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Description

The following analytic identifies a suspicious file creation in known cron table directories. This event is commonly abuse by malware, adversaries and red teamers to persist on the target or compromised host. crontab or cronjob is like a schedule task in windows environment where you can create an executable or script on the known crontab directories to run it base on its schedule. This Anomaly query is a good indicator to look further what file is added and who added the file if to consider it legitimate file.

  • Type: Anomaly
  • Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud
  • Datamodel: Endpoint
  • Last Updated: 2021-12-17
  • Author: Teoderick Contreras, Splunk
  • ID: 023f3452-5f27-11ec-bf00-acde48001122

ATT&CK

ID Technique Tactic
T1053.003 Cron Execution, Persistence, Privilege Escalation
T1053 Scheduled Task/Job Execution, Persistence, Privilege Escalation

| tstats `security_content_summariesonly` count min(_time) as firstTime max(_time) as lastTime FROM datamodel=Endpoint.Filesystem where Filesystem.file_path IN ("*/etc/cron*", "*/var/spool/cron/*") by Filesystem.dest Filesystem.file_create_time Filesystem.file_name Filesystem.process_guid Filesystem.file_path 
| `drop_dm_object_name(Filesystem)` 
| `security_content_ctime(lastTime)` 
| `security_content_ctime(firstTime)` 
| `linux_add_files_in_known_crontab_directories_filter`

Associated Analytic Story

How To Implement

To successfully implement this search, you need to be ingesting logs with the file name, file path, and process_guid executions from your endpoints. If you are using Sysmon, you can use the Add-on for Linux Sysmon from Splunkbase.

Required field

  • _time
  • Filesystem.dest
  • Filesystem.file_create_time
  • Filesystem.file_name
  • Filesystem.process_guid
  • Filesystem.file_path

Kill Chain Phase

  • Privilege Escalation

Known False Positives

Administrator or network operator can create file in crontab folders for automation purposes. Please update the filter macros to remove false positives.

RBA

Risk Score Impact Confidence Message
25.0 50 50 a file $file_name$ is created in $file_path$ on $dest$

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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