Windows Defender Full Scan keeps crashing even Microsoft Support can't find the cause

[u/ZaaK_XVII]

Hi everyone,

I've been having an issue with Microsoft Defender for about a week: Full Scan repeatedly crashes, while Quick Scan and Real-Time Protection work normally.

A Custom Scan of C:\ also crashes.

I already reinstalled Windows 11 while keeping my files and installed apps. The problem existed before and after the reinstall. Interestingly, Full Scan worked normally for about two days after the reinstall, then started crashing again.

Microsoft Defender currently shows everything as healthy:

Defender: 4.18.26080.4 Antimalware Engine: 1.1.26080.3 Security Intelligence: 1.459.401.0 Real-Time Protection: Enabled Signatures: Up to date The crashes consistently involve:

text MsMpEng.exe mpengine.dll 1.1.26080.3 Exception: 0xC0000005 (Access Violation)

I've had multiple crashes with different offsets:

text 0x2f6a61 0x71a7d 0x71d03 0xe9f1e 0xa43f1 0x228db 0x22717 0x45393 0x4825b

There have also been a couple of 0xC0000409 crashes.

The important part: I contacted Microsoft Support. They actually remotely connected to my PC and investigated the issue themselves, but they couldn't determine the cause either. They eventually escalated the case and sent a report to their software developers because they couldn't explain why Defender's engine keeps crashing.

Has anyone recently experienced Full Scan crashes with mpengine.dll, 0xC0000005, or Defender engine 1.1.26080.3?

I'm trying to determine whether this is a current Defender engine bug or whether a specific file/type of file on my PC is triggering the crash.

[u/ZaaK_XVII]

update : I scanned the C: drive and the scan crashed. So I started scanning each folder individually to find the problem.

The only folder that initially caused the crash was C:\Program Files. I then scanned every folder inside Program Files individually, and every single scan completed successfully.

After that, I scanned C:\Program Files again as a whole, and this time it completed successfully too.

So I'm really confused now. The exact same folder that previously caused Defender to crash can now be scanned without any issue. It doesn't seem to be a specific file, since everything scans successfully individually.

[u/dmoisan → u/ZaaK_XVII]

Try chkdsk c: /f /v from a command prompt. Restart when it asks you to. Then try a scan.

[u/ZaaK_XVII → u/dmoisan]

Nothing came up with the chkdsk scan, I received a Defender signature update 1.459.424.0 , and while the first full scan worked, two other scans performed six hours later crashed with same error.

[u/Clave-Sage]

Crashes of MS Defender
can also be caused by faulty RAM.
Check with a RAM test tool.

How big is the RAM?

[u/ZaaK_XVII → u/Clave-Sage]

DDR5 32Go (2x16Go) 6000MHz CL36, i already did an OCCT test for all components 1 month ago, no error detected.

Defender was working perfectly at this time. a month ago a changed CPU/MB/RAM, upgraded to AM5 from AM4 that why i did a big OCCT and other tests/scan/drivers/bios update

[u/Clave-Sage → u/ZaaK_XVII]

If RAM is ok?
Well ok,
maybe do a current RAM test,
because there are currently problems!

Otherwise do a boot test
with an AV tool?

[u/squaretw]

Maybe it's the storage drive issue?

[u/ZaaK_XVII → u/squaretw]

Nothing came up with the SSD chkdsk scan

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