WCF: Adding BehaviorExtensions generates a security exception

Question

When adding a <behaviorExtensions> section to the web.config file of my project I get a security exception (see below). What can be the problem? I follow this solution to another issue: link

Adding this:

<extensions>
  <behaviorExtensions>
    <add name="customWebHttp" type="CloudService4_WebRole.CustomHttpBehaviorExtensionElement, CloudService4_WebRole, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
  </behaviorExtensions>
</extensions>

results in:

Exception Details: System.Security.SecurityException: Request for the permission of type 'System.Configuration.ConfigurationPermission, System.Configuration, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' failed.