CVE-2019-20840 in LibVNCServer
Summary
by MITRE
An issue was discovered in LibVNCServer before 0.9.13. libvncserver/ws_decode.c can lead to a crash because of unaligned accesses in hybiReadAndDecode.
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Analysis
by VulDB Data Team • 06/18/2020
The vulnerability identified as CVE-2019-20840 represents a critical memory access issue within LibVNCServer version 0.9.12 and earlier. This flaw exists in the websockets implementation of the VNC server software, specifically within the ws_decode.c file where the hybiReadAndDecode function processes incoming websocket data. The root cause stems from improper handling of unaligned memory accesses that occur during the decoding process of websocket frames, creating a potential denial of service condition that can be exploited by remote attackers.
The technical implementation of this vulnerability manifests when the hybiReadAndDecode function attempts to read and decode websocket data without proper alignment checking. This function processes websocket frames that follow the hybi protocol specification, which governs the encoding and decoding of websocket data streams. When malformed or specially crafted websocket data arrives at the server, the function fails to properly validate memory access patterns, leading to unaligned memory reads that can cause segmentation faults or crashes in the application. This issue directly maps to CWE-121, which describes stack-based buffer overflow conditions, and CWE-122, which covers heap-based buffer overflow scenarios, though the specific manifestation here involves memory alignment rather than direct buffer overflows.
The operational impact of CVE-2019-20840 extends beyond simple service disruption to potentially enable more sophisticated attack vectors. Remote attackers can exploit this vulnerability by establishing websocket connections to vulnerable LibVNCServer instances and sending malformed websocket frames designed to trigger the unaligned memory access conditions. This can result in immediate service termination, requiring system administrators to restart the VNC server processes and potentially disrupting legitimate user sessions. The vulnerability affects any system running vulnerable versions of LibVNCServer, including desktop environments, remote management systems, and virtualization platforms that rely on VNC protocols for remote access. From an adversarial perspective, this vulnerability aligns with ATT&CK technique T1210, which involves exploitation of remote services through memory corruption, and T1499, which covers network denial of service attacks.
Mitigation strategies for CVE-2019-20840 primarily focus on upgrading to LibVNCServer version 0.9.13 or later, which includes patches addressing the unaligned memory access issue. System administrators should implement comprehensive patch management procedures to ensure all vulnerable systems are updated promptly. Additionally, network-level firewalls can be configured to restrict access to VNC server ports, limiting exposure to potential attackers. Monitoring systems should be enhanced to detect unusual websocket traffic patterns that might indicate exploitation attempts. Organizations should also consider implementing intrusion detection systems that can identify and alert on malformed websocket frames attempting to trigger memory access violations. The fix implemented in version 0.9.13 involves proper memory alignment checks within the websocket decoding functions, ensuring that all memory accesses are properly aligned before processing websocket data frames.