CVE-2001-0167 in WinVNC
Summary
by MITRE
Buffer overflow in AT&T WinVNC (Virtual Network Computing) client 3.3.3r7 and earlier allows remote attackers to execute arbitrary commands via a long rfbConnFailed packet with a long reason string.
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Analysis
by VulDB Data Team • 05/05/2025
The vulnerability identified as CVE-2001-0167 represents a critical buffer overflow flaw in AT&T WinVNC client versions 3.3.3r7 and earlier, fundamentally compromising the security integrity of remote desktop connections. This issue arises from inadequate input validation within the rfbConnFailed packet handling mechanism, where the software fails to properly sanitize the length of reason strings transmitted during connection failure scenarios. The flaw exists in the client-side implementation of the Virtual Network Computing protocol, which is widely used for remote desktop access and system administration across enterprise environments.
The technical exploitation of this vulnerability occurs when a remote attacker crafts a malicious rfbConnFailed packet containing an excessively long reason string that exceeds the allocated buffer space within the WinVNC client application. This buffer overflow condition allows attackers to overwrite adjacent memory locations, potentially leading to arbitrary code execution with the privileges of the affected user. The vulnerability specifically targets the client-side parsing logic that processes connection failure notifications, making it particularly dangerous in environments where remote desktop services are extensively deployed. According to CWE classification, this represents a classic buffer overflow weakness categorized under CWE-121, which involves stack-based buffer overflow conditions that can be exploited through improper input handling.
The operational impact of CVE-2001-0167 extends beyond simple privilege escalation, as it can enable attackers to gain complete control over systems running vulnerable WinVNC clients. This vulnerability directly aligns with ATT&CK technique T1071.004, which covers protocol tunneling and remote access tools, allowing adversaries to establish persistent access to compromised systems. Organizations utilizing WinVNC for remote administration face significant risk exposure, particularly in environments where unpatched systems are prevalent. The vulnerability's exploitability is enhanced by the fact that it requires no authentication for exploitation, making it particularly dangerous in network environments where WinVNC clients may be exposed to untrusted networks. Attackers can leverage this flaw to execute malicious code, establish backdoors, or perform reconnaissance activities without requiring legitimate credentials, effectively bypassing traditional authentication mechanisms.
Mitigation strategies for this vulnerability primarily focus on immediate patch deployment, as the official fix addresses the buffer overflow through proper input length validation and memory boundary checks. Organizations should implement comprehensive network segmentation to limit exposure of WinVNC client installations to trusted networks only, while also monitoring for suspicious rfbConnFailed packet patterns in network traffic. The implementation of network-based intrusion detection systems can help identify potential exploitation attempts by monitoring for oversized reason strings in VNC protocol communications. Additionally, administrators should consider migrating to more modern remote desktop solutions that have undergone rigorous security assessments and maintain active support cycles, as WinVNC has not received security updates since 2001. System hardening measures including disabling unnecessary VNC client functionality, implementing strict firewall rules, and conducting regular vulnerability assessments can further reduce the attack surface for this and similar vulnerabilities.