How to fix MU Online DEP (Data Execution Prevention) errors. If the game crashes on launch, shows "This application has been closed to protect your computer", or just stops at startup, Windows DEP may be blocking main.exe.

DEP is a Windows security feature that blocks harmful code from running, but it sometimes blocks older or modded game clients like MU.

What this guide does: It shows you how to add MU's game file (main.exe) to the DEP exceptions so Windows leaves it alone.

Steps: add MU to the DEP exception list

1. Open System Properties: press Windows Key + Pause/Break, or right-click This PC and choose Properties.
2. Click Advanced system settings (left sidebar).
3. Open Performance Settings: In System Properties, click Settings under the Performance section. Go to the Data Execution Prevention tab.
4. Enable DEP for All Programs Except Those You Select: Choose "Turn on DEP for all programs and services except those I select".
5. Add the MU Client Game File: Click Add…, browse to your MUOnline installation folder, select main.exe, click Open / Apply / OK.
6. Restart your PC.

Is this safe? Yes. You are not turning DEP off for everything, only excluding MU Online from it. This is a common fix for private server clients and older games.

Still stuck? Open a ticket in the right Discord channel.