CVE-2016-9942 in LibVNCServer
Summary
by MITRE
Heap-based buffer overflow in ultra.c in LibVNCClient in LibVNCServer before 0.9.11 allows remote servers to cause a denial of service (application crash) or possibly execute arbitrary code via a crafted FramebufferUpdate message with the Ultra type tile, such that the LZO payload decompressed length exceeds what is specified by the tile dimensions.
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Analysis
by VulDB Data Team • 10/27/2022
The vulnerability identified as CVE-2016-9942 represents a critical heap-based buffer overflow within the LibVNCServer library's LibVNCClient component. This flaw exists in versions prior to 0.9.11 and specifically affects the processing of FramebufferUpdate messages with Ultra type tiles. The vulnerability stems from inadequate input validation during the decompression of LZO payloads, creating a scenario where remote attackers can manipulate tile dimensions to trigger memory corruption. The issue occurs when the decompressed payload length exceeds the bounds specified by the tile dimensions, leading to unauthorized memory access patterns that can result in application instability or potential code execution.
The technical exploitation of this vulnerability involves crafting a malicious FramebufferUpdate message that contains an Ultra type tile with manipulated dimensions. When the LibVNCClient processes this malformed message, the decompression routine fails to properly validate that the decompressed data fits within the allocated memory buffer specified by the tile dimensions. This mismatch creates a classic heap overflow condition where data written beyond the allocated buffer boundaries can overwrite adjacent memory structures, potentially corrupting critical program state or executing arbitrary code. The vulnerability is particularly dangerous because it can be triggered remotely through network communication, making it accessible to attackers without physical access to the target system.
From an operational perspective, this vulnerability poses significant risks to systems utilizing LibVNCServer for remote desktop access and management. The potential for denial of service attacks means that legitimate users could be denied access to critical systems, while the possibility of arbitrary code execution creates opportunities for full system compromise. Organizations relying on VNC-based remote access solutions are particularly vulnerable, as attackers can exploit this flaw to gain unauthorized access to networked systems. The vulnerability affects both client and server implementations, making it a comprehensive threat across the entire VNC communication stack. Security incidents involving this vulnerability could result in extended downtime, data breaches, or complete system compromise depending on the attacker's objectives and the target system's security posture.
Mitigation strategies for CVE-2016-9942 primarily involve upgrading to LibVNCServer version 0.9.11 or later, which includes proper input validation and bounds checking for decompressed LZO payloads. System administrators should also implement network segmentation and access controls to limit exposure to potentially malicious VNC servers, particularly in environments where untrusted remote connections are permitted. Additional protective measures include monitoring network traffic for suspicious FramebufferUpdate messages and implementing intrusion detection systems that can identify malformed VNC protocol communications. Organizations should conduct thorough vulnerability assessments to identify all systems utilizing affected versions of LibVNCServer and establish patch management procedures to ensure timely remediation. The vulnerability aligns with CWE-121, heap-based buffer overflow, and represents a potential ATT&CK technique involving remote code execution through protocol manipulation.