DOM Clobbering

from HTML injection to controlling JS variables

xanhacks - HitchHack 2023

https://slides.com/xanhacks/dom-clobbering/

image: https://portswigger.net/research/dom-clobbering-strikes-back

Definition

Define (and occasionally manipulates)

Javascript variables from HTML

You do not write JS code, you can only manipulate a variable that has not been defined.

HTML Injection != Javascript Injection

  • Rich text input
  • Can be found in:
    • Forum post
    • Email
    • Discord message
  • You can define:
    • title, bold, italic, link, images...
    • but obviously no JS script

index.html

script.js

HTML Injection

Vulnerable JS (sink)

Attack scenarios

Example n°1 - Redirection

Text

Attack scenarios

index.html

script.js

HTML Injection

Vulnerable JS (sink)

Example n°2 - JS Execution

Attack scenarios

Prerequisites : HTML Injection & JS Sink

Goal : JavaScript execution

 

Advantages

  • Often bypass CSP (Content-Security-Policy)
  • DOM Clobbering attacks are almost unknown

Disadvantage

  • Very rare to get an HTML injection AND a sink

Theory

What to clobber?

Some HTML attributes

https://github.com/xanhacks/DOM-Clobbering-Generator/

What to clobber?

id or name

We can also create a variable using the name property, but this works only for:

  • embed, form, iframe, image, img, object

What to clobber?

Three levels

We can use form to clobber variables at a depth of 3

first.second.third

What to clobber?

More than three levels

We can use iframe to clobber variables at a depth of 4 and more

first.second.third.fourth
first.second.third.fourth.fifth...

Real world use-case

[html-janitor] Arbitrary HTML can bypass the sanitization process

https://hackerone.com/reports/308158

Practice DOM Clobbering attacks

  • https://portswigger.net/web-security/dom-based/dom-clobbering
  • https://www.root-me.org/fr/Challenges/Web-Client/DOM-Clobbering

Challenges

Whoami

@xanhacks

https://www.offensiveweb.com/

End! Question?

DOM Clobbering

By xanhacks

DOM Clobbering

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