CVE-2019-8265 in UltraVNC
Summary
by MITRE
UltraVNC revision 1207 has multiple out-of-bounds access vulnerabilities connected with improper usage of SETPIXELS macro in VNC client code, which can potentially result in code execution. This attack appears to be exploitable via network connectivity. These vulnerabilities have been fixed in revision 1208.
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Analysis
by VulDB Data Team • 05/28/2020
The CVE-2019-8265 vulnerability affects UltraVNC version 1207, specifically targeting the VNC client implementation where multiple out-of-bounds access conditions exist due to improper usage of the SETPIXELS macro. This vulnerability represents a critical security flaw that could enable remote code execution when exploited through network connectivity. The issue stems from insufficient bounds checking within the client-side code that processes graphical pixel data during remote desktop sessions. The SETPIXELS macro, which is designed to handle pixel manipulation operations, fails to validate input parameters properly, creating opportunities for attackers to craft malicious payloads that trigger buffer overflows or memory corruption conditions. These memory corruption vulnerabilities can be particularly dangerous as they may allow attackers to execute arbitrary code with the privileges of the affected VNC client process, potentially leading to full system compromise.
The technical exploitation of this vulnerability requires network connectivity and leverages the inherent trust model of VNC protocols where remote connections are established without sufficient input validation. When a remote attacker sends specially crafted pixel data to a vulnerable UltraVNC client, the improper macro usage causes memory corruption that can be leveraged to overwrite critical memory regions. This type of vulnerability aligns with CWE-125, which describes out-of-bounds read conditions, and CWE-787, which covers out-of-bounds write conditions. The attack vector is classified as network-based since the vulnerability can be triggered through remote connections without requiring local access. The exploitation process typically involves sending malformed pixel data that causes the client to attempt memory operations beyond allocated buffer boundaries, potentially leading to stack corruption or heap corruption depending on the specific implementation details of the affected code paths.
The operational impact of CVE-2019-8265 extends beyond simple denial of service scenarios as the potential for remote code execution makes this vulnerability particularly dangerous in enterprise environments where VNC is commonly deployed for remote administration. Organizations using UltraVNC for remote desktop access are at risk of unauthorized access, data exfiltration, and persistent backdoor installation if attackers successfully exploit these vulnerabilities. The vulnerability affects systems where UltraVNC clients are running and listening for remote connections, making it especially concerning for organizations that rely on remote desktop protocols for system administration. Attackers could potentially use this vulnerability to establish persistent access to target systems, escalate privileges, or deploy additional malware components through the compromised VNC client. The vulnerability's classification under the ATT&CK framework would fall under T1071.004 for application layer protocol usage and potentially T1059 for command and scripting interpreter execution if successful exploitation occurs.
The remediation for CVE-2019-8265 requires immediate upgrade to UltraVNC revision 1208 or later, which includes patches that address the improper macro usage and implement proper bounds checking for pixel data operations. Organizations should also implement network segmentation and access controls to limit exposure of VNC services to untrusted networks, as the vulnerability can be exploited remotely without authentication. Additional security measures include monitoring for unusual network traffic patterns related to VNC connections, implementing network intrusion detection systems to identify potential exploitation attempts, and conducting regular security assessments of remote desktop implementations. Security teams should also consider disabling unnecessary VNC services and implementing alternative secure remote access solutions such as VPN-based connections or zero-trust network access protocols. The vulnerability highlights the importance of proper input validation and bounds checking in graphics processing code, particularly in remote desktop software where graphical data manipulation is a core function. Organizations should also review their patch management processes to ensure timely deployment of security updates and maintain inventory of all VNC installations across their networks to prevent similar vulnerabilities from remaining unpatched.