CVE-2019-20839 in LibVNCServer
Summary
by MITRE
libvncclient/sockets.c in LibVNCServer before 0.9.13 has a buffer overflow via a long socket filename.
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Analysis
by VulDB Data Team • 06/18/2020
The vulnerability identified as CVE-2019-20839 resides within the LibVNCServer library, specifically in the libvncclient/sockets.c component, and affects versions prior to 0.9.13. This represents a critical buffer overflow flaw that can be exploited through manipulation of socket filename parameters. The issue manifests when the library processes socket filenames that exceed predetermined buffer limits, creating potential avenues for arbitrary code execution or system compromise. The vulnerability is particularly concerning as it affects a widely used remote desktop protocol implementation that serves numerous applications and systems across various network environments.
The technical flaw stems from inadequate input validation and buffer management within the socket filename handling code path. When a socket filename exceeds the allocated buffer space, the library fails to properly bounds-check the input data, leading to memory corruption that can overwrite adjacent memory locations. This type of vulnerability falls under the CWE-121 buffer overflow category, specifically classified as a stack-based buffer overflow when the vulnerable code operates on stack-allocated buffers. The flaw represents a classic example of unsafe string handling where functions like strcpy or sprintf are used without proper size checking against the destination buffer capacity, creating a predictable memory corruption scenario that attackers can leverage.
The operational impact of this vulnerability extends significantly across environments utilizing LibVNCServer for remote desktop functionality. Systems that rely on VNC protocols for remote administration, desktop sharing, or virtual machine management become vulnerable to exploitation. Attackers could potentially execute malicious code with the privileges of the affected process, leading to complete system compromise or denial of service conditions. The vulnerability is particularly dangerous in enterprise environments where VNC servers are commonly deployed for remote IT support, as it could allow unauthorized access to sensitive corporate infrastructure. Network monitoring systems and security tools that depend on LibVNCServer components may also be at risk, potentially leading to broader security breaches within the affected network perimeter.
Mitigation strategies for CVE-2019-20839 should prioritize immediate patching of all affected systems to LibVNCServer version 0.9.13 or later, which includes the necessary buffer overflow protections. Organizations should conduct comprehensive inventory assessments to identify all systems utilizing vulnerable versions of the library, including embedded systems, virtual machines, and network appliances that may incorporate VNC functionality. Network segmentation and access controls should be enhanced to limit exposure of VNC services to trusted networks only, while implementing additional monitoring for suspicious socket connection patterns. Security teams should also consider deploying intrusion detection systems that can identify potential exploitation attempts targeting buffer overflow vulnerabilities. The remediation process should include thorough testing of patched environments to ensure that the update does not introduce compatibility issues with existing VNC implementations, and organizations should maintain updated vulnerability management processes to prevent similar issues in other components of their infrastructure stack.