How to Fix Remote Desktop and Microsoft Authenticator MFA Error 500121
Question details
The user is unable to access Remote Desktop and Microsoft 365 services due to MFA Error 500121, as all verification methods including the Authenticator app, phone calls, and text messages fail.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Attempting to log into Microsoft 365 or Remote Desktop with Multi-Factor Authentication enabled.
- Observed behavior
- Login fails with Error 500121. Alternative verification methods do not work, and reinstalling the Microsoft Authenticator app fails because the account cannot be successfully re-added without verification.
Check all your devices and browsers to see if you have any active, authenticated Microsoft web sessions open. Bypassing the login screen through an existing session is the fastest way to reset your settings.
Reset Security Info via an Authenticated Web Session
If you are still logged into your Microsoft account on any browser, you can manually remove and re-register your Authenticator app without needing to pass an MFA prompt.
This solution only works if you have a browser window or device that is already authenticated and does not require a new login.
Open your browser where the session is active and navigate to https://myaccount.microsoft.com/.
Click on the 'Security info' tab from the left-hand navigation menu.
Locate the 'Microsoft Authenticator' entry in your list of methods, click 'Delete', and confirm the removal.
Click 'Add sign-in method', choose 'Authenticator app', and follow the on-screen prompts to register your phone's authenticator again.
Switch Networks and Verify Push Notifications
Network restrictions or phone settings can sometimes block MFA prompts from successfully reaching your device.
Contact Your Microsoft 365 IT Administrator
If all verification methods are locked and no active sessions exist, your IT administrator must intervene from the server side.
Edit Documents Offline While Resolving MFA Issues
If you are temporarily locked out of Microsoft 365 due to MFA Error 500121 and need to edit documents urgently, WPS Office is an excellent free alternative. It works entirely offline without requiring complex server-side logins and offers seamless compatibility with your existing Office files.
- 1. Download WPS Office: Visit the official WPS website and download the free installation package for your operating system.
- 2. Install the Software: Run the downloaded installer and follow the straightforward setup wizard to install the suite.
- 3. Open Your Documents: Launch WPS Office and open your existing Word, Excel, or PowerPoint files directly from your hard drive to start editing immediately.
Frequently Asked Questions
Why am I encountering MFA Error 500121?
Error 500121 indicates that the multi-factor authentication request failed or was denied. This typically happens due to network issues blocking the prompt, an expired session, disabled push notifications, or server-side policy misconfigurations.
Will reinstalling the Microsoft Authenticator app fix the login error?
No, uninstalling and reinstalling the app usually will not fix the issue. If you are locked out, you will be unable to pass the verification required to re-add your account to the freshly installed app without an administrator's assistance.
What should I do if my phone calls and text messages are also failing?
If alternative verification methods like SMS or voice calls are failing alongside the Authenticator app, you must contact your organization's Microsoft 365 IT administrator. They can reset your MFA methods globally or issue a Temporary Access Pass to restore your access.
