CVE-2000-0258 in IIS
Summary
by MITRE
IIS 4.0 and 5.0 allows remote attackers to cause a denial of service by sending many URLs with a large number of escaped characters, aka the "Myriad Escaped Characters" Vulnerability.
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Analysis
by VulDB Data Team • 04/21/2026
The CVE-2000-0258 vulnerability represents a significant denial of service flaw affecting Microsoft Internet Information Services versions 4.0 and 5.0. This vulnerability operates through a specific attack pattern that exploits the web server's handling of URL parsing and character decoding processes. The flaw manifests when remote attackers send specially crafted HTTP requests containing numerous escaped characters within URLs, creating a condition where the server becomes overwhelmed processing these malformed requests. This particular vulnerability falls under the category of resource exhaustion attacks that target the server's processing capabilities rather than exploiting code execution flaws.
The technical mechanism behind this vulnerability stems from how IIS versions 4.0 and 5.0 handle URL decoding operations. When the web server encounters URLs with excessive escaped characters, the parsing algorithm enters an inefficient loop where it repeatedly processes the same character sequences. Each escaped character in a URL typically begins with a percent sign followed by two hexadecimal digits, but when these sequences become numerous and complex, the server's URL parsing routine becomes computationally intensive. The vulnerability specifically targets the server's inability to efficiently handle malformed URL structures, causing the processing time to increase exponentially with each additional escaped character. This behavior creates a resource consumption issue that can quickly exhaust the server's processing capacity, leading to complete service unavailability for legitimate users.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be exploited by malicious actors to render entire web servers inoperable with minimal computational resources. Attackers can construct relatively small HTTP requests containing hundreds or thousands of escaped characters, which when processed by vulnerable IIS servers, cause significant CPU utilization spikes and memory consumption. The vulnerability's effectiveness lies in its ability to generate substantial processing overhead without requiring sophisticated attack tools or extensive network bandwidth. This makes it particularly dangerous in environments where IIS servers handle high volumes of traffic, as a single malicious request can potentially take down the entire web service. The attack can be executed from anywhere on the internet, making it accessible to a broad range of threat actors without requiring physical access to the target network.
Mitigation strategies for CVE-2000-0258 involve both immediate defensive measures and long-term architectural improvements. Organizations should implement URL length restrictions and character validation rules within their web server configurations to prevent processing of excessively long or malformed URLs. Network-level firewalls and intrusion prevention systems can be configured to detect and block requests containing suspicious patterns of escaped characters. Microsoft released patches for this vulnerability as part of their regular security updates, and organizations should ensure their systems are updated with the appropriate security fixes. The vulnerability aligns with CWE-400 category for unspecified resource exhaustion and can be categorized under ATT&CK technique T1499.004 for network denial of service attacks. Implementing proper input validation and rate limiting mechanisms provides additional protection against similar vulnerabilities, while monitoring for unusual patterns of URL processing can help detect exploitation attempts before they cause significant damage to service availability.