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Underpass is a beginner-friendly CTF from Hack The Box. It’s a great challenge for testing essential skills like basic reconnaissance and Linux privilege escalation. Initial Recon As usual, we begin with an nmap scan. I performed a SYN scan using the -sS flag for detailed output, but a regular TCP connect scan (-sT) would work just as well. To be thorough, I also ran a full UDP scan. Since these scans can be time-consuming and I’m taking these screenshots after completing the challenge, I specified the port numbers manually to speed up the process.