CVE-2019-8260 in UltraVNC
Summary
by MITRE
UltraVNC revision 1199 has a out-of-bounds read vulnerability in VNC client RRE decoder code, caused by multiplication overflow. This attack appears to be exploitable via network connectivity. This vulnerability has been fixed in revision 1200.
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Analysis
by VulDB Data Team • 07/26/2023
The vulnerability identified as CVE-2019-8260 affects UltraVNC client software at revision 1199, specifically within the Remote Rendering Extension RRE decoder component. This issue represents a critical security flaw that could potentially allow remote attackers to execute arbitrary code or cause denial of service conditions. The vulnerability arises from improper input validation within the RRE decoding logic, which processes compressed screen updates sent from VNC servers to clients. The flaw manifests as an out-of-bounds read condition that occurs during the processing of malformed RRE data packets, creating a scenario where memory access violations can occur. The vulnerability has been classified under CWE-129 as an Improper Validation of Array Index, specifically related to insufficient bounds checking in array access operations.
The technical root cause of this vulnerability stems from a multiplication overflow condition within the RRE decoder's memory allocation calculations. When processing incoming RRE-encoded data, the client software performs mathematical operations to determine buffer sizes needed for decompression. The multiplication overflow occurs when the product of two integer values exceeds the maximum representable value for the data type, resulting in a wraparound behavior that produces an unexpectedly small or negative value. This malformed calculation leads to insufficient buffer allocation, causing subsequent memory reads to access invalid memory locations beyond the allocated bounds. The vulnerability is particularly concerning because it can be triggered through network connectivity, meaning attackers can exploit it remotely without requiring local system access or authentication.
The operational impact of CVE-2019-8260 extends beyond simple denial of service scenarios, as the out-of-bounds read condition creates opportunities for more sophisticated exploitation techniques. Remote attackers can craft malicious RRE data packets that, when processed by vulnerable UltraVNC clients, could lead to information disclosure, arbitrary code execution, or system crashes. The vulnerability affects any system running UltraVNC client software at revision 1199 that connects to potentially malicious VNC servers or intermediate network devices that could be compromised. This includes enterprise environments where VNC clients are commonly used for remote desktop access, making the impact particularly severe for organizations relying on this technology for system administration and remote access operations.
Mitigation strategies for this vulnerability should prioritize immediate software updates to revision 1200 or later, which contains the necessary patches to address the multiplication overflow and out-of-bounds read conditions. Organizations should implement network segmentation and access controls to limit exposure of VNC client systems to untrusted networks. Security monitoring should be enhanced to detect unusual RRE data patterns or connection attempts that might indicate exploitation attempts. The vulnerability aligns with ATT&CK technique T1071.004 for Application Layer Protocol: DNS and T1046 for Network Service Scanning, as attackers may attempt to identify vulnerable systems through network reconnaissance. Additionally, implementing network-based intrusion detection systems with signatures for known RRE attack patterns can provide early warning capabilities. System administrators should also consider disabling unnecessary VNC client functionality and implementing strict firewall rules that limit VNC client communication to trusted sources only.