CVE-1999-0348 is a medium-severity vulnerability affecting Microsoft Internet Information Server (IIS) version 4.0. The issue arises from the way IIS handles ASP (Active Server Pages) caching when two virtual servers are configured to share the same physical directory. Specifically, the caching mechanism does not properly isolate cached content between these virtual servers, leading to unintended information disclosure. This vulnerability allows one virtual server to access sensitive information cached by another virtual server, potentially exposing confidential data. The vulnerability is classified under CWE-200 (Information Exposure), indicating that sensitive information is released to unauthorized parties. The CVSS v2 score of 5.0 reflects a network-accessible vulnerability with low attack complexity, no authentication required, and partial confidentiality impact without affecting integrity or availability. No patches are available for this vulnerability, and no known exploits have been reported in the wild. The root cause is the shared caching context in IIS 4.0 when virtual servers point to the same physical directory, which was a common configuration in early web hosting environments to conserve resources or simplify management. This flaw can lead to cross-virtual-server data leakage, undermining the isolation expected between hosted sites on the same IIS instance.

For European organizations, the impact of this vulnerability depends largely on the continued use of IIS 4.0 in production environments, which is unlikely given the age of the software. However, legacy systems or archival servers still running IIS 4.0 could be at risk. The primary impact is confidentiality loss, where sensitive data from one virtual server could be exposed to another, potentially leaking customer information, internal business data, or proprietary content. This could lead to regulatory compliance issues under GDPR, especially if personal data is exposed without consent. The vulnerability does not affect integrity or availability, so service disruption or data tampering is not a concern here. Given that no authentication is required and the attack can be performed remotely over the network, any publicly accessible IIS 4.0 server configured with shared physical directories between virtual hosts is vulnerable. European organizations with multi-tenant IIS hosting environments or legacy web infrastructure should be aware of this risk. Although no known exploits exist, the vulnerability's nature makes it a potential vector for information leakage in environments where virtual server isolation is critical.

Since no official patch is available for IIS 4.0, European organizations should consider the following specific mitigations: 1) Avoid configuring multiple virtual servers to share the same physical directory to prevent cache overlap. 2) Upgrade IIS to a supported and patched version where this caching isolation issue is resolved. 3) If upgrading is not immediately possible, isolate virtual servers by assigning distinct physical directories or separate IIS instances to ensure cache separation. 4) Implement network-level access controls to restrict access to legacy IIS servers only to trusted internal users or networks. 5) Conduct thorough audits of legacy IIS configurations to identify shared directory setups and remediate accordingly. 6) Monitor web server logs for unusual access patterns that could indicate attempts to exploit information leakage. 7) Consider deploying web application firewalls (WAFs) that can detect and block suspicious requests targeting legacy IIS vulnerabilities. These mitigations go beyond generic advice by focusing on configuration changes and architectural isolation specific to this vulnerability's root cause.