CVE-2018-0491 in Tor
Summary
by MITRE
A use-after-free issue was discovered in Tor 0.3.2.x before 0.3.2.10. It allows remote attackers to cause a denial of service (relay crash) because the KIST implementation allows a channel to be added more than once in the pending list.
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Analysis
by VulDB Data Team • 06/25/2024
The vulnerability identified as CVE-2018-0491 represents a critical use-after-free flaw within the Tor network's relay implementation that was present in versions 0.3.2.x prior to 0.3.2.10. This issue stems from a fundamental design flaw in how the KIST (Keep-Alive Interval Scheduling Table) component manages channel connections within the Tor relay architecture. The vulnerability manifests when the system allows a channel to be added multiple times to a pending list structure, creating a scenario where memory references become invalid after the initial allocation is freed.
The technical exploitation of this vulnerability occurs through a specific race condition in the channel management subsystem where concurrent operations permit duplicate entries in the pending channel list. When the system processes these duplicate entries, it attempts to free memory associated with the channel object while simultaneously referencing it in subsequent operations. This creates a use-after-free condition that can be reliably triggered by remote attackers through carefully crafted network communications designed to manipulate the channel addition process. The flaw operates at the core networking layer of Tor relays, specifically within the channel establishment and maintenance protocols that govern how relays communicate with each other in the Tor network.
The operational impact of this vulnerability extends beyond simple denial of service, as it can be leveraged to cause complete relay crashes that disrupt the Tor network's integrity and availability. When a Tor relay experiences this crash, it removes itself from the network's routing infrastructure, potentially breaking circuits and affecting the anonymity services that depend on stable relay connections. The vulnerability affects the fundamental reliability of the Tor network's distributed architecture, as compromised relays can no longer participate in the routing of anonymous traffic. This makes the vulnerability particularly dangerous in the context of the Tor network's security model, where maintaining a stable and trustworthy relay infrastructure is paramount for user anonymity and network resilience.
Mitigation strategies for CVE-2018-0491 require immediate patch deployment to all affected Tor relay instances, updating to version 0.3.2.10 or later where the KIST implementation has been corrected to prevent duplicate channel additions in pending lists. System administrators should implement monitoring protocols to detect potential exploitation attempts and establish automated alerting for relay crashes. The fix addresses the root cause by introducing proper synchronization mechanisms and validation checks that ensure channels are only added once to pending lists, preventing the race condition that leads to memory corruption. From a cybersecurity perspective, this vulnerability aligns with CWE-416, which describes the use of freed memory condition, and represents a typical example of how improper resource management can lead to denial of service attacks in network infrastructure components. The remediation process also includes implementing network segmentation and access controls to limit the potential impact of compromised relays within the broader Tor ecosystem.