CVE-2017-5884 in gtk-vnc
Summary
by MITRE
gtk-vnc before 0.7.0 does not properly check boundaries of subrectangle-containing tiles, which allows remote servers to execute arbitrary code via the src x, y coordinates in a crafted (1) rre, (2) hextile, or (3) copyrect tile.
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Analysis
by VulDB Data Team • 09/02/2020
The vulnerability identified as CVE-2017-5884 affects the gtk-vnc library version 0.7.0 and earlier, representing a critical security flaw in virtual network computing implementations. This issue resides within the library's handling of graphical tile updates during remote desktop sessions, specifically impacting the processing of subrectangle-containing tiles that are fundamental to efficient graphics transmission in VNC protocols. The vulnerability stems from insufficient boundary validation mechanisms that fail to properly verify coordinate parameters when processing different tile encoding types including rre (rich region encoding), hextile, and copyrect formats. These encoding methods are standard components of the VNC protocol specification and are designed to optimize bandwidth usage by transmitting only changed regions of the screen rather than full screen updates. When a remote VNC server sends maliciously crafted tile data with invalid source x and y coordinates, the library's inadequate input validation allows these malformed parameters to propagate through the processing pipeline, potentially leading to memory corruption and arbitrary code execution on the client system.
The technical nature of this vulnerability aligns with CWE-129, which addresses insufficient validation of length of inputs to ensure they are within acceptable bounds, and also relates to CWE-787, concerning out-of-bounds write operations. The flaw operates through a classic buffer overflow mechanism where malicious coordinate values cause the library to access memory locations beyond allocated boundaries during tile rendering operations. The vulnerability is particularly dangerous because it can be exploited remotely without requiring authentication, making it a significant threat to systems running vulnerable versions of gtk-vnc. Attackers can craft specially formatted VNC server responses that contain invalid coordinates in tile data structures, triggering the boundary checking failure. The impact extends beyond simple code execution to potentially allow full system compromise, as the vulnerable code path involves graphics rendering operations that may have elevated privileges or direct access to system resources. This vulnerability affects any application or service that relies on gtk-vnc for VNC client functionality, including remote desktop management tools, virtual machine consoles, and network monitoring applications.
The operational impact of CVE-2017-5884 is severe and multifaceted, particularly in enterprise environments where remote desktop access is common and security controls may be insufficient. Organizations using vulnerable versions of gtk-vnc in their remote access infrastructure face significant risk of unauthorized system compromise, data exfiltration, and persistent backdoor establishment. The vulnerability can be exploited in various attack scenarios including targeted attacks against specific systems, mass scanning campaigns, and supply chain compromises where vulnerable software packages are distributed to multiple endpoints. From an attack chain perspective, this vulnerability maps to multiple ATT&CK techniques including T1071.004 for application layer protocol usage, T1059 for command and script interpreter execution, and T1105 for remote file execution. The exploitation process typically begins with a VNC server establishing connection to a vulnerable client, followed by the transmission of malicious tile data that triggers the buffer overflow condition. Network administrators should be aware that this vulnerability can be leveraged for lateral movement within networks, as compromised systems can serve as launching points for further attacks. The vulnerability's remote exploitability makes it particularly concerning for public-facing services or systems exposed to untrusted networks, where the attack surface is expanded significantly.
Mitigation strategies for CVE-2017-5884 center on immediate version updates to gtk-vnc 0.7.0 or later, which contain the necessary boundary checking fixes. Organizations should conduct comprehensive inventory assessments to identify all systems running vulnerable versions of the library, including embedded systems, virtualization platforms, and remote access tools. Network segmentation and access controls should be implemented to limit exposure of VNC services to trusted networks only, reducing the attack surface for remote exploitation attempts. Security monitoring should be enhanced to detect unusual VNC traffic patterns or malformed tile data that might indicate exploitation attempts. System hardening measures including disabling unnecessary VNC services, implementing strong authentication mechanisms, and deploying intrusion detection systems can provide additional layers of protection. The fix implemented in version 0.7.0 addresses the core issue by adding proper boundary validation checks for all tile coordinate parameters, ensuring that source x and y coordinates are validated against expected ranges before processing. Organizations should also consider implementing network-based controls such as firewall rules that restrict VNC traffic to necessary ports and IP addresses, along with regular security assessments to identify other potential vulnerabilities in their remote access infrastructure. Regular patch management processes should be established to ensure timely deployment of security updates across all systems using vulnerable libraries.