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Comment by _urga | Hacker News Reader

That's not referring to stylistic concerns, and it's not bashing the DOM per se.

The context of "The DOM is a mess!" on slide 27 is specifically in terms of security, namely "DOM Clobbering" where an attacker can rewrite DOM methods from underneath you, and impedance mismatch owing to parser differences and bugs ("HTML elements implemented in completely different ways, different attribute handling" in the context of defending against XSS).

It's an honest assessment that's more a statement of fact than anything intended to be hurtful. It's not even a harsh statement of truth at that. I find it hard to believe that Chrome or Firefox engineers would find that offensive. I think they would well agree.

DOMPurify is really fantastic security work. It would make for a brilliant contribution to the DOM.

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> namely "DOM Clobbering" where an attacker can rewrite DOM methods from underneath you

I don't see that as a valid security concern in this case. Yes, it will break your code or do unintended things. In order for this to happen an attacker must have access to the page in your user's security context, which means some other preventable security violation has already transpired. This applies equally with any application/language. Even if you could freeze the DOM such that nothing can be assigned to object properties then you might be able to ward off DOM clobbering, but there is still a malicious user in your security context reading all your secure and private details. If you prevent the malicious agent from access this security concern with the DOM is eliminated.

In other words whether or not DOM clobbering occurs a prerequisite security violation is necessary and hardening the DOM won't provide the necessary solution.

Aside from malicious third parties intentionally writing over event handler assignments DOM clobbering really comes down to poor code management, which is the real security problem here. That makes this a stylistic concern. Additional layers of concerns isn't going to make people instantly less lazy. There are better ways to solve for this.

> HTML elements implemented in completely different ways

HTML is not the DOM. These are separate and unrelated technologies that are maintained in very different specifications. This separation is not an accident. It is by design. I know this is a contentious point, about HTML and the DOM being far separated.

"I don't see [DOM Clobbering] as a valid security concern in this case"

It is for sure a valid security concern when doing client-side XSS filtering, which is what the presentation is about. And no, DOM Clobbering does not require an attacker to "have access to the page in your user's security context". Fastmail have an introduction here: https://fastmail.blog/2015/12/20/sanitising-html-the-dom-clo.... Simply put, there's no way to do safe client-side XSS filtering without addressing DOM Clobbering as a valid security concern.

"hardening the DOM won't provide the necessary solution."

And the author is not suggesting or waiting for that. On the contrary, the premise is that XSS sanitizers need to be client-side exactly because the DOM is not hardened and has so many different implementations (even across browser versions). It's counter-intuitive I know, but server-side XSS sanitizers really can't address cross-browser parser differences safely. So again, it's not a question of "stylistic concerns" or "code management" but of doing secure XSS filtering wherever it is best done.

"There are better ways to solve for this."

And if you go on to the next slide, 28, the point is that despite the difficulties, this has been solved in DOMPurify, which should be added to the DOM so that developers can finally have a first-class client-side XSS sanitizer, without having to trust DOMPurify as third-party code.

There are not many people who know more about client-side XSS filtering than Mario Heiderich. And I know of no better client-side solution than DOMPurify.

How does the security aspect of DOM clobbering occur without injecting malicious code into a page?

Again, see Fastmail's introduction: https://fastmail.blog/2015/12/20/sanitising-html-the-dom-clo...

No code injection is required. DOM Clobbering simply presents an ambiguous view of the content being sanitized.

The article makes some false assumptions. My first job out of college was writing HTML in Email. HTML embedded in email presented in webmail was the toughest. That is learning CSS through the school of hard knocks, particularly when IE7 was released with a different box model.

Again, the problem here is injection, specifically HTTP injection. Email doesn't have an injection problem because it has a more robust protocol: RFC 2821, 2822 and their descendants. To make emails pretty somebody had the really bad idea of embedding HTML in email messaging. HTML is reliant upon the simplified architecture of the HTTP protocol. When you want that pretty content in email you make an HTTP request and some server issues a response.

If they simply took the HTML out of email this security problem would be instantly solved for email. Therefore this isn't an email problem. It isn't even an HTML problem. Its a problem of unregulated HTTP requests.

> HTMl is essentially just a serialisation format for the Document Object Model (DOM)

They are separate things.

I can speak to all of this with confidence. I passed the Security +, CASP, and CISSP exams on the first try just from reading a book. I did security for the military for 10 years, have been developing web technologies for 20 years, and have been writing JavaScript/TypeScript for more than a decade.

The real problem is that lazy developers are punishing their users under pressure from business marketing leaders. There are two simple solutions to this problem:

1. Don't do stupid things that punish your users.

2. Create a web standard ACL that limits all HTTP traffic to/from a browser.

These are both sane and simple solutions. Nobody wants them because bad developers don't want to own the liability for implementing somebody (probably a marketing executive) else's bad decisions. Also, because an ACL standard in the browser would kill the web media business.

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