For Exchange, this Rollup fixes unexpected authentication prompts when using OWA published using Forefront Threat Management Gateway (TMG) 2010 in conjunction with RSA authentication and Forms-based Authentication (FBA). For a full list of changes, consult knowledgebase article kb2735208.
This again proves TMG isn’t “dead” since it received it’s End-of-Life status. So again, when you got TMG, don’t panic: For TMG, support will end on April 14th, 2015 and extended support will end on April 14th, 2020. You have some time to look into alternatives.
You can request Forefront TMG SP2 RU3 directly from support here.