Threat actors can take advantage of a weakness that affects Microsoft Defender antivirus on Windows to learn locations excluded from scanning and plant malware there.
The issue has persisted for at least eight years, according to some users, and affects Windows 10 21H1 and Windows 10 21H2.
Like any antivirus solution, Microsoft Defender lets users add locations (local or on the network) on their systems that should be excluded from malware scans.
People commonly make exclusions to prevent antivirus from affecting the functionality of legitimate applications that are erroneously detected as malware.
Since the list of scanning exceptions differs from one user to another, it is useful information for an attacker on the system, since this gives them the locations where they can store malicious fileswithout fear of being detected.
Security researchers discovered that the list of locations excluded from Microsoft Defender scanning is unprotected and any local user can access it.
Regardless of their permissions, local users can query the registry and learn the paths that Microsoft Defender is not allowed to check for malware or dangerous files.
Antonio Cocomazzi, a SentinelOne threat researcher who is credited for reporting theRemotePotato0vulnerability,points outthat there is no protection for this information, which should be considered sensitive, and that running the “reg query” command reveals everything that Microsoft Defender is instructed not to scan, be it files, folders, extensions, or processes.
Another security expert,Nathan McNulty, confirmed that the issue is present on Windows 10 versions 21H1 and 21H2 but it does not affect Windows 11.
McNulty also confirmed that one can grab the list of exclusions from the registry tree with entries that store Group Policy settings. This information is more sensitive as it provides exclusions for multiple computers.
A security architect versed in protecting the Microsoft stack,McNulty warnsthat Microsoft Defender on a server has “automatic exclusions that get enabled when specific roles or features are installed” and these do not cover custom locations.
Although a threat actor needs local access to get the Microsoft Defender exclusions list, this is far from being a hurdle. Many attackers are already on compromised corporate networks looking for a way to move laterally as stealthily as possible.
By knowing the list of Microsoft Defender exclusions, a threat actor that already compromised a Windows machine can then store and execute malware from the excluded folders without fear of being spotted.
In tests done by BleepingComputer, a malware strain executed from an excluded folder ran unhindered on the Windows system and triggered no alert from Microsoft Defender.
We used a sample of Conti ransomware and when it executed from a normal location Microsoft Defender kicked in and blocked the malware.
After placing Conti malware in an excluded folder and running it from there, Microsoft Defender did not show any warning and did not take any action, allowing the ransomware to encrypt the machine.
This Microsoft Defender weakness is not new and has been highlighted publicly in the past by Paul Bolton:
A senior security consultant says that theynoticed the issue about eight years agoand recognized the advantage it provided to a malware developer.
"Always told myself that if I was some kind of malware dev I would just lookup the WD exclusions and make sure to drop my payload in an excluded folder and/or name it the same as an excluded filename or extension" -Aura
Given that it's been this long and Microsoft has yet to address the problem, network administrators should consult the documentation for properly configuring Microsoft Defenderexclusions on servers and local machinesvia group policies.
Microsoft Defender Firewall comes with the security suite
As an additional layer of protection, you get a firewall that helps keep sneaky hackers from gaining access to your device.
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Windows Defender and Microsoft Security Essentials are powerful scanning tools that find and remove malware from your PC.What are the disadvantages of Windows Defender? ›
- Lacks integrated dashboard for all devices using Windows Defender.
- No accountability if the computer is infected by malware.
- Limited features for large scale use.
- Slows down installation of frequently-used applications.
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How to protect against trojans. Use the following free Microsoft software to detect and remove it: Microsoft Defender Antivirus for Windows 10 and Windows 8.1, or Microsoft Security Essentials for previous versions of Windows. Microsoft Safety Scanner.How do I remove all malware from my computer? ›
- Step 1: Disconnect from the internet. ...
- Step 2: Enter safe mode. ...
- Step 3: Check your activity monitor for malicious applications. ...
- Step 4: Run a malware scanner. ...
- Step 5: Fix your web browser. ...
- Step 6: Clear your cache.
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|Malware Detection Test Results||Supported Platforms|
|Norton||100%||Windows, Mac, Android, iOS|
|McAfee||100%||Windows, Mac, Android, iOS|
|TotalAV||99%||Windows, Mac, Android, iOS|
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Open your Windows Security settings. Select Virus & threat protection > Scan options. Select Windows Defender Offline scan, and then select Scan now.Does wiping your PC remove all malware? ›
Running a factory reset, also referred to as a Windows Reset or reformat and reinstall, will destroy all data stored on the computer's hard drive and all but the most complex viruses with it. Viruses can't damage the computer itself and factory resets clear out where viruses hide.Can you tell if your computer is infected with malware? ›
Scan your device for malware.
Run a malware or security Delete anything it identifies as a problem. You may have to restart your device for the changes to take effect. Run your scan again to make sure everything is clear. If the scan shows there are no more issues, you've likely removed the malware.
Rootkits. The rootkit malware is dangerous and extremely hard to detect.Which malware can spy on you? ›
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Significant performance impact
AV-Comparatives' October 2021 Performance Test found Microsoft Defender to have a much stronger effect on performance than AVG AntiVirus FREE — hindering usability when copying files, archiving and unarchiving data, and installing apps.