Clear it in 3 steps
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Start the environment
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Investigate the target
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Submit the flag
This system has several cron configurations (/etc/cron.d, /etc/crontab, user crontabs, and more). Inspect each configuration file to find where the flag is written.
Check the contents of /etc/cron.d/
crontab -l shows your own crontab too
Pay attention to the arguments passed to cron job commands