CVE-2004-0847 in ASP.NET
Summary
by MITRE
The Microsoft .NET forms authentication capability for ASP.NET allows remote attackers to bypass authentication for .aspx files in restricted directories via a request containing a (1) "\" (backslash) or (2) "%5C" (encoded backslash), aka "Path Validation Vulnerability."
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Analysis
by VulDB Data Team • 07/07/2025
The vulnerability described in CVE-2004-0847 represents a critical path validation flaw within Microsoft's ASP.NET framework that specifically affects the forms authentication mechanism. This security weakness exists in the way the system processes directory traversal attempts when validating user authentication for web applications. The vulnerability stems from insufficient input validation of URL paths, particularly when backslash characters are present in the request URI. Attackers can exploit this by crafting malicious requests that contain either a literal backslash character or its URL-encoded equivalent %5C, which allows them to bypass authentication checks and gain unauthorized access to restricted .aspx files. The flaw is particularly dangerous because it operates at the core authentication layer of ASP.NET applications, potentially enabling attackers to access sensitive administrative pages or restricted resources without proper credentials. This issue affects the fundamental security model of web applications built on the Microsoft .NET platform, undermining the trust model that authentication systems are designed to provide.
The technical implementation of this vulnerability exploits the underlying path validation logic within ASP.NET's authentication subsystem. When the framework processes incoming requests containing backslash characters in the path, it fails to properly normalize or validate the path structure, allowing attackers to manipulate the authentication flow. The system's path resolution mechanism does not adequately sanitize input containing these special characters, which can cause the authentication module to misinterpret the requested resource path. This misinterpretation results in the bypass of directory access controls that should normally prevent unauthorized access to restricted content. The vulnerability operates by leveraging the way ASP.NET handles path resolution during authentication checks, where the presence of backslash characters can cause the system to traverse directories in unexpected ways. According to CWE-22, this maps to a Path Traversal vulnerability, where improper input validation allows attackers to access files or directories outside the intended scope. The attack vector specifically targets the authentication bypass mechanism rather than directly exploiting a code execution flaw, making it particularly insidious as it operates within the legitimate security framework.
The operational impact of CVE-2004-0847 extends beyond simple unauthorized access, creating potential for significant data breaches and system compromise within affected environments. Organizations running ASP.NET applications with vulnerable forms authentication configurations face the risk of attackers gaining access to sensitive administrative interfaces, user data, or confidential application resources. The vulnerability can be exploited across multiple attack surfaces including web applications that implement directory-based access controls, authentication portals, and any ASP.NET application that relies on forms authentication for access management. Security implications include potential privilege escalation, data exfiltration, and the ability to modify or delete sensitive application components. The attack can be executed remotely without requiring any special privileges or local system access, making it particularly dangerous for web-facing applications. From an ATT&CK perspective, this vulnerability maps to techniques involving privilege escalation and defense evasion, as attackers can bypass authentication mechanisms to access restricted system resources. The impact is particularly severe in environments where ASP.NET applications are used for business-critical functions or contain sensitive user information, as the vulnerability can lead to complete system compromise if not properly addressed.
Mitigation strategies for CVE-2004-0847 require immediate implementation of both software patches and configuration hardening measures. Microsoft released security updates that address the path validation issue in affected versions of the .NET Framework, and organizations should prioritize installing these patches as part of their vulnerability management process. In addition to patching, administrators should implement input validation controls at the web server level, particularly for URL paths containing special characters including backslashes. Web application firewalls can be configured to block or normalize requests containing potentially malicious path sequences before they reach the application server. The implementation of proper directory access controls and the use of secure authentication frameworks that properly sanitize input paths can help prevent exploitation of this vulnerability. Organizations should also conduct thorough security assessments of their ASP.NET applications to identify any additional path traversal vulnerabilities that may exist in their codebase. Configuration reviews should include validation of authentication settings, directory permissions, and input handling mechanisms to ensure that path validation occurs at multiple layers of the application stack. Regular security monitoring and log analysis should be implemented to detect any suspicious authentication bypass attempts that may indicate exploitation of this vulnerability. The remediation process should also include comprehensive testing to ensure that the applied fixes do not introduce regressions in application functionality while maintaining the security posture against path traversal attacks.