.NET AntiXSS Library

NOTE: This content is a work in progress and all contribution is welcome. Please contact Jeff Knutson with questions, ideas, corrections, etc.

Overview

Cross site scripting (XSS) continues to show up on the as a top vulnerability every year. While very pervasive and dangerous, this vulnerability is possible to mitigate with reasonable developer effort. This page is dedicated to helping mitigate this vulnerability in regards to the Microsoft .NET Framework.

Attack Vectors

The primary XSS attack vectors are:

• Reflected XSS
• Persistent XSS

Please see Cross-site Scripting (XSS) for more detail regarding reflected and persistent XSS attacks.

Options

• Microsoft AntiXSS Library
  • Available in ASP.NET 4.5 in the System.Web.Security.AntiXss namespace
  • Available prior to ASP.NET 4.5 via NuGet: (https://www.nuget.org/packages/AntiXSS/) Install-Package AntiXSS (currently v4.2.1 as of 4/12/2014)
    • Using Microsoft AntiXSS as the default encoder in ASP.NET instructions (Phil Haack has a good link on this already: http://haacked.com/archive/2010/04/06/using-antixss-as-the-default-encoder-for-asp-net.aspx/)
• Microsoft Web Protection Library (WPL) - via http://wpl.codeplex.com/workitem/17246
  • there seem to be known issues with this library: http://blog.securityps.com/2012/12/alternatives-to-microsofts-wpl-sanitizer.html
• OWASP Anti-Samy Library (https://www.owasp.org/index.php/Category:OWASP_AntiSamy_Project_.NET)
  • Not recently maintained (good option for to get it up to date and relevant!!!)
• HtmlSanitizer
  • An open-source .Net library.
  • The HTML is cleaned with a white list approach.
  • All allowed tags and attributes can be configured.
  • The library is unit tested with the OWASP XSS Filter Evasion Cheat Sheet
  • https://github.com/mganss/HtmlSanitizer

XSS References

• https://www.owasp.org/index.php/Cross-site_Scripting_(XSS)
• http://www.troyhunt.com/2010/05/owasp-top-10-for-net-developers-part-2.html
• http://www.troyhunt.com/2011/12/free-ebook-owasp-top-10-for-net.html
• http://pluralsight.com/training/Courses/TableOfContents/owasp-top10-aspdotnet-application-security-risks (NOTE: This is a subscription course from PluralSight - I am not advocating any website or product, but found this to be useful and informative)
• http://devproconnections.com/aspnet/microsoft-adds-antixss-tool-aspnet-45
• https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet
• RSnake's XSS Cheatsheet (in XML format; nice for programatic consumption!)
• http://lockmedown.com/preventing-xss-in-asp-net-made-easy (A comprehensive guide to cover the full web stack against XSS)
• TODO add more references here -- Troy Hunt's material is excellent - I'm sure more people in the world have content which would be valuable here to reference...

TODO

Now

• Look at the Microsoft implementations
• See what work has already been done in the OWASP space for XSS
• See what other work has been done for XSS (both .NET and other technology stacks)
• Illustrate vulnerabilities and how to mitigate them (e.g. WebGoat.NET)
• See if we can get the OWASP Anti-Samy project back into relevance

Future

• Dream big here!