CVE-2021-28089 in Tor
Summary
by MITRE • 03/19/2021
Tor before 0.4.5.7 allows a remote participant in the Tor directory protocol to exhaust CPU resources on a target, aka TROVE-2021-001.
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Analysis
by VulDB Data Team • 04/03/2021
The vulnerability identified as CVE-2021-28089 represents a critical resource exhaustion flaw within the Tor network's directory protocol implementation. This vulnerability affects Tor versions prior to 0.4.5.7 and allows remote attackers to consume excessive CPU resources on target Tor nodes, potentially leading to service disruption and network degradation. The issue stems from inadequate validation and handling of directory protocol messages that control how Tor nodes communicate and share network information.
The technical flaw manifests through the directory protocol's insufficient input validation mechanisms that process relay descriptors and other network information. When a malicious participant sends specially crafted directory protocol messages, the target Tor node processes these messages in a manner that consumes disproportionate CPU cycles. The vulnerability operates at the network protocol level where legitimate Tor directory services are designed to handle large volumes of data but fail to properly rate limit or validate incoming requests. This creates a scenario where an attacker can exploit the protocol's design to cause sustained CPU resource exhaustion without requiring authentication or privileged access.
The operational impact of this vulnerability extends beyond simple denial of service, as it can compromise the overall stability and performance of the Tor network. Targeted attacks can cause specific Tor relay nodes to become unresponsive or significantly degraded, affecting the network's ability to route traffic effectively. The vulnerability affects all Tor nodes that participate in the directory protocol, making it particularly dangerous for relay operators who may not immediately detect the resource exhaustion attacks. Network operators and users experience degraded performance as compromised nodes consume resources that should be available for legitimate network operations.
Mitigation strategies for CVE-2021-28089 require immediate patching of Tor software to version 0.4.5.7 or later, which includes proper input validation and rate limiting mechanisms for directory protocol messages. Network administrators should implement monitoring systems to detect unusual CPU usage patterns that may indicate exploitation attempts. The vulnerability aligns with CWE-400, which covers unrestricted resource consumption, and maps to ATT&CK technique T1499.004 for network denial of service attacks. Additional defensive measures include implementing network-level rate limiting, deploying intrusion detection systems, and configuring Tor nodes with appropriate resource constraints to prevent single points of failure. Organizations should also consider implementing automated alerting for unusual directory protocol activity and regularly review their Tor node configurations to ensure compliance with security best practices.