CVE-1999-1087 in Internet Explorer
Summary
by MITRE
Internet Explorer 4 treats a 32-bit number ("dotless IP address") in the a URL as the hostname instead of an IP address, which causes IE to apply Local Intranet Zone settings to the resulting web page, allowing remote malicious web servers to conduct unauthorized activities by using URLs that contain the dotless IP address for their server.
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Analysis
by VulDB Data Team • 04/20/2026
This vulnerability in Internet Explorer 4 represents a critical security flaw that stems from improper URL parsing and hostname resolution handling. The issue occurs when a web browser encounters a 32-bit number without dots in a URL, which should be interpreted as an IP address but instead gets treated as a hostname. This misclassification creates a dangerous security boundary that allows remote malicious servers to exploit the browser's zone management system. The vulnerability specifically affects how IE processes URLs containing dotless IP addresses, where the browser incorrectly applies Local Intranet Zone security settings to pages served from these addresses. This misapplication of security policies creates an attack surface where remote web servers can manipulate the browser's security context to bypass normal protection mechanisms.
The technical implementation of this flaw involves the browser's URL parsing engine failing to properly distinguish between numeric IP address formats and hostname strings. When IE encounters a URL containing a 32-bit number without dots, it treats this value as a hostname rather than an IP address, which triggers the browser's zone detection logic to classify the destination as part of the Local Intranet Zone. This zone typically has less restrictive security settings compared to the Internet Zone, allowing for more permissive behavior regarding active content execution, file system access, and cross-domain requests. The vulnerability essentially creates a bypass mechanism where malicious actors can craft URLs that appear to point to external servers but are interpreted by the browser as local network resources, thereby gaining access to security contexts that should remain restricted.
The operational impact of this vulnerability extends beyond simple privilege escalation to encompass potential data exfiltration, cross-site scripting attacks, and unauthorized access to internal network resources. Attackers can leverage this flaw to craft malicious URLs that, when visited by victims using vulnerable IE versions, cause the browser to execute code with elevated privileges typical of the Local Intranet Zone. This creates opportunities for session hijacking, credential theft, and exploitation of internal network services that might otherwise be protected by firewall rules and network segmentation. The vulnerability particularly affects corporate environments where Local Intranet Zone settings might grant access to internal file shares, web services, or administrative interfaces that are normally restricted from external access. Additionally, this flaw can be combined with other techniques such as social engineering to create more sophisticated attacks that exploit user trust in familiar web addresses.
Security professionals should consider this vulnerability in the context of the CWE (Common Weakness Enumeration) framework, specifically relating to CWE-20, which addresses "Improper Input Validation" and CWE-16, which covers "Configuration". The ATT&CK framework would categorize this under T1190 "Exploit Public-Facing Application" and potentially T1071.004 "Application Layer Protocol: DNS" when considering the broader attack chain. Mitigation strategies must include immediate patching of affected IE versions, implementation of proper URL validation at network boundaries, and configuration of browser security settings to disable automatic zone assignment for numeric IP addresses. Organizations should also consider implementing network segmentation and access controls that prevent direct access to internal resources from external networks, while monitoring for suspicious URL patterns that might indicate exploitation attempts. The vulnerability highlights the importance of robust input validation and proper security context management in web browser implementations, serving as a reminder of the critical need for comprehensive security testing of URL parsing and zone management functions.