CVE-2018-6307 in LibVNC
Summary
by MITRE
LibVNC before commit ca2a5ac02fbbadd0a21fabba779c1ea69173d10b contains heap use-after-free vulnerability in server code of file transfer extension that can result remote code execution.
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Analysis
by VulDB Data Team • 06/20/2023
The CVE-2018-6307 vulnerability represents a critical heap use-after-free flaw within the LibVNC library's server implementation, specifically affecting the file transfer extension functionality. This vulnerability exists in LibVNC versions prior to commit ca2a5ac02fbbadd0a21fabba779c1ea69173d10b, making it a significant concern for systems relying on VNC (Virtual Network Computing) server implementations. The flaw manifests in the server code's handling of file transfer operations, where improper memory management leads to conditions that could be exploited by remote attackers. The vulnerability's severity stems from its potential to enable remote code execution, which represents one of the most dangerous attack vectors in networked applications. This type of memory corruption vulnerability is particularly dangerous because it can allow attackers to execute arbitrary code on the target system with the privileges of the VNC server process, potentially leading to complete system compromise.
The technical root cause of this vulnerability lies in improper memory deallocation and subsequent usage within the file transfer extension's server-side code. When the VNC server processes file transfer requests, it manages memory allocations for handling file data and associated metadata. The use-after-free condition occurs when the application frees memory that is still referenced or when memory is accessed after it has been deallocated. This specific flaw in the file transfer extension suggests that the server code fails to properly track memory references during file transfer operations, particularly when handling multiple concurrent file transfers or when processing malformed file transfer requests. The vulnerability is classified under CWE-416, which specifically addresses use-after-free conditions, and represents a classic example of memory safety issues that plague C and C++ based applications. The flaw demonstrates poor memory management practices where the application does not properly validate the lifecycle of allocated memory blocks during file transfer operations.
The operational impact of CVE-2018-6307 extends beyond simple remote code execution, as it can enable attackers to gain persistent access to systems running vulnerable LibVNC implementations. Attackers can exploit this vulnerability by establishing a connection to the VNC server and initiating a file transfer operation that triggers the memory corruption condition. The remote code execution capability allows adversaries to execute arbitrary commands on the target system, potentially leading to privilege escalation, data exfiltration, or further network infiltration. This vulnerability is particularly concerning for enterprise environments where VNC servers are commonly used for remote administration, as it can be exploited without requiring authentication in many configurations. The attack surface includes any system running LibVNC server software with the file transfer extension enabled, making it a widespread concern across various operating systems and network environments. The vulnerability's exploitation aligns with ATT&CK technique T1059, which covers command and scripting interpreter usage, and T1078, which addresses valid accounts and legitimate credentials usage, as attackers can leverage the executed code to establish persistent access.
Mitigation strategies for CVE-2018-6307 require immediate action to update LibVNC implementations to versions containing the fix committed at ca2a5ac02fbbadd0a21fabba779c1ea69173d10b. Organizations should prioritize patching all systems running vulnerable LibVNC versions, particularly those with exposed VNC server instances on public networks. Network segmentation and firewall rules should be implemented to restrict access to VNC server ports, limiting exposure to authorized users only. Additional defensive measures include disabling the file transfer extension if it is not required for operations, implementing intrusion detection systems to monitor for suspicious VNC traffic patterns, and conducting thorough network scans to identify all vulnerable systems. Security teams should also consider implementing application whitelisting policies to prevent execution of unauthorized code and establish monitoring procedures for unusual command execution patterns. The vulnerability highlights the importance of regular security updates and proper memory management practices in networked server applications, emphasizing that even seemingly minor functionality like file transfer extensions can contain critical security flaws that can be exploited for complete system compromise.