The following analytic identifies one source endpoint failing to authenticate with multiple invalid domain 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. As attackers progress in a breach, mistakes will be made. In certain scenarios, adversaries may execute a password spraying attack using an invalid list of users. Event 4768 is generated every time the Key Distribution Center issues a Kerberos Ticket Granting Ticket (TGT). Failure code 0x6 stands for
client not found in Kerberos database (the attempted user is not a valid 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
- Last Updated: 2021-04-14
- Author: Mauricio Velazco, Splunk
- ID: 001266a6-9d5b-11eb-829b-acde48001122
Kill Chain Phase
1 2 3 4 5 6 7 8 `wineventlog_security` EventCode=4768 Result_Code=0x6 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 | `windows_invalid_users_failed_authentication_via_kerberos_filter`
The SPL above uses the following Macros:
windows_invalid_users_failed_authentication_via_kerberos_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.
List of fields required to use this analytic.
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.
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, multi-user systems and missconfigured systems.
Associated Analytic Story
|49.0||70||70||Potential Kerberos based password spraying attack from $Client_Address$|
The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.
source | version: 1