CVE-1999-0867 is a vulnerability affecting Microsoft Internet Information Services (IIS) versions 2.0, 2.5, 3.0, and 4.0. The issue is a denial of service (DoS) condition triggered by sending a flood of HTTP requests containing malformed headers to the IIS server. Specifically, the server fails to properly handle these malformed HTTP headers, which can cause the service to crash or become unresponsive. This vulnerability stems from improper input validation (CWE-20), where the server does not adequately check the structure and content of incoming HTTP headers before processing them. Since IIS is a widely used web server platform, especially in the late 1990s and early 2000s, this vulnerability could be exploited remotely without any authentication or user interaction, simply by overwhelming the server with crafted HTTP requests. The CVSS score of 5.0 (medium severity) reflects that while the vulnerability does not impact confidentiality or integrity, it can significantly affect availability by causing service outages. A patch addressing this issue was released by Microsoft as part of security bulletin MS99-029. No known exploits have been reported in the wild, but the vulnerability remains a concern for legacy systems still running these IIS versions. Given the age of the affected software, modern IIS versions are not impacted by this vulnerability.

For European organizations, the primary impact of CVE-1999-0867 is the potential disruption of web services hosted on vulnerable IIS versions. Organizations relying on legacy IIS servers for critical web applications or internal services could experience denial of service conditions, leading to downtime and loss of availability. This could affect business operations, customer access, and internal communications. Although the vulnerability does not compromise data confidentiality or integrity, the unavailability of web services can have significant operational and reputational consequences. Additionally, in sectors such as government, finance, and healthcare, where web service availability is critical, such disruptions could lead to regulatory non-compliance or loss of trust. However, given the age of the affected IIS versions, most European organizations are unlikely to be running these outdated systems in production environments. The risk is primarily to legacy or unsupported systems that have not been updated or decommissioned.

European organizations should prioritize upgrading or replacing any IIS servers running versions 2.0, 2.5, 3.0, or 4.0 with supported, modern versions of IIS or alternative web server platforms. Applying the official Microsoft patch MS99-029 is essential for any remaining legacy systems to remediate this vulnerability. Network-level protections such as web application firewalls (WAFs) and intrusion prevention systems (IPS) can be configured to detect and block malformed HTTP headers and abnormal request floods, reducing exposure to exploitation attempts. Rate limiting and traffic anomaly detection should be implemented to mitigate denial of service attacks. Regular vulnerability scanning and asset inventory management will help identify any legacy IIS deployments. Organizations should also enforce strict network segmentation to isolate legacy systems and monitor logs for unusual HTTP traffic patterns. Finally, decommissioning unsupported IIS versions and migrating services to secure, maintained platforms is the most effective long-term mitigation.