CVE-2019-8271 in UltraVNC
Summary
by MITRE
UltraVNC revision 1211 has a heap buffer overflow vulnerability in VNC server code inside file transfer handler, which can potentially result code execution. 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 • 05/28/2020
The vulnerability identified as CVE-2019-8271 represents a critical heap buffer overflow flaw within the UltraVNC remote desktop software version 1211. This vulnerability specifically resides within the VNC server code's file transfer handler component, making it particularly dangerous as it targets the core functionality that enables remote file operations. The flaw exists in the way the software processes file transfer requests, where insufficient bounds checking allows an attacker to manipulate memory allocation through crafted input data. The vulnerability affects the file transfer functionality that is integral to UltraVNC's remote desktop capabilities, potentially allowing malicious actors to execute arbitrary code on the target system.
The technical nature of this vulnerability places it under CWE-121, which describes heap-based buffer overflow conditions where insufficient bounds checking allows attackers to write past the end of allocated buffer space. The attack vector requires only network connectivity, making it particularly severe as it can be exploited remotely without requiring physical access to the target system. The vulnerability's exploitation potential stems from the fact that the file transfer handler processes data from network connections without adequate input validation, creating opportunities for attackers to craft malicious payloads that exceed buffer boundaries and overwrite adjacent memory locations. This memory corruption can lead to arbitrary code execution, allowing attackers to gain full control over the affected system.
The operational impact of this vulnerability extends beyond simple remote code execution, as it can enable attackers to establish persistent access to systems running vulnerable versions of UltraVNC. The file transfer handler component is commonly used in legitimate remote administration scenarios, making this attack surface particularly attractive to threat actors who may leverage it for reconnaissance, privilege escalation, or lateral movement within networks. Organizations using UltraVNC for remote desktop services face significant risk exposure, particularly in environments where the software is deployed without proper network segmentation or additional security controls. The vulnerability's exploitation requires minimal prerequisites, making it an attractive target for both automated attacks and targeted campaigns, especially when combined with other attack techniques from the MITRE ATT&CK framework that focus on remote service exploitation and privilege escalation.
The remediation for this vulnerability involves upgrading to UltraVNC revision 1212 or later, which includes proper bounds checking and memory management fixes within the file transfer handler component. Security practitioners should implement network segmentation to limit access to UltraVNC services, disable unnecessary file transfer functionality when possible, and monitor network traffic for anomalous file transfer activities. Additionally, organizations should consider implementing network-based intrusion detection systems that can identify potential exploitation attempts targeting this specific vulnerability pattern. The fix addresses the root cause by implementing proper input validation and memory boundary checks within the affected code path, ensuring that file transfer operations cannot result in heap corruption. Organizations should also conduct comprehensive vulnerability assessments to identify all systems running vulnerable versions of UltraVNC and ensure proper patch management processes are in place to maintain system security.