CVE-2019-8955 in Tor
Summary
by MITRE
In Tor before 0.3.3.12, 0.3.4.x before 0.3.4.11, 0.3.5.x before 0.3.5.8, and 0.4.x before 0.4.0.2-alpha, remote denial of service against Tor clients and relays can occur via memory exhaustion in the KIST cell scheduler.
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Analysis
by VulDB Data Team • 07/19/2023
The vulnerability identified as CVE-2019-8955 represents a critical denial of service weakness affecting multiple versions of the Tor anonymization network. This flaw specifically targets the KIST cell scheduler component within Tor's implementation, creating a condition where malicious actors can trigger memory exhaustion attacks against both client and relay nodes. The vulnerability impacts Tor versions prior to 0.3.3.12, 0.3.4.11, 0.3.5.8, and 0.4.0.2-alpha, making it a widespread concern across the Tor ecosystem. The KIST cell scheduler is responsible for managing the flow control of data cells within the Tor network's communication protocol, and its memory management flaw creates an exploitable condition that can be leveraged by remote attackers.
The technical exploitation of this vulnerability occurs through carefully crafted malicious traffic that causes the KIST scheduler to consume excessive memory resources. When Tor clients or relays process maliciously formatted KIST cells, the scheduler fails to properly limit memory allocation, leading to progressive memory exhaustion. This memory consumption pattern can cause the affected Tor nodes to become unresponsive, effectively disrupting their ability to route traffic through the network. The vulnerability is particularly dangerous because it can be exploited remotely without requiring authentication or special privileges, making it accessible to any attacker who can send traffic to the target Tor node. The flaw demonstrates a classic memory exhaustion attack pattern that aligns with CWE-400, which catalogs weaknesses related to resource exhaustion in software systems.
The operational impact of CVE-2019-8955 extends beyond individual node failures to potentially compromise the overall integrity and availability of the Tor network. When multiple nodes become compromised through memory exhaustion, it creates cascading failures that can degrade network performance and reduce the anonymity set available to users. The vulnerability affects both client and relay implementations, meaning that attackers can target either endpoint in the network, making it particularly challenging to defend against. From an adversary perspective, this vulnerability provides a straightforward method for conducting distributed denial of service attacks against Tor infrastructure, which could severely impact the network's ability to provide anonymous communication services. The attack vector operates at the network protocol level, making it difficult to detect and mitigate through traditional network security measures.
Mitigation strategies for CVE-2019-8955 primarily focus on upgrading to patched versions of Tor software, which contain improved memory management within the KIST scheduler component. System administrators should prioritize updating their Tor installations to versions 0.3.3.12, 0.3.4.11, 0.3.5.8, or 0.4.0.2-alpha respectively, depending on their current version. Network monitoring should include detection of unusual memory consumption patterns that could indicate exploitation attempts. The vulnerability's classification under ATT&CK technique T1499.004, which covers network denial of service, indicates that defensive measures should incorporate both preventive updates and reactive monitoring capabilities. Organizations using Tor services should also consider implementing rate limiting and traffic analysis to identify and block malicious KIST cell patterns before they can cause memory exhaustion. The fix implemented in patched versions typically involves stricter memory allocation limits and improved input validation for KIST cell processing, addressing the root cause of the memory exhaustion condition that made the vulnerability exploitable.