CVE-2003-0768 in ASP.NET
Summary
by MITRE
Microsoft ASP.Net 1.1 allows remote attackers to bypass the Cross-Site Scripting (XSS) and Script Injection protection feature via a null character in the beginning of a tag name.
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Analysis
by VulDB Data Team • 03/13/2017
Microsoft ASP.NET 1.1 contains a critical security vulnerability that undermines its core Cross-Site Scripting (XSS) protection mechanisms through a sophisticated bypass technique involving null character manipulation. This vulnerability resides in the HTML encoding and validation logic that processes user input before rendering web content, specifically targeting the way the framework handles tag names and their subsequent validation routines. The flaw exploits a fundamental weakness in the input sanitization process where the presence of a null character at the beginning of a tag name can cause the validation system to incorrectly interpret the tag structure, effectively neutralizing the security controls designed to prevent malicious script injection. The vulnerability manifests when ASP.NET processes HTML content containing malformed tag names with leading null characters, allowing attackers to craft payloads that bypass the built-in protection mechanisms.
The technical execution of this attack leverages the specific behavior of the .NET Framework's HTML parsing engine, which fails to properly normalize or validate tag names that begin with null characters. When a null character (0x00) appears at the start of a tag name, the parsing logic may misinterpret the tag structure, causing the security validation to either skip critical checks or apply incorrect validation rules. This occurs because the validation routines in ASP.NET 1.1 do not adequately handle null-terminated strings or properly sanitize input before processing, creating a gap in the security boundary that attackers can exploit. The vulnerability specifically affects the HTTP response encoding and validation components, where the framework's attempt to prevent script execution through XSS protection fails when encountering these specially crafted null character sequences.
The operational impact of CVE-2003-0768 extends beyond simple script injection attacks, as it represents a fundamental flaw in the security architecture of ASP.NET 1.1 applications. Attackers can leverage this vulnerability to execute malicious scripts in the context of a victim's browser, potentially leading to session hijacking, data theft, or unauthorized actions within the application. The bypass mechanism allows threat actors to circumvent security measures that would normally prevent malicious input from being executed, making it particularly dangerous for web applications handling sensitive user data or implementing authentication mechanisms. This vulnerability affects the core security model of ASP.NET applications, potentially compromising the integrity of user sessions and the confidentiality of data transmitted through web forms and user input fields.
Organizations running ASP.NET 1.1 applications must implement immediate mitigations to address this vulnerability, including deploying the official Microsoft security patches that correct the HTML parsing and validation logic. The recommended approach involves upgrading to patched versions of the .NET Framework or implementing additional input validation layers that explicitly check for and reject null character sequences in tag names. Security teams should also consider implementing web application firewalls that can detect and block suspicious null character patterns in HTTP requests, as well as enhancing logging and monitoring capabilities to identify potential exploitation attempts. From a compliance perspective, this vulnerability aligns with CWE-79 (Cross-site Scripting) and represents a critical weakness in the application's input validation and output encoding controls. The ATT&CK framework categorizes this as a technique involving input validation bypass, where attackers exploit implementation flaws in security controls to achieve unauthorized access or data manipulation, emphasizing the need for robust application-level protections and proper security architecture design.