CVE-1999-0154 is a vulnerability affecting Microsoft Internet Information Server (IIS) versions 2.0 and 3.0, which were released in the mid-1990s. The vulnerability allows remote attackers to read the source code of Active Server Pages (ASP) by appending a dot ('.') character to the end of the URL when requesting an ASP page. Normally, IIS processes ASP pages server-side and returns only the rendered HTML output to the client, thus protecting the underlying server-side script code. However, due to improper handling of URLs ending with a dot, IIS 2.0 and 3.0 fail to process the ASP script and instead serve the raw source code of the ASP file. This exposure can reveal sensitive information such as database connection strings, credentials, business logic, or other proprietary code embedded in the ASP scripts. The vulnerability requires no authentication and can be exploited remotely over the network with low complexity. The CVSS score of 5.0 (medium severity) reflects the partial confidentiality impact (source code disclosure) without affecting integrity or availability. No patches are available for these legacy IIS versions, and no known exploits are currently in the wild. Given the age of the affected software, this vulnerability is primarily of historical interest but remains a cautionary example of early web server security issues.

For European organizations, the direct impact of this vulnerability today is minimal because IIS 2.0 and 3.0 are obsolete and no longer in active use in production environments. However, if legacy systems running these IIS versions are still operational within any organization, the exposure of ASP source code could lead to significant confidentiality breaches. Attackers could harvest sensitive information such as database credentials or proprietary business logic, potentially enabling further attacks like database compromise or application-level exploitation. This could undermine trust, lead to data breaches, and cause regulatory compliance issues under GDPR if personal data is involved. Additionally, disclosure of source code can facilitate reverse engineering and targeted attacks. Although the vulnerability does not affect integrity or availability directly, the confidentiality breach alone can have serious consequences. European organizations with legacy infrastructure or insufficient patch management practices should be aware of this risk.

Given that no patches are available for IIS 2.0 and 3.0, the primary mitigation is to upgrade to a supported and secure version of IIS or migrate to modern web server platforms. Organizations should conduct thorough inventories to identify any legacy IIS installations and decommission or isolate them from external networks. If legacy systems must remain operational, strict network segmentation and access controls should be enforced to limit exposure. Additionally, web application firewalls (WAFs) or reverse proxies can be configured to block suspicious URL patterns such as those ending with a dot. Regular security audits and vulnerability assessments should be performed to detect such misconfigurations or exposures. Finally, secure coding practices and minimizing sensitive information in source code can reduce the impact of potential disclosures.