CVE-1999-1087 is a high-severity vulnerability affecting Microsoft Internet Explorer versions 4.0 and 4.0.1. The issue arises from how Internet Explorer 4 processes URLs containing a 32-bit number representing a "dotless IP address." Instead of interpreting this number as a standard IP address, IE treats it as a hostname. This misinterpretation causes the browser to apply Local Intranet Zone security settings to the web page loaded from such URLs. Typically, the Local Intranet Zone has more permissive security policies compared to the Internet Zone, allowing actions such as running ActiveX controls, executing scripts, or accessing local resources with fewer restrictions. An attacker controlling a malicious web server can exploit this behavior by crafting URLs with dotless IP addresses pointing to their server. When a user visits such a URL, IE mistakenly applies the relaxed Local Intranet Zone policies, enabling the attacker to perform unauthorized activities like executing malicious scripts or ActiveX controls, potentially leading to compromise of the user's system. This vulnerability does not require user authentication and can be exploited remotely over the network without user interaction beyond visiting a malicious URL. Microsoft has released patches addressing this vulnerability, documented in security bulletin MS98-016. No known exploits have been reported in the wild, but the vulnerability's nature and the high CVSS score (7.5) indicate a significant risk if left unpatched.

For European organizations, the impact of CVE-1999-1087 could be substantial, especially for those still operating legacy systems or environments where Internet Explorer 4 is in use (e.g., legacy industrial control systems, archival systems, or isolated networks). Exploitation could lead to unauthorized code execution, data theft, or lateral movement within internal networks due to the relaxed security context applied by the browser. This could compromise confidentiality, integrity, and availability of sensitive corporate or governmental data. Although modern browsers have replaced IE4, some legacy applications or intranet portals might still rely on it, particularly in sectors with long hardware/software lifecycles such as manufacturing, utilities, or government agencies. The vulnerability could also facilitate targeted attacks against European organizations by enabling attackers to bypass security zones and execute malicious payloads with fewer restrictions. Given the high CVSS score and the potential for privilege escalation within the browser context, organizations could face data breaches, espionage, or disruption of critical services if exploited.

1. Immediate patching: Apply the security updates provided by Microsoft in bulletin MS98-016 to all affected Internet Explorer 4 installations. 2. Browser upgrade: Migrate away from Internet Explorer 4 to modern, supported browsers that do not exhibit this vulnerability. 3. Network controls: Implement network-level filtering to block access to known malicious IP addresses or URLs using dotless IP address formats. 4. User education: Train users to avoid clicking on suspicious links, especially those with unusual URL formats such as dotless IP addresses. 5. Zone policy enforcement: Review and tighten Local Intranet Zone security settings via Group Policy to minimize the permissions granted, reducing the impact if exploitation occurs. 6. Legacy system isolation: Segregate legacy systems running IE4 from critical network segments to limit potential lateral movement. 7. Monitoring and detection: Deploy web proxy and endpoint monitoring solutions to detect anomalous URL patterns or unexpected browser behavior indicative of exploitation attempts.