CU5 for BizTalk 2013 is available as per this announcement  and can be found here at kb 3201943

One of the main fixes that we were wanting is KB 3121493 FIX: WCF-HTTP send doesn’t retry or suspend when it receives an error 500 in BizTalk Server 2013 R2, as that bug meant you had to check the https status code manually and then treat it as an error as mentioned in my BizTalk 2013 R2 known bugs, issues & quirks

Other fixes are a mixed bag for SAP/NCO, IDOC, MIME, MQSeries, Windows Sharepoint Service.

A list of all CU’s for all versions of BizTalk back to 2006 R2 can be found at kb 2555986 Service Pack and cumulative update list for BizTalk Server

Also see How to check the CU level of BizTalk Server quickly