0.4.6.5 or later to mitigate this risk, as the fix addresses the core issue in the relay message validation logic and implements proper authentication mechanisms for stream termination messages.
Additional mitigations include monitoring network traffic for suspicious relay message patterns and implementing network segmentation to reduce the attack surface. Security teams should also consider deploying intrusion detection systems that can identify anomalous relay message sequences that may indicate exploitation attempts. The vulnerability serves as a reminder of the critical importance of proper message validation and authentication in distributed security protocols, particularly those designed to protect user privacy and anonymity.