Microsoft moves to counter macro-based malware threats in Office 365 apps

Microsoft this week announced that it is integrating its Antimalware Scan Interface (AMSI) in its Office 365 client apps. The integration will allow AMSI to detect malicious macros and scripts in Office documents, stop them from executing, and flag them for further inspection from antivirus applications (via OnMSFT.

"Macro-based threats have always been a prevalent entry point for malware, but we have observed a resurgence in recent years. Continuous improvements in platform and application security have led to the decline of software exploits, and attackers have found a viable alternative infection vector in social engineering attacks that abuse functionalities like VBA macros," Microsoft says in a blog post announcing the new feature.

In addition to making AMSI detection mechanisms available in Office 365 client apps, Microsoft is ensuring any antivirus application has access to its open interface.

When a potentially high-risk function or method (a trigger; for example, CreateProcess or ShellExecute) is invoked, Office halts the execution of the macro and requests a scan of the macro behavior logged up to that moment, via the AMSI interface. The AMSI provider (e.g., antivirus software) is invoked synchronously and returns a verdict indicating whether or not the observed behavior is malicious.

The list of high-risk functions or triggers are meant to cover actions at various stages of an attack chain (e.g., payload download, persistence, execution, etc.) and are selected based on their prevalence among malicious and benign macros. The behavior log sent over AMSI can include information like suspicious URLs from which malicious data was downloaded, suspicious file names known to be associated with malware, and others.

Upon detection of malicious behavior, Microsoft says it stops the macro execution immediately and notifies the user via the Office app interface. The application's session is then shut down to prevent any further damage.

This is an important addition to the Office 365 suite as macro-based attacks continue to become more prevalent. If you'd like to dive into all of the nitty-gritty details, Microsoft has a more technical rundown of how AMSI works through the Office 365 client applications in its full blog post. AMSI integration is now available in Word, Excel, PowerPoint, Access, Visio, and Publisher for Office 365 Monthly Channel releases.