Windows Unusual NTLM Authentication Users By Destination

name: Windows Unusual NTLM Authentication Users By Destination
id: 1120a204-8444-428b-8657-6ea4e1f3e840
version: 10
creation_date: '2024-03-16'
modification_date: '2026-05-13'
author: Steven Dick
status: production
type: Anomaly
description: The following analytic detects when a device is the target of numerous NTLM authentications using a null domain. This activity generally results when an attacker attempts to brute force, password spray, or otherwise authenticate to a domain joined Windows device from a non-domain device. This activity may also generate a large number of EventID 4776 events in tandem, however these events will not indicate the attacker or target device.
data_source:
    - NTLM Operational 8004
    - NTLM Operational 8005
    - NTLM Operational 8006
search: |
    `ntlm_audit`
    EventCode = 8004
    SChannelName=*
    WorkstationName=*
    ```CIM alignment, remove leading \\ from some auth attempts ```
    | eval src = replace(WorkstationName,"\\\\","")
    | eval dest = SChannelName, user = UserName
    
    ``` Remove NTLM auths to self, improves accuracy for certain applications ```
    | where SChannelName!=src
    
    | stats count min(_time) as firstTime
                  max(_time) as lastTime
                  dc(eval(upper(user))) as unique_count by dest
    
    | eventstats avg(unique_count) as unique_avg
                 stdev(unique_count) as unique_std
    
    ```adjust formula for sensitivity```
    | eval upperBound_unique=(1+unique_avg+unique_std*3)
    
    | eval isOutlier=CASE(unique_count > upperBound_unique, 1, true(), 0)
    | where isOutlier==1
    | `security_content_ctime(firstTime)`
    | `security_content_ctime(lastTime)`
    | `windows_unusual_ntlm_authentication_users_by_destination_filter`
how_to_implement: The following analytic detects when an unusual number of NTLM authentications is attempted against the same destination. This activity generally results when an attacker attempts to brute force, password spray, or otherwise authenticate to a domain joined Windows device using an NTLM based process/attack. This same activity may also generate a large number of EventID 4776 events as well.
known_false_positives: Vulnerability scanners, print servers, and applications that deal with non-domain joined authentications. Recommend adjusting the upperBound_unique eval for tailoring the correlation to your environment, running with a 24hr search window will smooth out some statistical noise.
references:
    - https://attack.mitre.org/techniques/T1110/003/
    - https://techcommunity.microsoft.com/t5/ask-the-directory-services-team/ntlm-blocking-and-you-application-analysis-and-auditing/ba-p/397191
    - https://techcommunity.microsoft.com/t5/microsoft-defender-for-identity/enriched-ntlm-authentication-data-using-windows-event-8004/m-p/871827
    - https://www.varonis.com/blog/investigate-ntlm-brute-force
    - https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-nrpc/4d1235e3-2c96-4e9f-a147-3cb338a0d09f
intermediate_findings:
    entities:
        - field: dest
          type: system
          score: 20
          message: The device [$dest$] was the target of $count$ NTLM authentications using $unique_count$ unique user accounts.
analytic_story:
    - Active Directory Password Spraying
asset_type: Endpoint
mitre_attack_id:
    - T1110.003
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: endpoint
security_domain: endpoint