Type packetstorm
Reporter Adrian Pastor
Modified 2007-04-07T00:00:00


The following are the technical details for the Microsoft .NET request filtering bypass vulnerability (BID 20753):  
ProCheckUp Security Bulletin   
This advisory has been published following consultation with UK CPNI (formally known as NISCC)   
Title: Microsoft ASP.NET request filtering can be bypassed allowing XSS and HTML injection attacks  
CERT: None  
Date found: 7 July, 2006  
The following client/server environment was tested and found vulnerable:  
- Microsoft Windows Server 2003 Standard Edition Build 3790.srv03_sp1_rtm.050324-1447 Service Pack 1  
- Microsoft IIS 6.0  
- Microsoft ASP .NET Framework Version 2.0.50727.42  
- Microsoft Internet Explorer 6.0.2900.2180.xpsp_sp2_gdr.050301-1519  
- Microsoft Internet Explorer 7.0.5450.4 Beta 3  
- Microsoft Internet Explorer 7.0.5730.11  
Severity: Medium  
Credits: request filtering bypass found by Richard Brain and further researched by Jan Fry and Adrian Pastor  
Vendor Status: N/A  
CVE Candidate: Not assigned  
By understanding how ASP .NET malicious request filtering functions, ProCheckUp has found that it is possible to bypass ASP .NET request filtering and perform XSS and HTML injection attacks.  
It was possible to perform redirect, cookie theft, and unrestricted HTML injection attacks against an ASP .NET application setup in a test environment. ProCheckUp has also found this issue to be exploitable while carrying out penetration tests on several customer's live environments.  
In order to exploit this flaw, an attacker would need to target a .NET server-side application which does not sanitize input parameters properly before being returned back to the web browser.  
Proof of concept:  
In the following examples 'vuln-search.aspx' is a script that solely relies on ASP .NET request filtering, and returns user-supplied input back to the browser.  
Alert box injection - simply provided for testing purposes (may cause DoS issues on Internet Explorer)  
Redirection Attack  
Cookie stealing  
Unrestricted HTML injection from external '.js' file  
where 'xss.js' could contain a snippet that overwrites the entire document's HTML body. i.e.:   
document.body.innerHTML = '<b>since we can now insert brakets without having to escape the request filtering, we\'re free to insert any HTML tags</b></br><form name="myform" action=""><input type="text" name="login"><br/><input type="password" name="password"></br><input type="submit" value="Login"></form>';myform.login.focus();  
Attackers can hijack user accounts through XSS and HTML injection attacks against vulnerable applications that solely rely on ASP .NET request filtering.  
Do not rely on ASP .NET filtering protection, sanitize all input parameters on server side applications. Follow a whitelisting approach when performing input filtering.  
Copyright 2007 Procheckup Ltd. All rights reserved.  
Permission is granted for copying and circulating this Bulletin to the Internet community for the purpose of alerting them to problems, if and only if, the Bulletin is not edited or changed in any way, is attributed to Procheckup, and provided such reproduction and/or distribution is performed for non-commercial purposes.  
Any other use of this information is prohibited. Procheckup is not liable for any misuse of this information by any third party.