CVE-2019-8276 in UltraVNC
Summary
by MITRE
UltraVNC revision 1211 has a stack buffer overflow vulnerability in VNC server code inside file transfer request handler, which can result in Denial of Service (DoS). This attack appears to be exploitable via network connectivity. This vulnerability has been fixed in revision 1212.
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Analysis
by VulDB Data Team • 05/29/2020
The vulnerability identified as CVE-2019-8276 represents a critical stack buffer overflow flaw within the UltraVNC remote desktop software version 1211. This security weakness resides in the VNC server component's file transfer request handler, specifically within the file transfer functionality that enables users to transfer files between remote systems. The flaw manifests when the server processes incoming file transfer requests from connected clients, creating a potential avenue for attackers to exploit the software's memory management mechanisms.
The technical implementation of this vulnerability stems from inadequate bounds checking within the file transfer request handler code. When processing file transfer operations, the VNC server fails to properly validate the size or length of incoming data buffers, allowing maliciously crafted input to overflow the allocated stack memory space. This buffer overflow condition occurs during the handling of file transfer requests, where the software does not enforce proper input validation on the size parameters or data structures used in the file transfer protocol. The flaw specifically affects the server-side processing of file transfer operations, making it exploitable through network connectivity as the attack vector requires no local access to the system.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it represents a potential pathway for more sophisticated attacks. While the immediate effect is a denial of service that can cause the VNC server to crash or become unresponsive, the underlying buffer overflow presents a broader security risk that could potentially be leveraged for arbitrary code execution depending on the system configuration and memory layout. Attackers with network access to vulnerable UltraVNC servers could exploit this weakness to disrupt services, causing availability issues for legitimate users who rely on the remote desktop functionality. The vulnerability affects systems running UltraVNC revision 1211 where file transfer capabilities are enabled, making it particularly concerning for organizations that utilize this remote access software for system administration tasks.
The remediation for this vulnerability was implemented in UltraVNC revision 1212, where developers addressed the buffer overflow by implementing proper bounds checking mechanisms within the file transfer request handler. This fix ensures that all incoming data is properly validated against predetermined size limits before being processed, preventing the overflow condition from occurring. Organizations should prioritize updating to revision 1212 or later to mitigate this vulnerability, as the fix directly addresses the root cause of the stack buffer overflow. Security practitioners should also consider implementing network segmentation and access controls to limit exposure of UltraVNC services to untrusted networks, while monitoring for potential exploitation attempts. This vulnerability aligns with CWE-121 Stack-based Buffer Overflow, which specifically addresses buffer overflows occurring in stack memory, and may be categorized under ATT&CK technique T1210 for exploitation of remote services. The vulnerability demonstrates the importance of proper input validation in network-facing applications and highlights the potential for seemingly benign functionality to become attack vectors when proper security controls are not implemented.