Detection: Unusual Number of Kerberos Service Tickets Requested

Updated Date: 2024-10-17 ID: eb3e6702-8936-11ec-98fe-acde48001122 Author: Mauricio Velazco, Dean Luxton, Splunk Type: Anomaly Product: Splunk Enterprise Security

Description

The following analytic identifies an unusual number of Kerberos service ticket requests, potentially indicating a kerberoasting attack. It leverages Kerberos Event 4769 and calculates the standard deviation for each host, using the 3-sigma rule to detect anomalies. This activity is significant as kerberoasting allows adversaries to request service tickets and crack them offline, potentially gaining privileged access to the domain. If confirmed malicious, this could lead to unauthorized access to sensitive accounts and escalation of privileges within the Active Directory environment.

Search

1`wineventlog_security` EventCode=4769 ServiceName!="*$" TicketEncryptionType=0x17 
2| bucket span=2m _time  
3| stats dc(ServiceName) AS unique_services values(ServiceName) as requested_services values(user_category) as user_category values(src_category) as src_category by _time, user, src 
4| eventstats avg(unique_services) as comp_avg , stdev(unique_services) as comp_std by user, src 
5| eval upperBound=(comp_avg+comp_std*3)  
6| eval isOutlier=if(unique_services > 2 and unique_services >= upperBound, 1, 0)  
7| search isOutlier=1 
8| `unusual_number_of_kerberos_service_tickets_requested_filter`

Data Source

Name	Platform	Sourcetype	Source
Windows Event Log Security 4769	Windows	`'xmlwineventlog'`	`'XmlWinEventLog:Security'`

Macros Used

Name	Value
wineventlog_security	`eventtype=wineventlog_security OR Channel=security OR source=XmlWinEventLog:Security`
unusual_number_of_kerberos_service_tickets_requested_filter	`search *`

unusual_number_of_kerberos_service_tickets_requested_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
T1558	Steal or Forge Kerberos Tickets	Credential Access
T1558.003	Kerberoasting	Credential Access

KillChainPhase.EXPLOITAITON

NistCategory.DE_AE

Cis18Value.CIS_10

FIN7

Indrik Spider

Wizard Spider

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

An single endpoint requesting a large number of kerberos service tickets is not common behavior. Possible false positive scenarios include but are not limited to vulnerability scanners, administration systems and missconfigured systems.

Associated Analytic Story

Active Directory Kerberos Attacks

Risk Based Analytics (RBA)

Risk Message	Risk Score	Impact	Confidence
User $user$ requested a service ticket for $unique_services$ services indicating a potential kerberoasting attack	64	80	80

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: 5