Download and install the latest Service Pack which contains all fixes from previous patches. If you are running an older version, download and install the latest version and then the patch.
Workaround for prior versions:
Comment out the following <modules> from the CRM AppFx web.config file. These modules are not currently in use and can be safely commented out while we evaluate other potential long-term changes.
<add name="WSFederationAuthenticationModule" type="System.IdentityModel.Services.WSFederationAuthenticationModule, System.IdentityModel.Services, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler" />
<add name="SessionAuthenticationModule" type="System.IdentityModel.Services.SessionAuthenticationModule, System.IdentityModel.Services, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler" />