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.

MITRE ATT&CK coverage

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

Rules detecting the same action

Other rules on this platform that filter on the same API call or 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 splunk

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/
drilldown_searches:
    - name: View the detection results for - "$user$"
      search: '%original_detection_search% | search  user = "$user$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$user$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$user$") | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: 7d
      latest_offset: "0"
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
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1621/o365_multiple_failed_mfa_requests/o365_multiple_failed_mfa_requests.log
          source: o365
          sourcetype: o365:management:activity
      test_type: unit

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

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

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

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)

O365 Multiple Failed MFA Requests For User

MITRE ATT&CK coverage

Rules detecting the same action

Rule body splunk

Stages and Predicates

Stage 1: `search`

Stage 2: `bucket`

Stage 3: `stats`

Stage 4: `where`

Stage 5: `search`

Indicators

Keyboard shortcuts

Search operators

O365 Multiple Failed MFA Requests For User

MITRE ATT&CK coverage

Rules detecting the same action

Rule body splunk

Stages and Predicates

Stage 1: search

Stage 2: bucket

Stage 3: stats

Stage 4: where

Stage 5: search

Indicators

Stage 1: `search`

Stage 2: `bucket`

Stage 3: `stats`

Stage 4: `where`

Stage 5: `search`