CVE-2019-8263 in UltraVNC
Summary
by MITRE
UltraVNC revision 1203 has out-of-bounds access vulnerability in VNC client inside RAW decoder, which can potentially result code in execution. This attack appears to be exploitable via network connectivity. This vulnerability has been fixed in revision 1204.
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Analysis
by VulDB Data Team • 07/26/2023
The CVE-2019-8263 vulnerability represents a critical out-of-bounds memory access flaw within the UltraVNC remote desktop software client, specifically within its RAW decoder component. This vulnerability exists in revision 1203 and demonstrates a classic buffer over-read condition that can be exploited to execute arbitrary code on affected systems. The flaw manifests during the processing of VNC client data streams where the RAW decoder fails to properly validate input boundaries before accessing memory locations, creating a potential attack vector that can be leveraged by remote adversaries.
This vulnerability operates through the VNC protocol's RAW encoding mechanism which is used to transmit screen updates and user input data between the client and server components. When the VNC client processes malformed or specially crafted RAW encoded data packets, the decoder routine attempts to access memory regions beyond the allocated buffer boundaries. This memory corruption can potentially be exploited to overwrite critical program structures or execute malicious code with the privileges of the running VNC client process. The attack surface is particularly concerning as it requires only network connectivity to exploit, making it accessible to remote attackers without requiring physical access or local credentials.
The operational impact of this vulnerability extends beyond simple code execution, as it can enable full system compromise when exploited successfully. Attackers can leverage this flaw to establish persistent access, escalate privileges, or deploy additional malicious payloads within the target environment. The vulnerability's classification as a remote code execution issue aligns with ATT&CK technique T1059.007 for command and scripting interpreter and T1071.004 for application layer protocols, as it enables attackers to execute arbitrary commands through the compromised VNC client. The affected system typically runs with the privileges of the user who initiated the VNC session, potentially allowing for privilege escalation if the session runs with elevated permissions.
Security practitioners should prioritize immediate patching of all affected UltraVNC installations to address this vulnerability. The fix implemented in revision 1204 includes proper input validation and boundary checking within the RAW decoder component, preventing the out-of-bounds memory access condition. Organizations should conduct comprehensive vulnerability assessments to identify all systems running vulnerable versions of UltraVNC and implement network segmentation to limit exposure. The vulnerability's presence in a widely used remote desktop solution makes it particularly attractive to threat actors, as it can be used to establish backdoors in enterprise networks or target individual user workstations. Additional mitigations include implementing network access controls, monitoring for unusual VNC traffic patterns, and maintaining up-to-date threat intelligence regarding exploitation attempts. This vulnerability demonstrates the critical importance of input validation in network protocols and serves as a reminder of the potential security implications of remote desktop software in enterprise environments. The flaw's classification aligns with CWE-125, which describes out-of-bounds read conditions, and represents a common vector for privilege escalation attacks in remote desktop environments.