CVE-2000-0457 in IIS
Summary
by MITRE
ISM.DLL in IIS 4.0 and 5.0 allows remote attackers to read file contents by requesting the file and appending a large number of encoded spaces (%20) and terminated with a .htr extension, aka the ".HTR File Fragment Reading" or "File Fragment Reading via .HTR" vulnerability.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 05/24/2025
The CVE-2000-0457 vulnerability represents a critical file reading flaw in Microsoft Internet Information Services versions 4.0 and 5.0 that specifically targets the ISM.DLL component. This vulnerability operates through a sophisticated manipulation of URL encoding techniques that exploits how IIS handles file requests with specific extensions. The flaw allows remote attackers to access sensitive file contents by crafting malicious requests that append numerous encoded space characters followed by a .htr extension. The vulnerability is categorized under CWE-22 as a "Path Traversal" attack vector, which directly relates to improper input validation and inadequate sanitization of file paths. This particular weakness in IIS implementation enables attackers to bypass normal access controls and retrieve file contents that should otherwise remain protected.
The technical execution of this vulnerability relies on the specific handling of .htr files within IIS's ISM.DLL module. When a request is made for a file with a .htr extension, the system processes the request through a fragment reading mechanism that fails to properly validate the encoded space characters. Attackers can exploit this by appending multiple %20 encoded spaces to a file path and then terminating with .htr, causing the server to interpret the request in a way that reveals file contents from unintended locations. The vulnerability operates at the application layer of the OSI model and specifically targets the web server's file system access controls. This type of attack falls under the ATT&CK technique T1083 for "File and Directory Discovery" and T1566 for "Phishing" as it can be used to gather sensitive information that might be used in further attacks. The flaw demonstrates a fundamental weakness in IIS's request parsing and path resolution logic, where encoded characters are not properly normalized or validated before file access operations occur.
The operational impact of this vulnerability extends far beyond simple information disclosure, as it can lead to complete system compromise when combined with other attack vectors. Attackers can use this technique to access configuration files, source code, database connection strings, and other sensitive data that may be stored in the web server's file system. The vulnerability affects any system running IIS 4.0 or 5.0 where ISM.DLL is installed and active, making it particularly dangerous in enterprise environments where these older versions might still be in use. The attack can be executed remotely without requiring authentication, making it a significant threat to web server security. This vulnerability can be classified under the Common Vulnerability Scoring System as having a high severity rating due to its remote exploitability and potential for data exposure. The attack pattern can be automated and scaled, allowing for mass reconnaissance of vulnerable systems and systematic extraction of sensitive information.
Mitigation strategies for CVE-2000-0457 involve both immediate patching and architectural defenses. Microsoft released security updates to address this specific vulnerability in later versions of IIS, and organizations should ensure they are running patched versions of the software. The recommended approach includes implementing proper input validation at the web server level, where encoded characters in URLs are normalized and validated before file access operations. Network-level defenses such as web application firewalls can be configured to detect and block requests with excessive encoded spaces or suspicious .htr file extensions. Organizations should also implement proper file system access controls and limit the permissions of web server processes to prevent unauthorized file access even if the vulnerability is exploited. The vulnerability highlights the importance of proper security configuration management and demonstrates the risks associated with running legacy web server software. Additionally, implementing security monitoring and log analysis can help detect exploitation attempts of this vulnerability, as the malicious requests will generate specific patterns in web server logs that can be correlated with potential security incidents.