CVE-2001-0004 in IIS
Summary
by MITRE
IIS 5.0 and 4.0 allows remote attackers to read the source code for executable web server programs by appending "%3F+.htr" to the requested URL, which causes the files to be parsed by the .HTR ISAPI extension, aka a variant of the "File Fragment Reading via .HTR" vulnerability.
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Analysis
by VulDB Data Team • 10/11/2025
This vulnerability affects Microsoft Internet Information Services versions 5.0 and 4.0, representing a critical security flaw in web server configuration that enables remote code execution through source code disclosure. The vulnerability stems from improper handling of file extensions and ISAPI extension processing within the web server architecture. When attackers append the encoded string "%3F+.htr" to a URL, they exploit a parsing mechanism that causes the web server to process the requested file through the .HTR ISAPI extension, which is designed for server-side include processing rather than general file serving.
The technical implementation of this vulnerability leverages the way IIS handles file extensions and the order of extension processing. The .HTR extension is intended for server-side includes and typically processes files with the .htr extension, but due to flawed extension handling, when a file with a different extension is requested with the appended .htr component, the server incorrectly routes the request to the .HTR ISAPI extension. This misconfiguration allows attackers to bypass normal file access controls and retrieve the source code of executable web programs, potentially exposing sensitive application logic, database connection strings, and other confidential information.
The operational impact of this vulnerability is severe as it provides attackers with complete source code disclosure without requiring authentication or elevated privileges. This information disclosure can lead to comprehensive understanding of application architecture, potential exploitation of other vulnerabilities, and unauthorized access to backend systems. The vulnerability specifically targets the .HTR ISAPI extension which is part of the server-side include processing functionality, making it particularly dangerous as it can expose not just the current file but potentially entire application frameworks and business logic. This type of vulnerability falls under CWE-502 which describes deserialization of untrusted data and CWE-200 which covers exposure of sensitive information.
Attackers can exploit this vulnerability through simple HTTP requests without requiring complex payloads or specialized tools, making it particularly dangerous as it can be leveraged by attackers with minimal technical expertise. The attack vector operates through the standard web request process, making it difficult to detect through traditional network monitoring. This vulnerability aligns with ATT&CK technique T1566 which covers spearphishing attachments and T1059 which covers command and script injection, as the source code disclosure can lead to further exploitation opportunities. The vulnerability also represents a weakness in the principle of least privilege and proper access control implementation within the web server configuration.
The recommended mitigation strategies include immediate patching of affected IIS versions, proper configuration of ISAPI extension mappings, and implementation of web application firewalls to detect and block suspicious URL patterns. Administrators should review and restrict access to sensitive file types, ensure proper file extension handling, and implement regular security assessments of web server configurations. The vulnerability highlights the importance of proper input validation and extension handling within web server environments, and organizations should consider implementing more restrictive ISAPI extension policies to prevent similar issues. Additionally, regular security updates and vulnerability assessments should be conducted to identify and remediate similar configuration flaws that could lead to information disclosure attacks.