O365 Multiple Failed MFA Requests For User

Status: production
Severity: medium
Group by: _time, aws::recipientAccountId, dest, user, vendor_product
Author: Mauricio Velazco, Splunk
Source: github.com/splunk/security_content

The following analytic identifies potential "MFA fatigue" attacks targeting Office 365 users by detecting more than nine Multi-Factor Authentication (MFA) prompts within a 10-minute timeframe. It leverages O365 management activity logs, focusing on Azure Active Directory events with the UserLoginFailed operation, a Success ResultStatus, and an ErrorNumber of 500121. This activity is significant as attackers may exploit MFA fatigue to gain unauthorized access by overwhelming users with repeated MFA requests. If confirmed malicious, this could lead to data breaches, unauthorized data access, or further compromise within the O365 environment. Immediate investigation is crucial.

Known false positives

Multiple Failed MFA requests may also be a sign of authentication or application issues. Filter as needed.

MITRE ATT&CK coverage

Tactic	Techniques
Credential Access	`T1621` Multi-Factor Authentication Request Generation

Telemetry coverage

Platform	Record / event type
Microsoft 365	UserLoginFailed

Rules detecting the same action

These rules filter on the same operation.

High Number of Login Failures from a single source (Splunk)
M365 Identity User Account Lockouts (Elastic)
M365 Identity User Brute Force Attempted (Elastic)
O365 Excessive SSO logon errors (Splunk)
O365 High Number Of Failed Authentications for User (Splunk)
O365 Multi-Source Failed Authentications Spike (Splunk)
O365 Multiple AppIDs and UserAgents Authentication Spike (Splunk)
O365 Multiple Users Failing To Authenticate From Ip (Splunk)

Rule body

name: O365 Multiple Failed MFA Requests For User
id: fd22124e-dbac-4744-a8ce-be10d8ec3e26
version: 12
creation_date: '2023-12-06'
modification_date: '2026-05-13'
author: Mauricio Velazco, Splunk
status: production
type: TTP
description: The following analytic identifies potential "MFA fatigue" attacks targeting Office 365 users by detecting more than nine Multi-Factor Authentication (MFA) prompts within a 10-minute timeframe. It leverages O365 management activity logs, focusing on Azure Active Directory events with the UserLoginFailed operation, a Success ResultStatus, and an ErrorNumber of 500121. This activity is significant as attackers may exploit MFA fatigue to gain unauthorized access by overwhelming users with repeated MFA requests. If confirmed malicious, this could lead to data breaches, unauthorized data access, or further compromise within the O365 environment. Immediate investigation is crucial.
data_source:
    - O365 UserLoginFailed
search: |-
    `o365_management_activity` Workload=AzureActiveDirectory Operation=UserLoginFailed ResultStatus=Success ErrorNumber=500121
      | bucket span=10m _time
      | stats dc(_raw) as mfa_prompts values(LogonError) as LogonError values(signature) as signature values(action) as action values(src) as src
        BY user _time vendor_account
           vendor_product dest
      | where mfa_prompts > 9
      | `o365_multiple_failed_mfa_requests_for_user_filter`
how_to_implement: You must install the Splunk Microsoft Office 365 Add-on and ingest Office 365 management activity events.
known_false_positives: Multiple Failed MFA requests may also be a sign of authentication or application issues. Filter as needed.
references:
    - https://attack.mitre.org/techniques/T1621/
finding:
    title: Multiple failed MFA requestes for $user$
    entity:
        field: user
        type: user
        score: 50
analytic_story:
    - Office 365 Account Takeover
    - Scattered Lapsus$ Hunters
asset_type: O365 Tenant
mitre_attack_id:
    - T1621
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: cloud
security_domain: identity

Stages and Predicates

Stage 1: `search`

`o365_management_activity` Workload=AzureActiveDirectory Operation=UserLoginFailed ResultStatus=Success ErrorNumber=500121

Stage 2: `bucket`

| bucket span=10m _time

Stage 3: `stats`

| stats dc(_raw) as mfa_prompts values(LogonError) as LogonError values(signature) as signature values(action) as action values(src) as src
    BY user _time vendor_account
       vendor_product dest

Stage 4: `where`

| where mfa_prompts > 9

Stage 5: `search`

| `o365_multiple_failed_mfa_requests_for_user_filter`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`ErrorNumber`	eq	`500121`	`field:"ErrorNumber" kind:eq value:"500121"`
`Operation`	eq	`UserLoginFailed`	`field:"Operation" kind:eq value:"UserLoginFailed"`
`ResultStatus`	eq	`Success`	`field:"ResultStatus" kind:eq value:"Success"`
`Workload`	eq	`AzureActiveDirectory`	`field:"Workload" kind:eq value:"AzureActiveDirectory"`
`mfa_prompts`	gt	`9`	`field:"mfa_prompts" kind:gt value:"9"`
`sourcetype`	eq	`o365:management:activity`	`field:"sourcetype" kind:eq value:"o365:management:activity"`