CVE-2017-18922 in LibVNCServer
Summary
by MITRE
It was discovered that websockets.c in LibVNCServer prior to 0.9.12 did not properly decode certain WebSocket frames. A malicious attacker could exploit this by sending specially crafted WebSocket frames to a server, causing a heap-based buffer overflow.
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Analysis
by VulDB Data Team • 05/15/2023
The vulnerability identified as CVE-2017-18922 resides within the LibVNCServer library version 0.9.11 and earlier, specifically affecting the websockets.c component responsible for WebSocket frame processing. This flaw represents a critical security issue that undermines the integrity of VNC server implementations relying on the affected library. The vulnerability stems from improper handling of WebSocket frame decoding logic, where the software fails to adequately validate or sanitize incoming WebSocket frames before processing them in memory. This deficiency creates a pathway for malicious actors to exploit the system through carefully crafted network traffic that manipulates the WebSocket frame structure in ways that bypass normal input validation mechanisms.
The technical exploitation of this vulnerability manifests as a heap-based buffer overflow condition that occurs when the websockets.c module processes malformed WebSocket frames. When a malicious attacker sends specially crafted WebSocket frames to a vulnerable VNC server, the insufficient input validation causes the application to write data beyond the bounds of allocated heap memory buffers. This overflow condition can lead to arbitrary code execution, memory corruption, or application crashes, depending on how the overflow manifests within the process memory space. The vulnerability is particularly concerning because it operates at the network protocol level where attackers can remotely inject malicious frames without requiring authentication or prior access to the system. This type of vulnerability falls under CWE-121 heap-based buffer overflow, which is classified as a critical weakness in memory safety and represents a common vector for remote code execution attacks.
The operational impact of CVE-2017-18922 extends beyond simple service disruption to potentially enable complete system compromise when exploited. VNC servers are commonly used for remote desktop access and system administration, making them attractive targets for attackers seeking persistent access to networks. The vulnerability can be exploited by attackers positioned anywhere on the network path between the client and server, as WebSocket frame processing occurs at the application layer where network traffic can be intercepted or manipulated. This attack surface is particularly dangerous in enterprise environments where VNC servers may be exposed to untrusted networks or where administrators might not maintain up-to-date software versions. The vulnerability also aligns with ATT&CK technique T1071.004 for application layer protocol usage, where attackers leverage protocol-specific weaknesses to gain unauthorized access. Organizations running vulnerable LibVNCServer implementations face risks including unauthorized remote access, data exfiltration, and potential lateral movement within their network infrastructure.
Mitigation strategies for this vulnerability center around immediate software updates to LibVNCServer version 0.9.12 or later, which contains the necessary patches to properly validate WebSocket frame decoding. System administrators should conduct comprehensive inventory audits to identify all systems running affected versions of LibVNCServer and prioritize patching operations based on risk assessment. Network segmentation and firewall rules should be implemented to restrict access to VNC services to trusted networks only, reducing the attack surface available to potential exploiters. Additionally, monitoring for unusual WebSocket traffic patterns and implementing intrusion detection systems can help identify exploitation attempts. The fix implemented in version 0.9.12 addresses the root cause by introducing proper bounds checking and input validation for WebSocket frames, ensuring that all incoming data is properly sanitized before processing. Organizations should also consider implementing additional security controls such as multi-factor authentication for VNC access, regular security assessments, and maintaining updated security patches across all network infrastructure components to prevent similar vulnerabilities from being exploited in the future.