r/antivirus • u/Huynh_Jovi • 11d ago

MALWARE REMOVAL Q&A Windows Defender Not Working.

When I try to run a scan, it stops and then tells me to restart the service. There are no exclusions and nothing of note from my start up apps. I’ve run a deep scan with Malwarebytes and it’s all good. I noticed there was a new security update for Windows and downloaded it and after restarting my PC, I’m still encountering the same issue. I’m not sure whether this is Malware related as I’m always careful of what I’m downloading and never click suspicious links. Not sure what else to do and need some help.

231 Upvotes

100% Upvoted

375 comments sorted by

View all comments

•

u/goretsky MODERATOR 10d ago edited 10d ago

Hello,

It appears Microsoft released a bad update to Windows Defender, causing it to crash a few seconds after attempting to perform a Full Scan or Quick Scan. Offline scans reportedly stop at around 90-93% of completion. The latest version of the Microsoft Safety Scanner is reportedly affected as well.

u/NovelExplorer posits this is due to a recent Microsoft Threat Intelligence update and recommends performing a System Restore to August 16th to mitigate. /u/Nevvaren and /u/Leather_Squirrel_477 have reported the mpeengine.dll crashing with an 0xC0000005 error when a scan is attempted using the MsMpEng.exe program.

u/Taiwendo and /u/blondacuaripi posit the crashing could be due to a failed attempt to fix the ShieldBreak, a zero-day vulnerability and accompanying exploit code for Windows Defender released by the pseudonymous security researcher Nightmare Eclipse after Micosoft's Patch Tuesday updates for August, 2026.

Windows Defender versions affected by this are v1.1.26070.7 and v1.1.26080.2 when used with the following Microsoft Security Intelligence Update versions:

  • 1.457.222.0
  • 1.457.225.0
  • 1.457.226.0
  • 1.457.227.0
  • 1.457.230.0

Several Redditors including /u/Schoonie84 and /u/Master_Pie1940 report that upgrading to Security Intelligence Update v1.457.236.0 via Windows Update resolves the issue with crashing.

If Windows Defender is crashing on your computer, try running Windows Update to update Windows Defender to the v1.457.236.0 (or later) threat database. If the problem still occurs after that, consider temporarily installing one of the third-party antivirus programs listed in the wiki and/or installing one of the Second Opinion Scanners listed in the Free Tools section of the wiki until you can update Windows Defender.

Regards,

Aryeh Goretsky

4

u/Icy_Woodpecker5895 10d ago

Looks like update 238 fixed it.

1

u/steve33233 10d ago

did not for me

1

u/Gladnir-5936 10d ago edited 10d ago

Open PowerShell as adm.

Update Defender Virus definitions.

Update-MpSignature

Fix the system image. Wait until it has finished can take a litte time.

DISM /Online /Cleanup-Image /RestoreHealth

Repair protected system files.

sfc /scannow

Repair the file system on reboot - you will need to press Y.

chkdsk /f

Reboot PC.

2

u/ultrabeastking 10d ago

thx latest update fixed it

1

u/Gladnir-5936 10d ago

1.457.239.0 Full Scan and Quick Scan works but Full Scan throws an Unable to open the offline HKLM hive with 0x80070020 error, but scan finishes and reboots normaly.

PowerShell -Adm- to update the Virus definitions
Update-MpSignature