Today sees the release of AntiXSS v4.2 in order to address MS12-007. As AntiXSS is a developer tool developers need to download the latest version, test, then deploy the web sites using the library. nuget has also updated – if you’ve added AntiXSS via nuget you’ll need to update the package.
It is recommended you test and apply the new version as soon as possible.
The vulnerability only affects the HTML sanitizer. The sanitizer has been changed to remove all CSS it encounters, this new behaviour means that if you were expect CSS formatting to remain after sanitization this is no longer the case.
In addition to the change necessary to correct the vulnerability there are a few new features;
<httpRuntime encoderType="Microsoft.Security.Application.AntiXssEncoder, AntiXssLibrary"/>
The nuget package does not swap out the encoder as nuget configuration transforms can’t be made framework version specific yet so you will have to do that manually. Source code will uploaded to codeplex within the next few days.
Remember that downloading the new version is not enough – you will need to update your projects to use the new version then publish them to your web servers.
This release has also merged code from the .NET framework, taking some of their hard work in integrating the core AntiXSS functions into v4.5. There are some performance improvements in encoding.