CVE-1999-0871 is a security vulnerability affecting Microsoft Internet Explorer versions 4.0 and 4.01, identified as the "Cross Frame Navigate" vulnerability. This flaw arises from improper enforcement of the browser's cross-frame security model, which is designed to prevent web pages from accessing content in frames or windows originating from different domains. Due to this weakness, a remote attacker can exploit the vulnerability to read files on the victim's system by leveraging the browser's frame navigation capabilities. Specifically, the attacker can craft malicious web content that navigates frames in a way that bypasses the intended same-origin policy restrictions, allowing unauthorized access to local files. The vulnerability does not allow modification of files or code execution but compromises confidentiality by exposing potentially sensitive data. The CVSS v2 score is 2.6 (low severity), reflecting that exploitation requires network access, has high attack complexity, no authentication, and impacts confidentiality only. Microsoft addressed this issue with security bulletin MS98-013, providing patches to correct the cross-frame security enforcement. Given the age of the vulnerability (published in 1998) and the affected product versions, this issue primarily concerns legacy systems still running Internet Explorer 4.0 or 4.01, which are long out of support and generally not in use in modern environments.

For European organizations, the direct impact of CVE-1999-0871 today is minimal due to the obsolescence of Internet Explorer 4.0 and 4.01. However, if legacy systems running these versions remain in operation, the vulnerability could allow attackers to read sensitive local files remotely, potentially exposing confidential information such as internal documents, credentials, or configuration files. This could lead to information leakage and facilitate further attacks. The risk is compounded in environments where legacy applications or industrial control systems depend on outdated browsers. Confidentiality breaches could affect compliance with European data protection regulations like GDPR if personal or sensitive data is exposed. Nonetheless, the low CVSS score and absence of known exploits in the wild suggest limited active threat. The main concern is ensuring legacy systems are either upgraded or isolated to prevent exploitation.

European organizations should prioritize decommissioning or upgrading any systems still running Internet Explorer 4.0 or 4.01 to supported, secure browser versions. If legacy systems must remain operational, they should be isolated from external networks and restricted to trusted internal use only. Applying the official Microsoft patch MS98-013 is critical to remediate the vulnerability on affected systems. Network-level controls such as web filtering and intrusion detection can help detect and block attempts to exploit this vulnerability. Additionally, organizations should conduct audits to identify any legacy browsers in use and implement strict access controls to limit exposure. User education to avoid visiting untrusted websites on legacy browsers can further reduce risk. Finally, consider migrating legacy applications to modern platforms that do not rely on outdated browsers.