CVE-2000-0246 in IIS
Summary
by MITRE
IIS 4.0 and 5.0 does not properly perform ISAPI extension processing if a virtual directory is mapped to a UNC share, which allows remote attackers to read the source code of ASP and other files, aka the "Virtualized UNC Share" vulnerability.
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Analysis
by VulDB Data Team • 10/18/2025
The CVE-2000-0246 vulnerability represents a critical security flaw in Microsoft Internet Information Services versions 4.0 and 5.0 that stems from improper handling of ISAPI extension processing when virtual directories are mapped to UNC (Universal Naming Convention) shares. This vulnerability operates at the intersection of web server configuration management and security controls, creating a dangerous condition where remote attackers can exploit the misconfiguration to gain unauthorized access to sensitive source code files. The flaw specifically manifests when IIS servers are configured with virtual directories that point to network shares, creating a pathway for information disclosure attacks that directly violates fundamental security principles of access control and data protection.
The technical root cause of this vulnerability lies in the way IIS handles authentication and authorization when processing requests to virtual directories mapped to UNC shares. When a user accesses a web application through a virtual directory pointing to a UNC path, the server fails to properly validate the security context and authentication state before processing ISAPI extensions. This misconfiguration allows attackers to bypass normal access controls and directly request ASP files or other sensitive web content, enabling them to read the source code of applications rather than executing them as intended. The vulnerability specifically impacts the authentication delegation mechanisms within IIS, creating a scenario where the server fails to properly enforce security boundaries between different access levels and user contexts.
The operational impact of CVE-2000-0246 extends far beyond simple information disclosure, as it provides attackers with access to complete application source code including database connection strings, business logic implementations, and other sensitive code elements that can be leveraged for further attacks. This vulnerability directly violates multiple security principles including the principle of least privilege and defense in depth, as it allows unauthorized access to resources that should be protected by proper authentication and authorization controls. The exposure of ASP source code can reveal critical information such as database credentials, application logic, and internal architecture details that significantly increase the attack surface for subsequent exploitation attempts. From an attacker perspective, this vulnerability maps to several ATT&CK tactics including T1566 (Phishing for Information) and T1083 (File and Directory Discovery) while aligning with CWE-200 (Information Exposure) and CWE-284 (Improper Access Control) classifications.
Mitigation strategies for this vulnerability must address both immediate configuration fixes and long-term architectural improvements to prevent similar issues from occurring in web server deployments. Organizations should immediately disable virtual directory mappings to UNC shares when possible, or implement proper authentication and authorization controls for such mappings. The recommended approach involves configuring appropriate access control lists on the UNC shares themselves, ensuring that only authorized users and systems can access the shared resources. Additionally, implementing proper network segmentation and firewall rules can help limit access to the affected web servers and reduce the potential impact of successful exploitation attempts. Security professionals should also consider implementing web application firewalls and monitoring solutions to detect and prevent exploitation attempts targeting this specific vulnerability. The vulnerability demonstrates the critical importance of proper security configuration management and the dangers of default configurations that may appear convenient but create security risks, aligning with best practices outlined in security frameworks such as NIST SP 800-53 and ISO 27001 for secure system configuration and access control management.