CVE-2001-0664 in Internet Explorer
Summary
by MITRE
Internet Explorer 5.5 and 5.01 allows remote attackers to bypass security restrictions via malformed URLs that contain dotless IP addresses, which causes Internet Explorer to process the page in the Intranet Zone, which may have fewer security restrictions, aka the "Zone Spoofing vulnerability."
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Analysis
by VulDB Data Team • 03/23/2025
The CVE-2001-0664 vulnerability represents a critical zone spoofing flaw in Microsoft Internet Explorer versions 5.5 and 5.01 that fundamentally compromised web browser security mechanisms. This vulnerability exploited a fundamental weakness in how Internet Explorer classified and assigned security zones to web resources based on URL parsing algorithms. The flaw specifically targeted the browser's handling of IP address formats, creating a pathway for malicious actors to manipulate the security context in which web content executed. The vulnerability falls under the CWE-284 access control weakness category, specifically addressing improper access control mechanisms in web browsers. The attack vector leveraged malformed URLs containing dotless IP addresses such as 192168101 instead of the standard 192.168.1.1 format, which caused Internet Explorer to misinterpret the resource location and assign it to the Intranet Zone rather than the Internet Zone.
The technical implementation of this vulnerability exploited the browser's URL parsing and zone assignment logic, where the absence of dots in IP addresses triggered an incorrect parsing sequence that bypassed normal security boundary enforcement. When Internet Explorer encountered these malformed URLs, it failed to properly validate the address format and instead treated the resource as if it were located within an internal network, thereby granting it elevated privileges and reduced security restrictions. This behavior violated fundamental security principles of zone-based access control, where resources should be classified according to their actual network location and trust boundaries. The vulnerability specifically targeted the browser's security zone classification engine, which is designed to apply different security policies based on the perceived trust level of web resources. The flaw enabled attackers to execute malicious code with intranet privileges, potentially accessing local network resources, bypassing firewall restrictions, and exploiting other security mechanisms that assume the content originates from an internal trusted source.
The operational impact of CVE-2001-0664 was significant, as it allowed remote attackers to effectively circumvent the security model that separates internal and external web resources. This vulnerability created a persistent threat that could be exploited across multiple network environments, particularly in corporate settings where internal network access controls were more permissive than external internet restrictions. The attack could result in unauthorized access to internal network resources, potential data exfiltration, and privilege escalation within the security boundaries of affected organizations. Security researchers noted that this vulnerability particularly affected enterprise environments where Internet Explorer was the primary browser and internal network access was configured with more relaxed security policies. The exploitation of this vulnerability could enable attackers to perform actions such as accessing internal file shares, executing network reconnaissance, or leveraging other security weaknesses that were typically restricted to external internet traffic.
Mitigation strategies for CVE-2001-0664 required immediate implementation of Microsoft security patches and updates to affected Internet Explorer versions, as well as organizational policy changes to address the underlying parsing vulnerability. Network administrators needed to implement additional security controls such as URL filtering, proxy server configurations, and enhanced browser security policies to prevent exploitation of malformed URLs. The vulnerability highlighted the importance of proper input validation and URL parsing in web browsers, leading to enhanced security practices in subsequent browser development cycles. Organizations were advised to disable unnecessary browser features, implement strict security policies, and deploy additional network monitoring to detect and prevent exploitation attempts. This vulnerability also underscored the need for robust security testing of URL parsing mechanisms and the importance of maintaining updated security configurations to prevent similar zone spoofing attacks in modern web browsers. The incident contributed to the evolution of security standards and practices that continue to influence browser security architecture today.