CVE-1999-0891 is a vulnerability identified in Microsoft Internet Explorer 5. The issue arises from the "download behavior" feature within the browser, which can be manipulated by remote attackers to read arbitrary files on a victim's system. This is achieved through a server-side redirect that exploits the way Internet Explorer 5 handles download behaviors, allowing an attacker to bypass normal access controls and retrieve sensitive files without authentication or user interaction. The vulnerability is classified under CWE-94, which relates to improper control of code generation, indicating that the flaw involves unsafe handling of input that leads to unauthorized file access. The CVSS score of 5.0 (medium severity) reflects that the attack vector is network-based, requires no authentication, and impacts confidentiality but not integrity or availability. Although this vulnerability is relatively old and affects a legacy product version, it highlights a critical security flaw in early web browsers where client-side behaviors could be exploited to compromise user data. A patch addressing this vulnerability was released by Microsoft in 1999 (MS99-040), which should be applied to mitigate the risk. There are no known exploits in the wild documented for this vulnerability, but the potential for arbitrary file reading remains a significant concern if unpatched systems are still in use.

For European organizations, the impact of this vulnerability primarily concerns the confidentiality of sensitive information stored on client machines running Internet Explorer 5. Although Internet Explorer 5 is an outdated browser and unlikely to be in widespread use today, legacy systems in certain sectors such as government, industrial control, or specialized enterprise environments might still operate it due to compatibility requirements. Exploitation could lead to unauthorized disclosure of confidential files, potentially exposing personal data, intellectual property, or internal documents. This could result in regulatory non-compliance under GDPR if personal data is compromised. Additionally, the breach of confidentiality could undermine trust and lead to reputational damage. However, the lack of impact on integrity and availability limits the scope of damage to data exposure rather than system disruption or data manipulation. The medium severity rating suggests that while the threat is serious, it is not critical, especially given the age of the affected software and the availability of patches.

European organizations should ensure that all systems have been updated to versions of Internet Explorer beyond 5.0 or replaced with modern, supported browsers. Specifically, any legacy systems still running Internet Explorer 5 must be patched using the Microsoft security bulletin MS99-040. Network administrators should implement strict network segmentation and firewall rules to limit access to legacy systems from untrusted networks. Additionally, organizations should conduct audits to identify any remaining instances of Internet Explorer 5 and plan for their upgrade or decommissioning. Employing endpoint detection and response (EDR) tools can help monitor for suspicious file access patterns indicative of exploitation attempts. User education should emphasize avoiding untrusted websites that could host malicious redirects. Finally, organizations should enforce the principle of least privilege on client machines to minimize the impact of any arbitrary file read attempts.