CVE-2018-20748 in LibVNC
Summary
by MITRE
LibVNC before 0.9.12 contains multiple heap out-of-bounds write vulnerabilities in libvncclient/rfbproto.c. The fix for CVE-2018-20019 was incomplete.
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Analysis
by VulDB Data Team • 07/04/2023
The vulnerability identified as CVE-2018-20748 affects LibVNC versions prior to 0.9.12 and represents a critical heap out-of-bounds write flaw within the libvncclient/rfbproto.c component. This issue arises from an incomplete fix for a previously discovered vulnerability CVE-2018-20019, creating a persistent security weakness that can be exploited by remote attackers to potentially execute arbitrary code or cause denial of service conditions. The vulnerability specifically impacts the Remote Framebuffer protocol implementation that LibVNC uses for network communication, making it particularly dangerous in environments where remote desktop access is utilized.
The technical flaw manifests through improper bounds checking during the processing of network packets received by the VNC client library. When handling certain malformed data structures in the RFB protocol, the code fails to validate array indices before writing data to heap memory locations, resulting in out-of-bounds memory corruption. This memory corruption can be leveraged to overwrite adjacent memory regions, potentially leading to arbitrary code execution or application crashes. The vulnerability is classified under CWE-787, which specifically addresses out-of-bounds write conditions in software implementations. The incomplete nature of the fix for CVE-2018-20019 suggests that developers may have addressed some but not all potential attack vectors, leaving residual weaknesses in the codebase.
The operational impact of this vulnerability extends across various systems that utilize LibVNC for remote desktop functionality, including enterprise environments, cloud infrastructure, and desktop management solutions. Attackers can exploit this vulnerability by establishing a malicious VNC connection to a vulnerable system, sending specially crafted packets that trigger the out-of-bounds write condition. The attack surface is particularly broad given that VNC remains a widely deployed remote access protocol in both corporate and public environments. Organizations using vulnerable versions of LibVNC are at risk of unauthorized access, data breaches, and system compromise, with potential lateral movement within networks where VNC services are exposed to untrusted networks.
Mitigation strategies for CVE-2018-20748 primarily focus on upgrading to LibVNC version 0.9.12 or later, which contains the complete fix for both the original vulnerability and the incomplete patch. System administrators should also implement network segmentation and access controls to limit exposure of VNC services to trusted networks only. Additional protective measures include disabling unnecessary VNC services, implementing strong authentication mechanisms, and monitoring network traffic for suspicious VNC protocol activity. From an ATT&CK framework perspective, this vulnerability maps to techniques involving remote service exploitation and privilege escalation, with potential for lateral movement through compromised VNC sessions. Organizations should also consider implementing intrusion detection systems that can identify anomalous VNC traffic patterns and conduct regular security assessments of remote access implementations to prevent exploitation of similar vulnerabilities in the future.