It_is_my_Birthday

Desccription :

I sent out 2 invitations to all of my friends for my birthday! I’ll know if they get stolen because the two invites look similar, and they even have the same md5 hash, but they are slightly different! You wouldn’t believe how long it took me to find a collision. Anyway, see if you’re invited by submitting 2 PDFs to my website. http://mercury.picoctf.net:50970/

Solution

as discription is given we can say that this challenge is about md5 collision. in simple term collision means two differnt strings with same md5 hash.

Browse the given link, website is asking for upload two different files in pdf formate.

when we do so we will get an error that md5 hash do not.

now we have to find two different file with same md5 hash, you can find it here https://www.mathstat.dal.ca/~selinger/md5collision/

change exe to pdf and aploade the files, and you will get response.

Click for response :

<?php

if (isset($_POST["submit"])) {
    $type1 = $_FILES["file1"]["type"];
    $type2 = $_FILES["file2"]["type"];
    $size1 = $_FILES["file1"]["size"];
    $size2 = $_FILES["file2"]["size"];
    $SIZE_LIMIT = 18 * 1024;

    if (($size1 < $SIZE_LIMIT) && ($size2 < $SIZE_LIMIT)) {
        if (($type1 == "application/pdf") && ($type2 == "application/pdf")) {
            $contents1 = file_get_contents($_FILES["file1"]["tmp_name"]);
            $contents2 = file_get_contents($_FILES["file2"]["tmp_name"]);

            if ($contents1 != $contents2) {
                if (md5_file($_FILES["file1"]["tmp_name"]) == md5_file($_FILES["file2"]["tmp_name"])) {
                    highlight_file("index.php");
                    die();
                } else {
                    echo "MD5 hashes do not match!";
                    die();
                }
            } else {
                echo "Files are not different!";
                die();
            }
        } else {
            echo "Not a PDF!";
            die();
        }
    } else {
        echo "File too large!";
        die();
    }
}

// FLAG: picoCTF{c0ngr4ts_u_r_1nv1t3d_73b0c8ad}

?>
<!DOCTYPE html>
<html lang="en">

<head>
    <title>It is my Birthday</title>


    <link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css" rel="stylesheet">

    <link href="https://getbootstrap.com/docs/3.3/examples/jumbotron-narrow/jumbotron-narrow.css" rel="stylesheet">

    <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>

    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>


</head>

<body>

    <div class="container">
        <div class="header">
            <h3 class="text-muted">It is my Birthday</h3>
        </div>
        <div class="jumbotron">
            <p class="lead"></p>
            <div class="row">
                <div class="col-xs-12 col-sm-12 col-md-12">
                    <h3>See if you are invited to my party!</h3>
                </div>
            </div>
            <br/>
            <div class="upload-form">
                <form role="form" action="/index.php" method="post" enctype="multipart/form-data">
                <div class="row">
                    <div class="form-group">
                        <input type="file" name="file1" id="file1" class="form-control input-lg">
                        <input type="file" name="file2" id="file2" class="form-control input-lg">
                    </div>
                </div>
                <div class="row">
                    <div class="col-xs-12 col-sm-12 col-md-12">
                        <input type="submit" class="btn btn-lg btn-success btn-block" name="submit" value="Upload">
                    </div>
                </div>
                </form>
            </div>
        </div>
    </div>
    <footer class="footer">
        <p>&copy; PicoCTF</p>
    </footer>

</div>

<script>
$(document).ready(function(){
    $(".close").click(function(){
        $("myAlert").alert("close");
    });
});
</script>
</body>

</html>

there is your flag.

Writeups

This repository contains writeups for different CTFs and other challenges.

It_is_my_Birthday

Desccription :

Solution

flag : picoCTF{c0ngr4ts_u_r_1nv1t3d_73b0c8ad}