🧠 Task Description

Challenge Name: Firewall3
Platform: pwn.college
Goal: From your host at 10.0.0.1, connect to the remote host at 10.0.0.2 on port 31337.
However, outbound traffic to port 31337 is currently blocked by a firewall rule.

🔍 Reconnaissance

Inspect the challenge logic from /challenge/run:

root@ip-10-0-0-1:~# cat /challenge/run
  • A drop_packets(31337) function uses iptables to block outgoing TCP traffic to port 31337: 
    iptables -A OUTPUT -p tcp --dport 31337 -j DROP
  • The server (10.0.0.2) listens on port 31337 and sends the flag if it receives a connection.
  • You are required to remove the outbound firewall rule and then connect.

Exploitation: Unblock Outbound Port 31337

Step 1: Flush the OUTPUT chain to remove the blocking rule

iptables -F OUTPUT

Step 2: Connect to the server

nc 10.0.0.2 31337

Successful output:

pwn.college{U9e7ao4glbM9Yew3MCVWPwDf7KC.0FM1AjNxwSM0IzMyEzW}

Flag

pwn.college{U9e7ao4glbM9Yew3MCVWPwDf7KC.0FM1AjNxwSM0IzMyEzW}