Windows Unusual Count Of Invalid Users Failed To Auth Using NTLM

name: Windows Unusual Count Of Invalid Users Failed To Auth Using NTLM
id: 15603165-147d-4a6e-9778-bd0ff39e668f
type: Anomaly
version: 10
status: production
date: '2026-03-10'
author: Mauricio Velazco, Splunk
data_source:
    - Windows Event Log Security 4776
description: The following analytic identifies a source endpoint failing to authenticate with multiple invalid users using the NTLM protocol. It leverages EventCode 4776 and calculates the standard deviation for each host, using the 3-sigma rule to detect anomalies. This behavior is significant as it may indicate a Password Spraying attack, where an adversary attempts to gain initial access or elevate privileges. If confirmed malicious, this activity could lead to unauthorized access or privilege escalation, posing a significant threat to the Active Directory environment. This detection is focused on domain controllers.
how_to_implement: To successfully implement this search, you need to be ingesting Domain Controller events. The Advanced Security Audit policy setting `Audit Credential Validation' within `Account Logon` needs to be enabled.
known_false_positives: A host failing to authenticate with multiple invalid domain users is not a common behavior for legitimate systems. Possible false positive scenarios include but are not limited to vulnerability scanners and missconfigured systems. If this detection triggers on a host other than a Domain Controller, the behavior could represent a password spraying attack against the host's local accounts.
references:
    - https://attack.mitre.org/techniques/T1110/003/
    - https://docs.microsoft.com/en-us/windows/security/threat-protection/auditing/audit-credential-validation
    - https://docs.microsoft.com/en-us/windows/security/threat-protection/auditing/event-4776
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 ("$user$") starthoursago=168  | 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: $info_min_time$
      latest_offset: $info_max_time$
search: |-
    `wineventlog_security` EventCode=4776 TargetUserName!=*$ Status=0xc0000064
      | bucket span=2m _time
      | stats dc(TargetUserName) AS unique_accounts values(TargetUserName) as user values(dest) as dest
        BY _time, Workstation
      | eventstats avg(unique_accounts) as comp_avg , stdev(unique_accounts) as comp_std
        BY Workstation
      | eval upperBound=(comp_avg+comp_std*3)
      | eval isOutlier=if(unique_accounts > 10 and unique_accounts >= upperBound, 1, 0)
      | search isOutlier=1
      | rename Workstation as src
      | `windows_unusual_count_of_invalid_users_failed_to_auth_using_ntlm_filter`
rba:
    message: Potential NTLM based password spraying attack from $src$
    risk_objects:
        - field: user
          type: user
          score: 20
        - field: src
          type: system
          score: 20
    threat_objects: []
tags:
    analytic_story:
        - Active Directory Password Spraying
        - Volt Typhoon
    asset_type: Endpoint
    mitre_attack_id:
        - T1110.003
    product:
        - Splunk Enterprise
        - Splunk Enterprise Security
        - Splunk Cloud
    security_domain: endpoint
tests:
    - attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1110.003/purplesharp_invalid_users_ntlm_xml/windows-security.log
          source: XmlWinEventLog:Security
          sourcetype: XmlWinEventLog
      name: True Positive Test