CVE-2004-1750 in RealVNC
Summary
by MITRE
RealVNC 4.0 and earlier allows remote attackers to cause a denial of service (crash) via a large number of connections to port 5900.
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Analysis
by VulDB Data Team • 06/30/2019
RealVNC version 4.0 and earlier implementations contain a critical vulnerability that enables remote attackers to execute denial of service attacks through excessive connection attempts to the default VNC port 5900. This vulnerability stems from inadequate connection handling mechanisms within the VNC server software, where the system fails to properly manage or limit incoming connection requests. The flaw manifests when an attacker establishes a large number of simultaneous connections to the target system, overwhelming the server's ability to process legitimate requests and ultimately causing the VNC service to crash or become unresponsive. This vulnerability directly relates to CWE-400, which addresses unchecked resource consumption, and represents a classic example of a resource exhaustion attack that targets the fundamental connection management capabilities of the software. The impact extends beyond simple service disruption as it can render remote desktop access completely unavailable to authorized users while potentially affecting other services running on the same system. Attackers can exploit this weakness with minimal technical expertise by simply connecting to port 5900 repeatedly, making it particularly dangerous in environments where VNC servers are exposed to untrusted networks. The vulnerability demonstrates poor input validation and resource management practices within the VNC protocol implementation, as the server does not implement proper connection rate limiting or connection queue management. This weakness aligns with ATT&CK technique T1499.004, which covers network denial of service attacks, and specifically targets the availability aspect of the CIA triad. Organizations using vulnerable RealVNC versions face significant operational risks including complete loss of remote access capabilities, potential business disruption, and increased attack surface for more sophisticated exploits. The vulnerability is particularly concerning in enterprise environments where VNC servers may be used for remote administration, system maintenance, or technical support operations. The flaw exists in the core protocol handling logic where the server fails to implement adequate safeguards against connection flooding attacks, allowing malicious actors to consume system resources rapidly and cause service degradation. This vulnerability can be easily exploited through automated tools and scripts, making it a preferred method for attackers seeking to disrupt remote access services without requiring advanced technical knowledge or significant computational resources. The lack of proper connection throttling mechanisms in RealVNC 4.0 and earlier versions creates a persistent security gap that can be leveraged for extended periods without detection. System administrators should immediately implement connection rate limiting at network boundaries, deploy intrusion prevention systems to detect and block excessive connection attempts, and upgrade to patched versions of RealVNC software. Additionally, implementing proper monitoring and alerting mechanisms can help detect unusual connection patterns that may indicate exploitation attempts. The vulnerability serves as a critical reminder of the importance of proper resource management and input validation in network services, particularly those handling multiple concurrent connections. Organizations should also consider implementing network segmentation strategies to limit exposure of VNC services to untrusted networks and ensure that only necessary systems have access to the VNC port 5900. Regular security assessments and vulnerability scanning should include verification of VNC service configurations to prevent exploitation of this and similar connection-based denial of service vulnerabilities. The incident highlights the necessity of maintaining up-to-date security patches and implementing defense-in-depth strategies that protect against various attack vectors including those targeting fundamental service availability.