CVE-2000-0408 in IIS
Summary
by MITRE
IIS 4.05 and 5.0 allow remote attackers to cause a denial of service via a long, complex URL that appears to contain a large number of file extensions, aka the "Malformed Extension Data in URL" vulnerability.
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Analysis
by VulDB Data Team • 10/16/2025
The CVE-2000-0408 vulnerability represents a classic denial of service flaw affecting Microsoft Internet Information Services versions 4.05 and 5.0. This vulnerability stems from insufficient input validation within the IIS web server's URL parsing mechanism, specifically when processing URLs containing an excessive number of file extensions. The flaw exploits a fundamental weakness in how the server handles malformed extension data, creating a scenario where legitimate requests can be manipulated to consume excessive system resources. This vulnerability operates at the application layer and demonstrates a critical design oversight in the URL processing pipeline of the affected IIS versions.
The technical implementation of this vulnerability involves constructing a URL with an abnormally large number of file extensions separated by periods, creating what appears to be a complex file path structure. When the IIS server attempts to parse such a malformed URL, the internal parsing algorithm enters an inefficient loop or recursive processing pattern that consumes disproportionate CPU cycles and memory resources. The server's inability to properly handle this edge case causes it to allocate excessive system resources while attempting to validate the URL structure, ultimately leading to resource exhaustion and service unavailability. This behavior aligns with CWE-400, which categorizes unchecked resource consumption as a significant vulnerability pattern.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be exploited by remote attackers without requiring authentication or special privileges. Attackers can craft malicious URLs that trigger the vulnerability with minimal effort, making it particularly dangerous in production environments where IIS servers handle high volumes of traffic. The denial of service condition can persist until the affected server is manually restarted or the system resources are reclaimed, potentially causing extended downtime for critical web applications. This vulnerability represents a significant risk to web infrastructure security and demonstrates the importance of robust input validation in web server implementations.
Organizations affected by this vulnerability should implement immediate mitigations including applying the relevant Microsoft security patches released for IIS 4.05 and 5.0, implementing URL filtering rules at the network level to block suspicious extension patterns, and configuring server-side resource limits to prevent excessive consumption. Network administrators should also consider deploying intrusion detection systems that can identify and block malformed URL patterns characteristic of this exploit. From an ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique for network denial of service, while the exploitation methodology aligns with T1595.001 for reconnaissance and T1590.003 for reconnaissance using multiple methods. The vulnerability highlights the critical need for comprehensive security testing of web server configurations and the importance of maintaining up-to-date security patches across all server infrastructure components.