CVE-2000-0258 is a vulnerability affecting Microsoft Internet Information Server (IIS) versions 4.0 and 5.0. This vulnerability, known as the "Myriad Escaped Characters" vulnerability, allows remote attackers to cause a denial of service (DoS) condition by sending a large number of URLs containing an excessive amount of escaped characters. Escaped characters in URLs are typically used to encode special characters, but when processed in large quantities, IIS 4.0 and 5.0 fail to handle them properly, leading to resource exhaustion or server instability. The vulnerability does not allow attackers to compromise confidentiality or integrity but impacts availability by causing the web server to become unresponsive or crash. The attack vector is network-based, requiring no authentication or user interaction, making it relatively easy to exploit remotely. The vulnerability is classified under CWE-20, which relates to improper input validation, indicating that IIS does not adequately validate or limit the number of escaped characters in incoming URL requests. Microsoft has released patches to address this issue, documented in security bulletin MS00-023. Although no known exploits have been reported in the wild, the high CVSS score of 7.5 reflects the significant impact on availability and the ease of exploitation. Given the age of the affected IIS versions, this vulnerability primarily concerns legacy systems that have not been updated or replaced.

For European organizations still operating legacy IIS 4.0 or 5.0 servers, this vulnerability poses a significant risk of denial of service attacks that can disrupt web services and business operations. The DoS condition can lead to downtime, loss of customer trust, and potential financial losses due to service unavailability. Critical infrastructure sectors, government agencies, and enterprises relying on legacy Microsoft web servers are particularly vulnerable. Since the attack requires no authentication and can be launched remotely, attackers can easily target exposed IIS servers over the internet. The impact is limited to availability, with no direct compromise of data confidentiality or integrity. However, prolonged outages could indirectly affect data access and operational continuity. Given the age of the vulnerability, most modern environments are unlikely to be affected, but organizations with legacy systems or insufficient patch management remain at risk.

1. Immediate patching: Apply the security update provided by Microsoft in bulletin MS00-023 to all affected IIS 4.0 and 5.0 servers. 2. Upgrade IIS: Migrate legacy IIS 4.0 and 5.0 servers to supported versions of IIS or modern web server platforms that are actively maintained and patched. 3. Network-level protections: Deploy web application firewalls (WAFs) or intrusion prevention systems (IPS) configured to detect and block requests with excessive escaped characters or malformed URLs. 4. Rate limiting: Implement rate limiting on web servers or perimeter devices to restrict the number of requests from a single source, mitigating potential DoS attempts. 5. Disable unnecessary services: If IIS 4.0 or 5.0 must be retained temporarily, disable unnecessary web services or restrict access to trusted networks only. 6. Monitoring and alerting: Establish monitoring for unusual spikes in URL requests or server resource usage to detect potential exploitation attempts early. 7. Incident response planning: Prepare response procedures for DoS incidents to minimize downtime and restore services promptly.