Windows Unusual NTLM Authentication Users By Source

Try in Splunk Security Cloud

Description

The following analytic detects when an unusual number of NTLM authentications is attempted by the same source. 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 in as well.

• Type: Anomaly

• Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud

• Last Updated: 2024-03-16
• Author: Steven Dick
• ID: 80fcc4d4-fd90-488e-b55a-4e7190ae6ce2

Annotations

ATT&CK

ATT&CK

ID	Technique	Tactic
T1110	Brute Force	Credential Access

T1110.003

Password Spraying

Credential Access

Kill Chain Phase

• Exploitation

NIST

• DE.AE

CIS20

• CIS 10

CVE

Search

`ntlm_audit` EventCode = 8004 SChannelName=* WorkstationName=* 
| eval src = replace(WorkstationName,"\\\\","")  ```CIM alignment, remove leading \\ from some auth attempts ``` 
| eval dest = SChannelName, user = UserName ``` CIM alignment``` 
| where SChannelName!=src ``` Remove NTLM auths to self, improves accuracy for certain applications ``` 
| `windows_unusual_ntlm_authentication_users_by_source_filter` 
| stats count min(_time) as firstTime max(_time) as lastTime dc(eval(upper(user))) as unique_count by src 
| eventstats avg(unique_count) as unique_avg , stdev(unique_count) as unique_std 
| eval upperBound_unique=(1+unique_avg+unique_std*3) ``` adjust formula for sensitivity``` 
| eval isOutlier=CASE(unique_count > upperBound_unique, 1, true(), 0) 
| where isOutlier==1 
| `security_content_ctime(firstTime)` 
| `security_content_ctime(lastTime)`

Macros

The SPL above uses the following Macros:

• ntlm_audit
• security_content_ctime

windows_unusual_ntlm_authentication_users_by_source_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Required fields

List of fields required to use this analytic.

• _time
• EventCode
• DomainName
• Security
• WorkstationName

How To Implement

The following analytic requires that NTLM Operational logs to be imported from the environment Domain Controllers. This requires configuration of specific auditing settings, see Microsoft references for further guidance. This analytic is specific to EventID 8004~8006.

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.

Associated Analytic Story

• Active Directory Password Spraying

RBA

Risk Score	Impact	Confidence	Message
25.0	25	100	The device [$src$] attempted $count$ NTLM authentications using $unique_count$ user accounts.

The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

Reference

• 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

Test Dataset

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range

source | version: 1