Microsoft Defender Will Soon Block Windows Password Theft

I’ve argued for a while that Microsoft Defender is free and does a great job of defending you against the evils of the Internet. It’s now about to get better. Microsoft is enabling a Microsoft Defender ‘Attack Surface Reduction’ security rule by default to block hackers’ attempts to steal Windows credentials from the LSASS process:

When threat actors compromise a network, they attempt to spread laterally to other devices by stealing credentials or using exploits. One of the most common methods to steal Windows credentials is to gain admin privileges on a compromised device and then dump the memory of the Local Security Authority Server Service (LSASS) process running in Windows. This memory dump contains NTLM hashes of Windows credentials of users who had logged into the computer that can be brute-forced for clear-text passwords or used in Pass-the-Hash attacks to login into other devices. While Microsoft Defender block programs like Mimikatz, a LSASS memory dump can still be transferred to a remote computer to dump credentials without fear of being blocked. 

To prevent threat actors from abusing LSASS memory dumps, Microsoft has introduced security features that prevent access to the LSASS process. One of these security features is Credential Guard, which isolates the LSASS process in a virtualized container that prevents other processes from accessing it. However, this feature can lead to conflicts with drivers or applications, causing some organizations not to enable it. As a way to mitigate Windows credential theft without causing the conflicts introduced by Credential Guard, Microsoft will soon be enabling a Microsoft Defender Attack Surface Reduction (ASR) rule by default. The rule, ‘ Block credential stealing from the Windows local security authority subsystem,’ prevents processes from opening the LSASS process and dumping its memory, even if it has administrative privileges. 

While enabling the ASR rule by default will significantly impact the stealing of Windows credentials, it is not a silver bullet by any means. This is because the full Attack Surface Reduction feature is only supported on Windows Enterprise licenses running Microsoft Defender as the primary antivirus. However, BleepingComputer’s tests show that the LSASS ASR rule also works on Windows 10 and Windows 11 Pro clients. Unfortunately, once another antivirus solution is installed, ASR is immediately disabled on the device. Furthermore, security researchers have discovered built-in Microsoft Defender exclusion paths allowing threat actors to run their tools from those filenames/directories to bypass the ASR rules and continue to dump the LSASS process. Mimikatz developer Benjamin Delpy told BleepingComputer that Microsoft probably added these built-in exclusions for another rule, but as exclusions affect ALL rules, it bypasses the LSASS restriction.

This is a great move on the part of Microsoft as this will make things better. But like everything else in the universe, there is a catch:

While enabling the ASR rule by default will significantly impact the stealing of Windows credentials, it is not a silver bullet by any means.

This is because the full Attack Surface Reduction feature is only supported on Windows Enterprise licenses running Microsoft Defender as the primary antivirus. However, BleepingComputer’s tests show that the LSASS ASR rule also works on Windows 10 and Windows 11 Pro clients.

Unfortunately, once another antivirus solution is installed, ASR is immediately disabled on the device.

Furthermore, security researchers have discovered built-in Microsoft Defender exclusion paths allowing threat actors to run their tools from those filenames/directories to bypass the ASR rules and continue to dump the LSASS process.

Still it’s a great first step by Microsoft. Let’s hope that they improve on this to remove the negatives and make this a positive for all.

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