CVE-2008-3843 in .NET Framework
Summary
by MITRE
Request Validation (aka the ValidateRequest filters) in ASP.NET in Microsoft .NET Framework with the MS07-040 update does not properly detect dangerous client input, which allows remote attackers to conduct cross-site scripting (XSS) attacks, as demonstrated by a query string containing a "<~/" (less-than tilde slash) sequence followed by a crafted STYLE element.
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Analysis
by VulDB Data Team • 08/02/2021
The vulnerability described in CVE-2008-3843 represents a critical flaw in the ASP.NET Request Validation mechanism within Microsoft .NET Framework versions affected by the MS07-040 update. This weakness specifically targets the ValidateRequest filters that are designed to prevent malicious input from being processed by web applications. The vulnerability allows attackers to bypass the standard input validation controls that are meant to protect against cross-site scripting attacks by exploiting a specific pattern in client input that the validation system fails to properly detect.
The technical implementation of this vulnerability stems from the improper detection of dangerous client input sequences within the ASP.NET request validation framework. Attackers can exploit this by crafting query strings containing the specific "<~/" sequence followed by a malicious STYLE element, which the validation filters fail to recognize as potentially harmful. This particular pattern circumvents the normal validation checks that would typically block such XSS attempts, allowing malicious code to be injected into web applications. The flaw exists in the parsing logic of the validation system, where certain combinations of characters are not properly identified as dangerous input patterns, creating a bypass mechanism that can be reliably exploited.
The operational impact of this vulnerability is severe and directly enables remote attackers to conduct cross-site scripting attacks against vulnerable web applications. When successfully exploited, the vulnerability allows attackers to inject malicious scripts into web pages viewed by other users, potentially leading to session hijacking, data theft, or unauthorized actions performed on behalf of users. The attack vector through query strings makes this particularly dangerous as it can be easily executed through standard web browser interactions without requiring complex exploitation techniques. This vulnerability essentially undermines the core security protection mechanisms that ASP.NET applications rely on to prevent XSS attacks, making it a significant concern for web application security.
The vulnerability aligns with CWE-79 which specifically addresses Cross-Site Scripting flaws in web applications, and demonstrates how input validation bypasses can lead to serious security consequences. From an ATT&CK perspective, this vulnerability maps to the T1190 technique for Exploit Public-Facing Application, as it represents a method for attacking web applications through input validation weaknesses. Organizations running affected versions of the .NET Framework are particularly vulnerable to this type of attack, as the vulnerability affects the fundamental security controls that protect web applications from malicious input. The impact is particularly severe because it affects the core framework functionality rather than individual application code, meaning that applications deployed on affected systems are automatically vulnerable regardless of their own security implementation.
The recommended mitigation strategies for this vulnerability include applying the MS07-040 security update from Microsoft, which specifically addresses the validation bypass issue in the ASP.NET framework. Organizations should also implement additional defensive measures such as comprehensive input sanitization at multiple layers, including application-level validation, proper output encoding, and implementation of Content Security Policy headers. Regular security assessments and vulnerability scanning should be conducted to identify any remaining instances of the vulnerability within web applications, particularly those using older versions of the .NET Framework that may not have received the specific patch. Additionally, developers should adopt secure coding practices that include thorough input validation and output encoding to reduce the risk of exploitation even if the framework-level vulnerability is not immediately patched.