Attack Surface Reduction feature in Windows Defender

Attack Surface Reduction is a feature of Windows Defender Exploit Guard that prevent actions that are used by exploit-seeking malware to infect computers. Windows Defender Exploit Guard is a new set of invasion prevention capabilities that Microsoft introduced as a part of Windows 10 v1709. The four components of Windows Defender Exploit Guard include:

One of the major capability, as mentioned above, is Attack Surface Reduction, that guard against common actions of malicious software that execute themselves on Windows 10 devices.

Let understand what is Attack Surface reduction and why it is so important.

Windows Defender Attack Surface Reduction feature

Emails and office applications are most crucial part of any enterprise’s productivity. They are the easiest way for cyber attackers to get entry to their PCs and networks and install malware. Hackers can directly use office macros and scripts to directly perform exploits that operate entirely in memory and are often undetectable by traditional Antivirus scans.

The worst thing is, that for a malware to get an entry, it just takes the user to enable macros on a legitimate-looking Office file, or to open an email attachment that can compromise the machine.

This is where Attack Surface Reduction comes to the rescue.

Advantages of Attack Surface Reduction

Attack Surface Reduction offers a set of built-in intelligence that can block the underlying behaviors used by these malicious documents to execute without hindering productive scenarios. By blocking malicious behaviors, independent of what the threat or exploit is, Attack Surface Reduction can protect enterprises from never before seen zero-day attacks, and balance their security risk and productivity requirements.

Windows Defender Attack Surface Reduction

ASR covers three main behaviors:

  1. Office apps
  2. Scripts and
  3. Emails

For Office apps, Attack Surface Reduction rule can:

  1. Block Office apps from creating executable content
  2. Block Office apps from creating child process
  3. Block Office apps from injecting code into another process
  4. Block Win32 imports from macro code in Office
  5. Block obfuscated macro code

Many a time malicious office macros can infect a PC by injecting and launching executables. Attack Surface Reduction can protect against this and also from DDEDownloader that has lately infected PCs across the World. This exploit uses the Dynamic Data Exchange popup in official documents to run a PowerShell downloader while creating a child process that ASR rule efficiently blocks!

For the script, Attack Surface Reduction rule can:

  • Block malicious JavaScript, VBScript, and PowerShell codes that have been obfuscated
  • Block JavaScript and VBScript from executing payload downloaded from internet

For email, ASR can:

  • Block execution of executable content dropped from email (webmail/mail-client)

Now a day, there has been a subsequent increase in spear-phishing and even an employee personal emails are targeted. ASR enables enterprise administrators to apply file policies on personal email for both webmail & mail-clients on company devices for protection from threats.

How Attack Surface Reduction works

ASR works through rules that are identified by their unique rule ID. In order to configure the state or mode for each rule, they can be managed with:

  • Group Policy
  • PowerShell
  • MDM CSPs

They can be used when only some rules are to be enabled or rules are to be enabled in individual mode.

For any line of business applications running within your enterprise, there is the capability to customize file and folder based exclusions if your applications include unusual behaviors that may be impacted by ASR detection.

Attack Surface Reduction requires Windows Defender Antivirus to be the main AV and it requires real-time protection feature to be enabled. Windows 10 Security baseline suggests most of the rules in block mode mentioned above should be enabled to secure your devices from any threats!

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Anand Khanse is the Admin of, a 10-year Microsoft MVP Awardee in Windows (2006-16) & a Windows Insider MVP. Please read the entire post & the comments first, create a System Restore Point before making any changes to your system & be careful about any 3rd-party offers while installing freeware.

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