CVE-2019-8270 in UltraVNC
Summary
by MITRE
UltraVNC revision 1210 has out-of-bounds read vulnerability in VNC client code inside Ultra decoder, which results in a denial of service (DoS) condition. This attack appear to be exploitable via network connectivity. This vulnerability has been fixed in revision 1211.
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Analysis
by VulDB Data Team • 05/28/2020
The vulnerability identified as CVE-2019-8270 represents a critical out-of-bounds read flaw within the UltraVNC client implementation that specifically affects revision 1210. This issue manifests in the ultra decoder component of the VNC client code, where insufficient input validation allows maliciously crafted data to trigger memory access violations. The vulnerability exists at the boundary between legitimate data processing and memory access operations, creating a scenario where the decoder fails to properly bounds-check data received from network connections. This flaw directly impacts the stability and availability of the VNC client application, as it can be triggered through network-based attacks without requiring any special privileges or authentication. The vulnerability has been classified under CWE-129, which specifically addresses insufficient bounds checking, a fundamental weakness that allows attackers to access memory locations outside the intended buffer boundaries.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it represents a potential entry point for more sophisticated attacks that could leverage the memory corruption to execute arbitrary code or escalate privileges. The fact that exploitation requires only network connectivity makes this vulnerability particularly dangerous in environments where VNC clients are exposed to untrusted networks or where users might inadvertently connect to malicious VNC servers. Attackers can craft specially formatted VNC protocol messages that, when processed by the vulnerable UltraVNC client, cause the application to read memory beyond its allocated bounds, resulting in application crashes or system instability. This vulnerability demonstrates the critical importance of proper input validation and memory management in remote desktop protocol implementations, where network-based attacks can have significant operational consequences.
Organizations utilizing UltraVNC revision 1210 should immediately implement mitigation strategies including network segmentation to prevent unauthorized access to VNC client systems, mandatory patching to upgrade to revision 1211 or later versions, and network monitoring to detect potential exploitation attempts. The vulnerability aligns with ATT&CK technique T1071.004, which covers application layer protocol traffic, and represents a classic example of how protocol parsing flaws can be exploited to create denial of service conditions. Security teams should also consider implementing network-based intrusion detection systems that can identify suspicious VNC protocol traffic patterns and establish secure remote access policies that limit exposure to potentially malicious network connections. The patch released in revision 1211 addresses the core issue by implementing proper bounds checking in the ultra decoder component, ensuring that all input data is validated before memory access operations are performed. This fix demonstrates the importance of maintaining up-to-date security patches and the potential for seemingly minor protocol implementation flaws to create significant operational risks in enterprise environments.