CVE-2000-0024 in IIS
Summary
by MITRE
IIS does not properly canonicalize URLs, potentially allowing remote attackers to bypass access restrictions in third-party software via escape characters, aka the "Escape Character Parsing" vulnerability.
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Analysis
by VulDB Data Team • 04/20/2026
The CVE-2000-0024 vulnerability represents a critical flaw in Microsoft Internet Information Services version 5.0 and earlier, specifically within the URL parsing mechanism that fails to properly canonicalize incoming requests. This vulnerability stems from the web server's inability to correctly process and normalize URL paths that contain escape sequences, creating opportunities for attackers to manipulate the request processing pipeline. The issue manifests when the IIS server encounters URL-encoded characters or special sequences that should be normalized but are instead processed in their encoded form, allowing unauthorized access to protected resources.
The technical root cause of this vulnerability lies in the improper handling of URL canonicalization within the IIS web server architecture. When a client submits a request containing escape characters such as backslashes or other special sequences, the server fails to properly resolve these sequences to their canonical form before performing access control checks. This parsing deficiency enables attackers to craft malicious URLs that appear to point to restricted directories or files while actually resolving to accessible locations within the server's file system. The vulnerability specifically affects the way IIS processes paths that contain sequences like double dots or backslash characters that should be normalized during URL processing.
From an operational impact perspective, this vulnerability creates significant security risks for organizations running affected IIS versions, as it allows remote attackers to bypass authentication and authorization mechanisms that are typically enforced by third-party applications and security modules. Attackers can exploit this weakness to access restricted areas of web applications, potentially gaining access to sensitive data, administrative interfaces, or system resources that should be protected. The vulnerability is particularly dangerous because it operates at the web server level, meaning that even applications with their own security controls can be bypassed if they rely on IIS's URL parsing for access restriction enforcement.
The attack surface for this vulnerability extends beyond simple file access restrictions, as it can be leveraged to circumvent various security controls implemented by third-party software running on the IIS platform. Security researchers have identified that this vulnerability aligns with CWE-174, which describes the weakness of insufficient canonicalization of path names, and can be mapped to ATT&CK technique T1078.1.1 for Valid Accounts and T1566.001 for Phishing as attackers often use this vulnerability as part of broader exploitation campaigns. The vulnerability also intersects with CWE-22, which covers improper limitation of a pathname to a restricted directory, making it a prime target for directory traversal attacks that can be amplified through the escape character parsing flaw.
Organizations affected by this vulnerability should implement immediate mitigations including upgrading to patched versions of IIS, applying the Microsoft security bulletin MS00-005, and implementing additional URL validation controls at the application level. Network segmentation and proper access control lists should be deployed to limit the impact of potential exploitation, while monitoring systems should be configured to detect unusual URL patterns that may indicate attempts to exploit this vulnerability. The remediation process requires careful consideration of third-party applications that may be affected by the URL canonicalization changes, as some software may rely on the vulnerable behavior for legitimate operations. Regular security assessments and penetration testing should be conducted to ensure that the implemented fixes do not introduce regressions in application functionality while maintaining effective protection against this specific class of attack.