AADSTS500121 Severity: Medium MFA authentication failed
The second factor (authenticator app, SMS, call) was not completed successfully. Happens with cancelled MFA, lost push, or wrong method.
Affected systems
Symptoms
- AADSTS500121 after the MFA prompt
- Authenticator app shows no push, or you dismissed it
- MFA SMS does not arrive
Possible causes
- Authenticator push was not confirmed (timeout)
- Wrong MFA method active (e.g. SMS instead of app)
- SMS phone number changed
- Authenticator app not yet registered on new phone
Solutions
1.Re-trigger MFA
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Try sign-in again, open the authenticator app at the MFA step and actively confirm the push.
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Check push notification permission for the authenticator app in phone settings.
2.Choose another MFA method
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The MFA prompt usually has a 'use another method' link.
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Pick SMS, phone call, or security key instead.
3.MFA setup at mysignins.microsoft.com
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Sign in to https://mysignins.microsoft.com.
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Open security methods, check phone number and authenticator app are current.
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Register the authenticator app on a new phone.
4.When all else fails: ask IT to reset
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If you lost your phone without a backup, you cannot fix it yourself.
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IT can reset MFA on your account, after which you set it up fresh on next sign-in.
AADSTS500121 is mostly an interaction or device issue, not a server error. The self-service at mysignins.microsoft.com handles most cases in two minutes.