Microsoft Defender crashes after Full scan — mpengine.dll 0xc0000005

Hi,

I made an account just because I am a little curious and a little worried.

I updated to the newest Defender platform version earlier today, and it all seemed fine. However, I opened League of Legends tonight and my Microsoft Defender crashed with an exception code of 0xc0000005. When I saw this, I got scared and shut down Vanguard.

Following this, I performed a full scan while checking my Event Viewer and noticed this:
File Name: \Device\HarddiskVolume7\Windows\System32\fcon.dll

The full scan came back clean, but after 5 seconds it crashed again with the same error and fcon.dll in audit failure again.

I am wondering if this is related to the newest platform version because I was able to open League fine just the other night and there were no crashes, but after this update it has crashed upon opening League and after a full scan.

I do not believe it is malware and have no plausible evidence to believe it is such.

Thank you for any help you can provide.

Open an elevated Command prompt (Run as Administrator)

Copy/paste and run the following...

sfc /SCANNOW

Run until there are no errors found and corrected.

Banjoman301 said:

Open an elevated Command prompt (Run as Administrator)

Copy/paste and run the following...

sfc /SCANNOW

Run until there are no errors found and corrected.

Click to expand...

Thank you for the quick response, I have just ran it and it came back clean, no errors were found.

I'm not sure if this will be of any help but, It has been an hour since the last crash and this is the health report I have received:
Endpoint Protection client health report (time in UTC):
Platform version: 4.18.26100.4
Engine version: 1.1.26100.4
Network Realtime Inspection engine version: 1.1.26100.4
Antivirus security intelligence version: 1.463.16.0
Antispyware security intelligence version: 1.463.16.0
Network Realtime Inspection security intelligence version: 1.463.16.0
RTP state: Enabled
OA state: Enabled
IOAV state: Enabled
BM state: Enabled
Antivirus security intelligence age: 0
Antispyware security intelligence age: 0
Last quick scan age: 0
Last full scan age: 0
Antivirus security intelligence creation time: 2026-10-28T20:03:00Z
Antispyware security intelligence creation time: 2026-10-28T20:03:00Z
Last quick scan start time: 2026-10-28T21:49:59Z
Last quick scan end time: 2026-10-28T21:50:49Z
Last quick scan source: 1
Last full scan start time: 2026-10-28T21:51:05Z
Last full scan end time: 2026-10-28T22:24:12Z
Last full scan source: 1
Product status: 0x00080000

All the version numbers are current.

What happens if you check for an update?

It says security intelligence is up to date.

Banjoman301 said:

All the version numbers are current.

What happens if you check for an update?

Click to expand...

It says "Security intelligence is up to date."

A part of me just feels like saying this likely has something to do with the most recent platform update as before this current one my system has been running flawlessly

It's getting pretty late for me so I will be heading to bed for now and check back in the morning.

Edit: Before going to bed I decided to restart my computer, there were no errors on boot and a quick scan finished fine with no errors, I didn't have time to check if a fullscan would cause the same error as before but I feel this may be useful information.

Some additional info on the issue in this search result (a couple of these you've already done)...

Microsoft Defender crashing with an exception code of 0xc0000005 (Access Violation) when you launch League of Legends is an active issue resulting from a conflict between Microsoft's anti-malware engine (mpengine.dll) and Riot's kernel-level anti-cheat driver (Vanguard). When League launches, Vanguard tightly hooks into protected memory, which causes Defender to trigger a memory access crash if the security platform was recently updated. [1]

You can resolve this issue and stabilize your system by following these prioritized troubleshooting steps:

1. Update Windows Security Intelligence (Defender Signatures)

Because this is an active bug where a recent security platform or signature update causes a crash, checking for a patch from Microsoft should be your first move.

• Go to Settings > Update & Security / Windows Update > Check for updates.
• Alternatively, search for Windows Security in your Start menu, navigate to Virus & threat protection, scroll down to Virus & threat protection updates, and click Check for updates. [1]

2. Set Exclusions for the Riot Client and League of Legends

Prevent Defender from scanning or conflicting with League's directories while launching. [1, 2]

1. Open Windows Security and click on Virus & threat protection.
2. Under Virus & threat protection settings, click Manage settings.
3. Scroll down to Exclusions and click Add or remove exclusions.
4. Click Add an exclusion and choose Folder. Add these two directories:
   • C:\Riot Games\ (or wherever your League of Legends/Riot Client is installed).
   • C:\Program Files\Riot Vanguard\ [1, 2]

3. Run the Riot Client and League as Administrator

Elevating permissions often bypasses memory-mapping access blocks between Windows and Vanguard: [1, 2]

• Right-click the Riot Client shortcut on your desktop and select Properties.
• Go to the Compatibility tab.
• Check Run this program as an administrator, then click Apply and OK.
• Do the same for the main executable located inside C:\Riot Games\League of Legends\Game\League of Legends.exe. [1, 2, 3, 4]

4. Fully Reinstall Vanguard

If Vanguard's files or local configurations are corrupted, it will repeatedly crash your primary system services. [1]

1. Close League of Legends and the Riot Client completely via your system tray or Task Manager.
2. Search Windows for Add or remove programs.
3. Find Riot Vanguard, click the three dots, and select Uninstall.
4. Restart your PC immediately.
5. Open the Riot Client; it will automatically prompt you to reinstall Vanguard. Once finished, restart your PC one more time to initialize the driver before trying to play. [1, 2]

5. Repair Damaged System Files

If Defender's file structure itself was corrupted during the initial crash, use Windows' deployment tools to repair it: [1]

1. Press the Windows Key, type cmd, right-click Command Prompt, and select Run as administrator.
2. Paste the following command and hit Enter: sfc /scannow
3. After that completes, paste this command and hit Enter: DISM /Online /Cleanup-Image /RestoreHealth
4. Restart your computer. [1, 2]