Clear it in 3 steps
Start the environment
Investigate the target
Submit the flag

Analyze the authentication log `auth.log` to find traces of a brute-force attack. Aggregate failed logins per source IP; the flag is near the successful login of the IP with the outlier failure count.
Count failed logins in auth.log per source IP
The IP with an extreme failure count is the attacker
The flag is near that IP's final successful (Accepted) login