Detection: Windows Unusual Count Of Invalid Users Fail To Auth Using Kerberos

Updated Date: 2024-05-31 ID: f122cb2e-d773-4f11-8399-62a3572d8dd7 Author: Mauricio Velazco, Splunk Type: Anomaly Product: Splunk Enterprise Security

Description

The following analytic identifies a source endpoint failing to authenticate with multiple invalid domain users using the Kerberos protocol. It leverages Event ID 4768, which is generated when the Key Distribution Center issues a Kerberos Ticket Granting Ticket (TGT) and detects failure code 0x6, indicating the user is not found in the Kerberos database. 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 and potential privilege escalation within the Active Directory environment.

1`wineventlog_security` EventCode=4768 TargetUserName!=*$ Status=0x6 
2| bucket span=5m _time 
3| stats dc(TargetUserName) AS unique_accounts values(TargetUserName) as user by _time, IpAddress 
4| eventstats avg(unique_accounts) as comp_avg , stdev(unique_accounts) as comp_std by IpAddress 
5| eval upperBound=(comp_avg+comp_std*3) 
6| eval isOutlier=if(unique_accounts > 10 and unique_accounts >= upperBound, 1, 0) 
7| search isOutlier=1 
8| `windows_unusual_count_of_invalid_users_fail_to_auth_using_kerberos_filter`

Data Source

Name Platform Sourcetype Source Supported App
Windows Event Log Security 4768 Windows icon Windows 'xmlwineventlog' 'XmlWinEventLog:Security' N/A

Macros Used

Name Value
wineventlog_security eventtype=wineventlog_security OR Channel=security OR source=XmlWinEventLog:Security
windows_unusual_count_of_invalid_users_fail_to_auth_using_kerberos_filter search *
windows_unusual_count_of_invalid_users_fail_to_auth_using_kerberos_filter is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Annotations

- MITRE ATT&CK
+ Kill Chain Phases
+ NIST
+ CIS
- Threat Actors
ID Technique Tactic
T1110.003 Password Spraying Credential Access
T1110 Brute Force Credential Access
KillChainPhase.EXPLOITAITON
NistCategory.DE_AE
Cis18Value.CIS_10
APT28
APT29
APT33
Chimera
HEXANE
Lazarus Group
Leafminer
Silent Librarian
APT28
APT38
APT39
DarkVishnya
Dragonfly
FIN5
Fox Kitten
HEXANE
OilRig
Turla

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Setting Value
Disabled true
Cron Schedule 0 * * * *
Earliest Time -70m@m
Latest Time -10m@m
Schedule Window auto
Creates Risk Event True
This configuration file applies to all detections of type anomaly. These detections will use Risk Based Alerting.

Implementation

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

Risk Based Analytics (RBA)

Risk Message Risk Score Impact Confidence
Potential Kerberos based password spraying attack from $IpAddress$ 49 70 70
The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

References

Detection Testing

Test Type Status Dataset Source Sourcetype
Validation ✅ Passing N/A N/A N/A
Unit ✅ Passing Dataset XmlWinEventLog:Security XmlWinEventLog
Integration ✅ Passing Dataset XmlWinEventLog:Security XmlWinEventLog

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: GitHub | Version: 2