CVE-2019-8280 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 execution. This attack appear to be exploitable via network connectivity. This vulnerability has been fixed in revision 1204.
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Analysis
by VulDB Data Team • 05/29/2020
The vulnerability identified as CVE-2019-8280 represents a critical out-of-bounds memory access flaw within the UltraVNC remote desktop software client. This security weakness specifically affects revision 1203 of the UltraVNC suite and resides within the RAW decoder component of the VNC client implementation. The vulnerability manifests as a buffer overflow condition that occurs when processing malformed data packets during the decoding process, creating a potential pathway for remote code execution. The flaw is particularly concerning because it can be exploited through network connectivity, meaning an attacker does not require local system access to attempt exploitation. This makes the vulnerability highly relevant in environments where remote desktop services are exposed to untrusted networks or where users might inadvertently connect to malicious VNC servers.
The technical implementation of this vulnerability stems from inadequate input validation within the RAW decoder module of the UltraVNC client. When the client receives VNC data streams containing specially crafted malformed packets, the decoder fails to properly bounds-check array accesses or validate data lengths before processing. This allows an attacker to manipulate memory layout by writing beyond allocated buffer boundaries, potentially overwriting critical program structures or executable code. The vulnerability aligns with CWE-129, which addresses improper validation of array indices, and CWE-787, which covers out-of-bounds write vulnerabilities. From an operational perspective, this flaw creates a significant risk for organizations relying on VNC for remote administration, as it could enable attackers to execute arbitrary code on target systems with the privileges of the VNC client process.
The exploitation potential of CVE-2019-8280 is particularly dangerous in enterprise environments where remote desktop protocols are commonly used for system administration and support operations. Attackers could leverage this vulnerability to gain unauthorized access to systems, escalate privileges, or establish persistent backdoors through the VNC client. The remote exploitability means that threat actors could target vulnerable systems from outside the organization's network perimeter, making traditional network segmentation measures insufficient for protection. Organizations using UltraVNC versions prior to 1204 face significant risk of compromise, especially in scenarios where the software is used in unsecured network environments or where users connect to untrusted VNC servers. The vulnerability also aligns with ATT&CK technique T1071.004, which covers protocol tunneling through VNC connections, potentially enabling attackers to use compromised VNC clients as pivot points for further network reconnaissance and lateral movement.
The remediation for this vulnerability is straightforward and involves upgrading to UltraVNC revision 1204 or later, which includes proper bounds checking and input validation within the RAW decoder module. Security administrators should prioritize this update across all systems running vulnerable versions of UltraVNC, particularly those used for remote administration or system support. Additional mitigations may include network segmentation to limit VNC client exposure, implementing firewall rules that restrict VNC connections to trusted IP addresses, and monitoring network traffic for suspicious VNC protocol activity. Organizations should also consider implementing network-based intrusion detection systems to identify potential exploitation attempts targeting this vulnerability. The fix demonstrates proper software engineering practices by addressing the root cause through improved input validation rather than attempting to patch the effects of the vulnerability after exploitation has occurred, which aligns with industry best practices for secure software development lifecycle implementation.