CVE-2006-1652 in UltraVNC
Summary
by MITRE
Multiple buffer overflows in (a) UltraVNC (aka Ultr@VNC) 1.0.1 and earlier and (b) tabbed_viewer 1.29 (1) allow user-assisted remote attackers to execute arbitrary code via a malicious server that sends a long string to a client that connects on TCP port 5900, which triggers an overflow in Log::ReallyPrint; and (2) allow remote attackers to cause a denial of service (server crash) via a long HTTP GET request to TCP port 5800, which triggers an overflow in VNCLog::ReallyPrint.
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Analysis
by VulDB Data Team • 06/22/2024
The vulnerability identified as CVE-2006-1652 represents a critical security flaw affecting UltraVNC and tabbed_viewer software versions up to 1.0.1 and 1.29 respectively. This issue manifests through multiple buffer overflow conditions that occur when these remote desktop applications process incoming network connections. The primary attack vector involves malicious servers sending excessively long strings to client applications connecting via TCP port 5900, which triggers a buffer overflow in the Log::ReallyPrint function. This flaw falls under the CWE-121 buffer overflow category, specifically classified as a stack-based buffer overflow that can lead to arbitrary code execution when exploited by remote attackers. The vulnerability is particularly dangerous because it requires minimal user interaction, as the attack can be initiated through automated connections without requiring user consent or specific actions.
The technical implementation of this vulnerability occurs within the VNC protocol implementation where the Log::ReallyPrint function fails to properly validate input string lengths before copying them into fixed-size buffers. When a malicious server sends a string exceeding the allocated buffer space on TCP port 5900, the excess data overflows into adjacent memory locations, potentially corrupting program execution flow. This overflow can be exploited to inject and execute malicious code with the privileges of the running VNC client process. The secondary vulnerability affects TCP port 5800 where HTTP GET requests can trigger a similar overflow in VNCLog::ReallyPrint, causing server crashes and denial of service conditions. Both scenarios demonstrate improper bounds checking and lack of input sanitization that are fundamental requirements for secure network application development. The vulnerability aligns with ATT&CK technique T1203 (Exploitation for Client Execution) and T1499 (Endpoint Denial of Service) through its ability to execute arbitrary code and cause system crashes.
The operational impact of CVE-2006-1652 extends beyond simple exploitation as it affects the integrity and availability of remote desktop services. Organizations relying on UltraVNC for remote administration face significant risks including unauthorized system access, data exfiltration, and complete system compromise. The vulnerability's remote nature means attackers can exploit it from anywhere on the internet without requiring physical access to target systems. The buffer overflow conditions create unstable application states that can result in unpredictable behavior, making system recovery complex and potentially requiring full system reinstallation. Security professionals should note that this vulnerability was particularly concerning because it affected widely deployed remote desktop software used in enterprise environments, educational institutions, and government organizations. The flaw demonstrates the critical importance of input validation in network services and highlights the dangers of legacy software that may not receive regular security updates.
Mitigation strategies for CVE-2006-1652 should include immediate software patching and version updates to address the buffer overflow conditions in both UltraVNC and tabbed_viewer applications. Organizations must implement network segmentation to restrict access to TCP ports 5900 and 5800, particularly when these services are not essential for business operations. Network access control lists should be configured to limit connections to these ports from trusted IP ranges only, and additional monitoring should be implemented to detect unusual traffic patterns that might indicate exploitation attempts. System administrators should consider disabling unnecessary VNC services and implementing alternative remote access solutions with better security track records. The vulnerability also underscores the importance of network intrusion detection systems that can identify malformed HTTP requests or unusual string lengths that might indicate buffer overflow attacks. Regular security assessments and vulnerability scanning should be conducted to identify unpatched systems that may be exposed to this and similar vulnerabilities. Organizations should also implement application whitelisting policies to prevent unauthorized versions of UltraVNC from running on their networks, ensuring that only properly patched and verified versions are permitted to execute.