Beware KB979917

If you are using claims based authentication on SharePoint 2010 you will eventually see the following error from the Health Analyzer.

Title Web Applications using Claims authentication require an update.  
Severity 1 – Error  
Category Security  
Explanation Web Applications using Claims authentication are at risk for a potential security vulnerability, which may allow users elevation of privileges. 
Remedy A security update is required on each server in the farm. For more information about this rule, see "".

The security update is KB979917 – QFE for Sharepoint issues – Perf Counter fix & User Impersonation and it introduces a pretty major change to the behavior of the affected SharePoint Web Applications that can break your code.

You can read about the ‘fixes’ in this update here:

The parts of interest here are:

Issue 1

You deploy some partially trusted Web parts on the SharePoint site. These Web parts have more permissions than they should have. This issue may create a security risk on the SharePoint site. For example, these Web parts may generate database requests or HTTP requests unexpectedly. This behavior creates a security risk.

Note Partially trusted Web parts are Web parts that are deployed to the Bin directory of a Web application.


Issue 1

This issue occurs because of an error in the ASP.NET 2.0 authentication component. The error causes the partially trusted Web parts to impersonate the application pool account. Therefore, the Web parts have full permission to access the SharePoint site.


Issue 1

This hotfix makes a new application setting available in ASP.NET 2.0. The new application setting is aspnet:AllowAnonymousImpersonation. You can enable this setting by adding the following section to the Web.config file:


<add key="aspnet:AllowAnonymousImpersonation" value="true" />


To enable this setting, you must have IIS 7 or IIS 7.5 running in Integrated mode. When this setting is enabled, the application runs under the security context of the IUSR identity.

This hotfix updates the .NET 2.0 framework’s System.Web assembly and makes a major change when aspnet:AllowAnonymousImpersonation is set to true in web.config. Instead of code running under the application pool account as is traditional, code now runs under NT Authority\IUSR – anonymous. If you have any code that depends on an authenticated identity or the specific app pool identity – SQL connections that use integrated security, for example, the code will break with Access Denied errors.

If you are doing any integration of legacy code based on ASP.NET 2.0, there is a good chance this security fix will break your code. The good news is you can turn it off by setting aspnet:AllowAnonymousImpersonation to false in web.config. The bad news is that, no matter how you change the value (by hand, PowerShell webconfigmod, or via a feature) if you edit the claim authentication provider settings in Central Admin, it will helpfully change it back to true.

So be careful, KB979917 is like a little bomb that has the potential to go off every once in a while to ruin your day and your user’s day.

Author: Doug Ware