CU 6 for BizTalk Server 2013 R2 is out, details are here.
There are 16 issues fixed in this CU.
There are 6 fixes to do with the SAP adapter to do with the NCo connection type. So if you use that and are having issues you will probably want to release this to your test environments ASAP and re-test.
There are 2 fixes to do with File receive locations using alternate credentials.
2 fixes for the MIME/sMime pipeline component.
1 Fix each for the JSON decoder, MQ Series adapter, FTP adapter, message tracking and BizTalk DTA.
For some reason they have not yet updated the page Service Pack and cumulative update list for BizTalk Server to include this CU as yet.