CVE-2000-0266 is a vulnerability affecting Microsoft Internet Explorer versions 5.0 and 5.01. The issue arises from the browser's inadequate enforcement of the cross-frame security policy. Specifically, a remote attacker can exploit this vulnerability by crafting a malicious Java applet that interacts with the JavaScript JSObject interface. This interaction allows the attacker to manipulate the Document Object Model (DOM) properties of an IFRAME element, setting its source to an arbitrary JavaScript URL. By doing so, the attacker can bypass the intended same-origin policy restrictions that normally prevent scripts from one domain from accessing or modifying content in frames from another domain. This bypass could potentially allow an attacker to execute JavaScript in the context of a trusted site, leading to partial confidentiality breaches, such as reading information that should be isolated within a frame. However, the vulnerability does not allow modification of data or denial of service, limiting its impact. The vulnerability was published in April 2000, and no patches or fixes were made available by Microsoft. The CVSS v2 base score is 2.6, indicating a low severity, with network attack vector, high attack complexity, no authentication required, partial confidentiality impact, and no integrity or availability impact. There are no known exploits in the wild documented for this vulnerability, and it primarily affects legacy systems running these outdated browser versions.

For European organizations, the direct impact of CVE-2000-0266 today is minimal due to the obsolescence of Internet Explorer 5.0 and 5.01. Modern browsers have long since replaced these versions, and most organizations have migrated to supported platforms. However, in legacy environments where these versions might still be in use—such as in certain industrial control systems, embedded devices, or legacy intranet applications—there is a risk that an attacker could leverage this vulnerability to bypass frame security policies. This could lead to unauthorized disclosure of sensitive information displayed within frames, potentially exposing confidential data. Although the impact on integrity and availability is negligible, confidentiality breaches could have compliance implications under regulations like GDPR if personal data is exposed. The low severity and lack of known exploits reduce the immediate threat level, but organizations with legacy systems should remain cautious. The vulnerability could also be used as part of a multi-stage attack chain if combined with other vulnerabilities.

Given the age of the vulnerability and the lack of patches, the most effective mitigation is to discontinue the use of Internet Explorer 5.0 and 5.01 entirely. Organizations should upgrade to modern, supported browsers that enforce robust same-origin and cross-frame security policies. For legacy systems that cannot be upgraded immediately, network-level controls should be implemented to restrict access to vulnerable systems, such as isolating them within segmented networks and applying strict firewall rules. Additionally, disabling Java applet support in browsers or Java runtime environments can prevent exploitation via malicious applets. Monitoring and logging web traffic for unusual frame manipulation attempts can provide early detection of exploitation attempts. Finally, educating users about the risks of visiting untrusted sites and running unverified applets can reduce the attack surface.