1. 先隨便丟東西上去，都說file too large😥😥 2. 看devTool也沒什麼發現 3. 仔細讀題目，提到MD5和collision=>找到會產生collision的兩個例子(像[這個](https://marc-stevens.nl/research/md5-1block-collision/ ))，下載後把檔名改成.pdf後上傳 4. 得到以下code，含flag ```php <?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> ```