DTOO127 - Add-ins to Office applications must be signed by a Trusted Publisher.
Office 2013 applications do not check the digital signature on application add-ins before opening them. Disabling or not configuring this setting may allow an application to load a dangerous add-in. As a result, malicious code could become active on user computers or the network.
Set the policy User Configuration -> Administrative Templates -> Microsoft Project 2013 -> Project Options -> Security -> Trust Center -> 'Require that application add-ins are signed by Trusted Publisher' to 'Enabled'.