Antimalware Engine 1.1.10502.0 was released to all Microsoft Security Essentials, Forefront Client Security, Forefront Endpoint Protection, Windows Intune Endpoint Protection, and System Center Endpoint Protection customers on 17 April 2014. Signature package 126.96.36.199 is the first that contains this engine.
This is due to an update that was shipped on April 15, 2014 that may have caused interrupted service for some customers using Microsoft security products. This was corrected via signature update, which automatically resolved the issue, and customers who have deployed the most recent signatures do not need to take any action. Customers who deployed workarounds such as disabling Behavior Monitoring or any other features should revert those changes.
While the issue primarily impacted customers running Microsoft security products on Windows XP and Windows Server 2003, it may have also impacted other supported operating system versions. Microsoft continues to supply antimalware engine and signature updates for Windows XP systems which is supported through July 14, 2015.