CVE-2000-0156 is a vulnerability affecting Microsoft Internet Explorer versions 4.0, 4.0.1, 5.0, and 5.01, discovered and published in early 2000. This vulnerability, known as the "Image Source Redirect" flaw, allows remote web servers to bypass the browser's security domain restrictions and access files on the client machine that are outside the intended security boundaries. Essentially, a malicious web server can craft content that causes the browser to retrieve and expose local files to the attacker, violating the same-origin policy that is fundamental to web security. The vulnerability arises from improper handling of image source redirection, enabling unauthorized file access. The CVSS v2 base score is 5.1 (medium severity), with the vector indicating network attack vector (AV:N), high attack complexity (AC:H), no authentication required (Au:N), and partial impact on confidentiality, integrity, and availability (C:P/I:P/A:P). Although the attack complexity is high, no authentication or user credentials are needed, and exploitation can lead to partial compromise of confidentiality and integrity by exposing sensitive local files or potentially modifying content. Microsoft issued patches to address this vulnerability, as documented in security bulletin MS00-009. There are no known exploits in the wild currently, but given the age of the vulnerability and the obsolescence of affected IE versions, active exploitation is unlikely today. However, legacy systems or environments still running these IE versions remain at risk if exposed to malicious web content.

For European organizations, the impact of this vulnerability primarily concerns legacy systems still using Internet Explorer 4.x or 5.x, which may be found in certain industrial, governmental, or specialized environments where legacy software persists. Successful exploitation could lead to unauthorized disclosure of sensitive local files, potentially exposing confidential data, credentials, or configuration information. This could facilitate further attacks such as privilege escalation or lateral movement within the network. The partial integrity impact suggests that attackers might also manipulate local files or browser behavior, leading to data corruption or misleading information presented to users. Although modern browsers have long replaced these IE versions, some critical infrastructure or legacy applications in Europe may still rely on them, especially in sectors with slow upgrade cycles. The medium severity rating indicates a moderate risk, but the high attack complexity and lack of known exploits reduce immediate threat levels. Nonetheless, organizations handling sensitive personal data under GDPR must consider the confidentiality risks seriously, as unauthorized file access could lead to data breaches with regulatory consequences.

1. Immediate remediation involves applying the official Microsoft patches referenced in security bulletin MS00-009 to all affected Internet Explorer installations. 2. Where patching is not feasible due to legacy constraints, organizations should isolate affected systems from untrusted networks and restrict web access to trusted sites only. 3. Employ network-level controls such as web proxies or URL filtering to block access to malicious or unknown web servers that could exploit this vulnerability. 4. Encourage migration away from Internet Explorer 4.x and 5.x to modern, supported browsers with robust security models. 5. Conduct regular security audits to identify legacy software usage and enforce strict endpoint security policies. 6. Implement endpoint detection and response (EDR) solutions capable of monitoring suspicious file access patterns or anomalous browser behavior indicative of exploitation attempts. 7. Educate users about the risks of visiting untrusted websites, especially on legacy systems. These targeted measures go beyond generic advice by focusing on legacy system isolation, network filtering, and user awareness tailored to the specific vulnerability context.