📊 Challenge Overview

Category	Details	Additional Info
🏆 Event	Nullcon Goa HackIM 2025 CTF	Event Link
🔰 Category	Web	🌐
💎 Points	500	Out of 500 total
⭐ Difficulty	🟡 Medium	Personal Rating: 5/10
👤 Author	@gehaxelt	Profile
🎮 Solves (At the time of flag submission)	75	solve rate
📅 Date	02-02-2025	Nullcon Goa HackIM 2025 CTF
🦾 Solved By	mH4ck3r0n3	Team: QnQSec

📝 Challenge Information

Long sessions must be secure, right? http://52.59.124.14:5008

🎯 Challenge Files & Infrastructure

Provided Files

1	Files: None

🔍 Initial Analysis

First Steps

Initially, the website appears as follows:

Taking a look at the source:

I understood that the presented PHP code has a vulnerability related to predictable session IDs:

1 2	define("ALPHA", str_split("abcdefghijklmnopqrstuvwxyz0123456789_-")); ini_set("error_reporting", 0);

• ALPHA: An array of possible characters used to build the session ID.
• Disable errors to prevent giving attackers any information.

1 2	include "flag.php"; // $FLAG $SEEDS = str_split($FLAG, 4);

The file flag.php contains the flag (e.g., $FLAG = "ENO{...}"). *$SEEDS is an array of substrings of the flag, each 4 characters long:

1 2 3 4 5 6 7 8 9

function session_id_secure($id) { global $SEEDS; mt_srand(intval(bin2hex($SEEDS[md5($id)[0] % (count($SEEDS))]), 16)); $id = ""; for($i = 0; $i < 1000; $i++) { $id .= ALPHA[mt_rand(0, count(ALPHA) - 1)]; } return $id; }

This function generates a custom session ID:

• Calculates the seed for the Mersenne Twister random number generator (MT) based on the first character of the MD5 hash of the combined input of username and password.
• Regenerates the session ID of 1000 characters using the characters defined in ALPHA.

The main vulnerability lies in the fact that the session ID is predictable if we know the flag segments (seed) used to initialize mt_rand. Since the seed is derived from segments of the flag, it is possible to determine the flag itself by understanding the behavior of the Mersenne Twister generator.

1 2 3 4 5 6 7

if (isset($_POST['username']) && isset($_POST['password'])) { session_id(session_id_secure($_POST['username'] . $_POST['password'])); session_start(); echo "Thank you for signing up!"; } else { echo "Please provide the necessary data!"; }

If the username and password are provided, a PHPSESSID is generated based on them. This allows an attacker to register multiple sessions and observe the generated session ID to deduce the seed. So, to summarize:

• Using mt_rand to generate sessions: Mersenne Twister is fast but not secure for cryptographic operations.
• Seed derived from flag segments: This allows using php_mt_seed to predict the seed and, consequently, deduce the flag.
• Predictability of the seed: Since the seed is calculated using a part of the flag (four characters at a time), it’s possible to iteratively decode the entire flag (once we have the first 4 characters, we know the flag starts with ENO{, which gives us the format of the flag).

Searching online, I found a tool for cracking the seed: (https://www.openwall.com/php_mt_seed/). Now that we have all this information, we can move on to the exploitation.

🔬 Vulnerability Analysis

Potential Vulnerabilities

• Predictable Session ID Vulnerability

🎯 Solution Path

Exploitation Steps

Initial setup

The exploitation is divided into four phases:

• Multiple Registration: Register usernames with random combinations of username and password.
• Session ID Retrieval: Observe the PHPSESSID value assigned by the server.
• Using php_mt_seed: Use php_mt_seed to find the seed used and trace back to the part of the flag that generated it.
• Iterate on the Flag: Repeat the process to reconstruct the full flag.

Exploitation

To perform the above-described procedure, I wrote an exploit in Python, which uses:

• php_mt_seed.c

To crack the seed and extract parts of the flag one by one. Once the seed is deduced, we can proceed with reconstructing the flag.

Flag capture

🛠️ Exploitation Process

Approach

The automated exploit literally follows the process described above:

1 2 3

cd php_mt_seed-4.0 make python exploit.py

•  Exploit
• php_mt_seed

🚩 Flag Capture

Flag

Proof of Execution

Screenshot of successful exploitation

🔧 Tools Used

Tool	Purpose
Python	Exploit
php_mt_seed	Seed Cracking

💡 Key Learnings

New Knowledge

I have learned that the mt_rand function in PHP is vulnerable because the seed used for random generation can be extracted to gather information.

Skills Improved

• Binary Exploitation
• Reverse Engineering
• Web Exploitation
• Cryptography
• Forensics
• OSINT
• Miscellaneous

📚 References & Resources

Learning Resources

• https://www.openwall.com/php_mt_seed/

📊 Final Statistics

Created: 02-02-2025 • Last Modified: 02-02-2025 Author: mH4ck3r0n3 • Team: QnQSec