The following analytic aims to detect unauthorized activities through suspicious file creation in recognized cron table directories, prevalent Unix-based locations for scheduling tasks. This behavior is often exploited by nefarious entities like malware or threat actors, including red teamers, to establish persistence on a targeted or compromised host. The analogy to Windows-based scheduled tasks helps explain the utility of a crontab or cron job. To enhance clarity and actionable intelligence, the anomaly query flags the anomaly, urging further investigation into the added file's details. A cybersecurity analyst should consider additional data points such as the user identity involved, the file's nature and purpose, file origin, timestamp, and any changes in system behavior post file execution. This comprehensive understanding aids in accurately determining the file's legitimacy, facilitating prompt and effective response actions.
- 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
Kill Chain Phase
- CIS 10
1 2 3 4 5 6 | 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`
The SPL above uses the following Macros:
linux_add_files_in_known_crontab_directories_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 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.
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.
Associated Analytic Story
|25.0||50||50||a file $file_name$ is created in $file_path$ on $dest$|
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