CVE-2018-20020 in LibVNC
Summary
by MITRE
LibVNC before commit 7b1ef0ffc4815cab9a96c7278394152bdc89dc4d contains heap out-of-bound write vulnerability inside structure in VNC client code that can result remote code execution
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Analysis
by VulDB Data Team • 06/19/2023
The vulnerability identified as CVE-2018-20020 represents a critical heap out-of-bounds write flaw within the LibVNC library's client-side implementation. This security defect exists in versions prior to commit 7b1ef0ffc4815cab9a96c7278394152bdc89dc4d and specifically targets the VNC client code structure handling mechanisms. The issue manifests when the client processes malformed VNC server responses, creating a condition where memory operations exceed allocated buffer boundaries. Such vulnerabilities are particularly dangerous in networked environments where untrusted remote entities can trigger exploitable conditions through crafted protocol messages.
The technical root cause of this vulnerability stems from inadequate input validation and memory management within the VNC client's packet processing routines. When the client receives certain malformed data structures from a VNC server, it fails to properly bounds-check array accesses or validate the size of incoming data before writing to heap-allocated memory regions. This flaw directly maps to CWE-787, which describes out-of-bounds write vulnerabilities that occur when a program writes data past the end of a buffer, and can be classified under CWE-129 as an insufficient input validation issue. The vulnerability operates at the intersection of memory safety and network protocol parsing, where the client's failure to validate data integrity leads to arbitrary memory corruption.
The operational impact of this vulnerability extends beyond simple denial-of-service scenarios, as it can enable remote code execution when exploited successfully. An attacker positioned on the network path between a vulnerable VNC client and server can craft malicious VNC responses that trigger the heap overflow condition, potentially allowing for arbitrary code execution with the privileges of the affected application. This threat model aligns with ATT&CK technique T1203, which covers exploitation of remote services, and T1059, covering command and scripting interpreters. The vulnerability affects any system running a vulnerable version of LibVNC, making it particularly concerning for enterprise environments where VNC clients are commonly deployed for remote administration and support operations.
Mitigation strategies for CVE-2018-20020 require immediate implementation of the patched LibVNC version containing the commit 7b1ef0ffc4815cab9a96c7278394152bdc89dc4d fix. Organizations should conduct comprehensive inventory assessments to identify all systems utilizing vulnerable LibVNC implementations, including desktop environments, remote access tools, and network monitoring applications. Network segmentation and access controls should be enforced to limit exposure of VNC clients to untrusted networks, while implementing intrusion detection systems to monitor for suspicious VNC protocol traffic patterns. Additionally, security teams should consider disabling VNC functionality where possible and implementing alternative secure remote access solutions such as SSH-based tunneling or enterprise-grade remote desktop protocols that have better security track records and more frequent security updates.