Detection: Linux Auditd Edit Cron Table Parameter

Updated Date: 2024-09-30 ID: f4bb7321-7e64-4d1e-b1aa-21f8b019a91f Author: Teoderick Contreras, Splunk Type: TTP Product: Splunk Enterprise Security

Description

The following analytic detects the suspicious editing of cron jobs in Linux using the crontab command-line parameter (-e). It identifies this activity by monitoring command-line executions involving 'crontab' and the edit parameter. This behavior is significant for a SOC as cron job manipulations can indicate unauthorized persistence attempts or scheduled malicious actions. If confirmed malicious, this activity could lead to system compromise, unauthorized access, or broader network compromise.

1`linux_auditd` type=SYSCALL SYSCALL=rename (comm IN ("crontab") OR exe IN ("*/crontab")) success=yes AND NOT (UID IN("daemon")) 
2| rename host as dest 
3| stats count min(_time) as firstTime max(_time) as lastTime by comm exe  SYSCALL UID ppid pid dest 
4| `security_content_ctime(firstTime)` 
5| `security_content_ctime(lastTime)`
6| `linux_auditd_edit_cron_table_parameter_filter`

Data Source

Name Platform Sourcetype Source
Linux Auditd Syscall Linux icon Linux 'linux:audit' '/var/log/audit/audit.log'

Macros Used

Name Value
linux_auditd sourcetype="linux:audit"
linux_auditd_edit_cron_table_parameter_filter search *
linux_auditd_edit_cron_table_parameter_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
T1053.003 Cron Execution
T1053 Scheduled Task/Job Persistence
KillChainPhase.EXPLOITAITON
KillChainPhase.INSTALLATION
NistCategory.DE_CM
Cis18Value.CIS_10
APT38
APT5
Rocke
Earth Lusca

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 Notable Yes
Rule Title %name%
Rule Description %description%
Notable Event Fields user, dest
Creates Risk Event True
This configuration file applies to all detections of type TTP. These detections will use Risk Based Alerting and generate Notable Events.

Implementation

To implement this detection, the process begins by ingesting auditd data, that consist SYSCALL, TYPE, EXECVE and PROCTITLE events, which captures command-line executions and process details on Unix/Linux systems. These logs should be ingested and processed using Splunk Add-on for Unix and Linux (https://splunkbase.splunk.com/app/833), which is essential for correctly parsing and categorizing the data. The next step involves normalizing the field names to match the field names set by the Splunk Common Information Model (CIM) to ensure consistency across different data sources and enhance the efficiency of data modeling. This approach enables effective monitoring and detection of linux endpoints where auditd is deployed

Known False Positives

Administrator or network operator can use this application for automation purposes. Please update the filter macros to remove false positives.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message Risk Score Impact Confidence
A SYSCALL - [$comm$] event was executed on host - [$dest$] to edit the cron table. 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 /var/log/audit/audit.log linux:audit
Integration ✅ Passing Dataset /var/log/audit/audit.log linux:audit

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