##Update 30 Aug 2013##
The MS13-061 security update has been re-released for Exchange Server 2013, the download links below have been updated.
##Update 15 Aug 2013##
The MS13-061 security update for Exchange Server 2013 has been pulled as it seems to cause an issue with the Content Index for mailbox databases. If you have already installed this update see KB2879739 for information on how to resolve this. This issue does not occur in Exchange 2010 or Exchange 2007.
A few hours ago Microsoft released a critical security update for all supported editions of Microsoft Exchange Server 2007, 2010, and 2013. This security update addresses vulnerabilities that exist in the WebReady Document Viewing and Data Loss Prevention features of Exchange that could allow remote code execution in the security context of the transcoding service on the Exchange server if a user previews a specially crafted file using Outlook Web App. For detailed information, see Microsoft Security Bulletin MS13-061
Microsoft recommends that customers apply the update immediately. The update can be downloaded at the following links: