Microsoft’s Open Office XML (OOXML) specification defines an XML-based format for Office documents (.docx, xlsx, .pptx) to replace older binary formats (.doc, .xls, .ppt). OOXML files are packed together ZIP archives compromised of various XML files, referred to as parts, containing properties that collectively define how a document is rendered. 
Properties within parts may reference shared public resources accessed via online URLs. For example, template properties reference a file, serving as a pre-formatted document blueprint, that is fetched when the document is loaded.
Adversaries may abuse this technology to initially conceal malicious code to be executed via documents (i.e. Scripting). Template references injected into a document may enable malicious payloads to be fetched and executed when the document is loaded. These documents can be delivered via other techniques such as Spearphishing Attachment and/or Taint Shared Content and may evade static detections since no typical indicators (VBA macro, script, etc.) are present until after the malicious payload is fetched.  Examples have been seen in the wild where template injection was used to load malicious code containing an exploit. 
Consider disabling Microsoft Office macros/active content to prevent the execution of malicious payloads in documents , though this setting may not mitigate the Forced Authentication use for this technique.
Because this technique involves user interaction on the endpoint, it's difficult to fully mitigate. However, there are potential mitigations including training users to identify social engineering techniques and spearphishing emails. Network/Host intrusion prevention systems, antivirus, and detonation chambers can be employed to prevent documents from fetching and/or executing malicious payloads. 
Analyze process behavior to determine if an Office application is performing actions, such as opening network connections, reading files, spawning abnormal child processes (ex: PowerShell), or other suspicious actions that could relate to post-compromise behavior.
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- Microsoft. (n.d.). Enable or disable macros in Office files. Retrieved September 13, 2018.