Detect Cron in Elastic Security
Adversaries may abuse the cron utility to perform task scheduling for initial or recurring execution of malicious code. The cron utility is a time-based job scheduler for Unix-like operating systems. The crontab file contains the schedule of cron entries to be run and the specified times for execution. Adversaries use cron in Linux, macOS, and ESXi environments to execute programs at system startup or on a scheduled basis for persistence, privilege escalation, or execution. Real-world malware families including Kinsing, Skidmap, GoldMax, NKAbuse, Rocke, and Anchor have all leveraged cron for persistence. In ESXi environments, cron jobs must be created directly via the crontab file (e.g., /var/spool/cron/crontabs/root).
MITRE ATT&CK
- Technique
- T1053 Scheduled Task/Job
- Sub-technique
- T1053.003 Cron
- Canonical reference
- https://attack.mitre.org/techniques/T1053/003/
Elastic Detection Query
sequence by host.name with maxspan=5m
[process where event.type == "start" and
(process.name in ("crontab", "cron") or process.args : "crontab") and
not process.parent.name in ("crond", "systemd", "init")]
[file where event.type in ("creation", "change") and
(file.path : ("/etc/crontab", "/var/spool/cron/*", "/etc/cron.d/*", "/etc/cron.daily/*", "/etc/cron.hourly/*", "/etc/cron.weekly/*", "/etc/cron.monthly/*", "/var/cron/tabs/*"))]
OR
process where event.type == "start" and
(
(process.name in ("crontab", "cron") and
process.args : ("-e", "-l", "-r")) and
(
process.command_line : ("*wget*", "*curl*", "*nc *", "*ncat*", "*netcat*",
"*bash -i*", "*/dev/tcp*", "*python -c*", "*perl -e*",
"*base64 -d*", "*base64 --decode*", "*openssl enc*",
"*chmod +x*", "*chmod 777*", "*.sh*", "*/tmp/*")
or process.parent.command_line : ("*wget*", "*curl*", "*nc *", "*/dev/tcp*",
"*/tmp/*", "*/dev/shm/*", "*base64*")
)
)
OR
file where event.type in ("creation", "change") and
file.path : ("/etc/crontab", "/var/spool/cron/*", "/etc/cron.d/*",
"/etc/cron.daily/*", "/etc/cron.hourly/*",
"/etc/cron.weekly/*", "/etc/cron.monthly/*", "/var/cron/tabs/*") and
process.name not in ("dpkg", "apt", "apt-get", "yum", "rpm", "ansible", "puppet", "chef-client", "salt-minion")Detects T1053.003 cron persistence abuse via three correlated signals: (1) a crontab command followed by a cron file write within 5 minutes, (2) crontab editing with suspicious command payloads (downloaders, reverse shells, temp paths), and (3) direct file writes to cron directories from non-package-manager processes. Covers Linux and macOS cron abuse patterns used by Kinsing, Skidmap, GoldMax, and similar malware families.
Data Sources
Required Tables
False Positives & Tuning
- Legitimate system administrators using crontab -e to schedule authorized maintenance tasks during change windows
- Configuration management tools (Ansible, Chef, Puppet, SaltStack) writing to /etc/cron.d/ as part of automated provisioning
- Package managers (apt, yum, rpm) installing software that includes cron job definitions in /etc/cron.d/ or /etc/cron.daily/
Other platforms for T1053.003
Testing Methodology
Validate this detection against 4 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Add Persistent Cron Job via crontab Command
Expected signal: Auditd: syscall write/open on /var/spool/cron/crontabs/<username>. Syslog/cron log: CMD (/tmp/argus_test.sh) entries every minute. Process creation event for crontab command. File creation event for /tmp/argus_test.sh.
- Test 2Add @reboot Persistence Entry to Crontab
Expected signal: Auditd: write to /var/spool/cron/crontabs/<username> with @reboot content. Syslog: crontab modification event. Process creation for crontab command. File creation for /tmp/argus_backdoor_test.sh with chmod +x.
- Test 3Direct Write to /etc/cron.d/ for System-Wide Persistence
Expected signal: Auditd: file creation syscall on /etc/cron.d/argus-test-job. File creation event with initiating process bash/sudo. Syslog: within 5 minutes, CRON execution of curl command will appear in /var/log/cron or /var/log/syslog.
- Test 4Cron Job with Base64-Encoded Payload Download
Expected signal: File creation event on /etc/cron.d/argus-encoded-test. Cron execution logs showing base64 and bash commands. If wget succeeds (requires listener), process creation for wget and the payload. Auditd syscall records for file write.
References (13)
- https://attack.mitre.org/techniques/T1053/003/
- https://www.cloudsek.com/blog/analysis-of-files-used-in-esxiargs-ransomware-attack-against-vmware-esxi-servers
- https://labs.sentinelone.com/20-common-tools-techniques-used-by-macos-threat-actors-malware/
- https://research.checkpoint.com/2019/speakup-a-new-undetected-backdoor-linux-threat/
- https://blog.aquasec.com/threat-alert-kinsing-malware-container
- https://www.trendmicro.com/en_us/research/19/i/skidmap-linux-malware-uses-rootkit-capabilities-to-hide-cryptocurrency-mining-payload.html
- https://blog.talosintelligence.com/2018/08/rocke-champion-of-monero-miners.html
- https://www.crowdstrike.com/blog/carbon-spider-skeleton-spider-target-esxi-servers-with-novel-ransomware/
- https://www.welivesecurity.com/2022/01/11/signed-sealed-delivered-securing-operational-technology/
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1053.003/T1053.003.md
- https://github.com/SigmaHQ/sigma/tree/master/rules/linux/auditd
- https://linux.die.net/man/5/crontab
- https://www.sans.org/white-papers/1693/
Unlock Pro Content
Get the full detection package for T1053.003 including response playbook, investigation guide, and atomic red team tests.