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Description

The following analytic identifies one source endpoint failing to authenticate with multiple valid users using the Kerberos protocol. This behavior could represent an adversary performing a Password Spraying attack against an Active Directory environment using Kerberos to obtain initial access or elevate privileges. Event 4771 is generated when the Key Distribution Center fails to issue a Kerberos Ticket Granting Ticket (TGT). Failure code 0x18 stands for wrong password provided (the attempted user is a legitimate domain user).
The detection calculates the standard deviation for each host and leverages the 3-sigma statistical rule to identify an unusual number of users. To customize this analytic, users can try different combinations of the bucket span time and the calculation of the upperBound field. This logic can be used for real time security monitoring as well as threat hunting exercises.
This detection will only trigger on domain controllers, not on member servers or workstations.
The analytics returned fields allow analysts to investigate the event further by providing fields like source ip and attempted user accounts.

  • Type: Anomaly
  • Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud
  • Datamodel: Endpoint
  • Last Updated: 2021-04-08
  • Author: Mauricio Velazco, Splunk
  • ID: 3a91a212-98a9-11eb-b86a-acde48001122

ATT&CK

ID Technique Tactic
T1110.003 Password Spraying Credential Access
T1110 Brute Force Credential Access
`wineventlog_security` EventCode=4771 Failure_Code=0x18 Account_Name!="*$" 
| bucket span=2m _time 
| stats dc(Account_Name) AS unique_accounts values(Account_Name) as tried_accounts by _time, Client_Address 
| eventstats avg(unique_accounts) as comp_avg , stdev(unique_accounts) as comp_std by Client_Address 
| eval upperBound=(comp_avg+comp_std*3) 
| eval isOutlier=if(unique_accounts > 10 and unique_accounts >= upperBound, 1, 0) 
| search isOutlier=1 
| `multiple_users_failing_to_authenticate_from_host_using_kerberos_filter`

Associated Analytic Story

How To Implement

To successfully implement this search, you need to be ingesting Domain Controller and Kerberos events. The Advanced Security Audit policy setting Audit Kerberos Authentication Service within Account Logon needs to be enabled.

Required field

  • _time
  • EventCode
  • Result_Code
  • Account_Name
  • Client_Address

Kill Chain Phase

  • Exploitation

Known False Positives

A host failing to authenticate with multiple valid domain users is not a common behavior for legitimate systems. Possible false positive scenarios include but are not limited to vulnerability scanners, missconfigured systems and multi-user systems like Citrix farms.

RBA

Risk Score Impact Confidence Message
49.0 70 70 Potential Kerberos based password spraying attack from $Client_Address$

Reference

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

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