Nmap scans the IP and sees that port 21 is open for FTP service (used for file upload and file management).
Connect to the IP using FTP.
Use anonymous ID to log in to FTP.
Use ls to see flag.txt.
Use get flag.txt to download it locally.
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Subsequent content is not disclosed.
Reason - contains flag.
Use ls in the local environment to view files.
See flag.txt file.
Use cat flag.txt to open the file.
See the key.