[_jzlw]

The author really needs to start with that. They say "the API that we are building" and assume I know who they are and what they're working on, all the way until the very bottom. I just assumed it's some open source library.

> HTML parsing is not stable and a line of HTML being parsed and serialized and parsed again may turn into something rather different

Are there any examples where the first approach (sanitize to string and set inner html) is actually dangerous? Because it's pretty much the only thing you can do when sanitizing server-side, which we do a lot.

Edit: I also wonder how one would add for example rel="nofollow noreferrer" to links using this. Some sanitizers have a "post process node" visitor function for this purpose (it already has to traverse the dom tree anyway).

[crote]

> Are there any examples where the first approach (sanitize to string and set inner html) is actually dangerous?

The article links to [0], which has some examples of instances in which HTML parsing is context-sensitive. The exact same string being put into a <div> might be totally fine, while putting it inside a <style> results in XSS.

[0]: https://www.sonarsource.com/blog/mxss-the-vulnerability-hidi...

[tobr]

> They say "the API that we are building" and assume I know who they are and what they're working on, all the way until the very bottom.

This is a common and rather tiresome critique of all kinds of blog posts. I think it is fair to assume the reader has a bit of contextual awareness when you publish on your personal blog. Yes, you were linked to it from a place without that context, but it’s readily available on the page, not a secret.

[_jzlw]

Well that's... certainly a take. But I have to disagree. Most traffic coming to blog posts is not from people who know you and are personally following your posts, they're from people who clicked a link to the article someone shared or found it while googling something.

It's not hard to add one line of context so readers aren't lost. Here, take this for example, combining a couple parts of the GitHub readme:

> For those who are unfamiliar, the Sanitizer API is a proposed new browser API being incubated in the Sanitizer API WICG, with the goal of bringing this to the WHATWG.

Easy. Can fit that in right after "this blog post will explain why", and now everyone is on the same page.

[swiftcoder]

> Most traffic coming to blog posts is not from people who know you and are personally following your posts

Do we have data to back that up? Anecdotally the blogs I have operated over the years tend to mostly sustain on repeat traffic from followers (with occasional bursts of external traffic if something trends on social media)

[rerdavies]

Your data sounds a bit anecdotal. :-P

Here's my anecdotal data. Number of blogs that I personally follow: zero. And yet, somehow, I end up reading a lot of blog posts (mostly linked from HN, but also from other places in my webosphere).

(More than a bit irritated by the "Do you have data to back that up" thing, given that you don't really have data to back up your position).

[swiftcoder]

> (More than a bit irritated by the "Do you have data to back that up" thing, given that you don't really have data to back up your position).

It wasn't necessarily a request for you personally to provide data. I'm curious if any larger blog operators have insight here.

"person who only reads the 0.001% of blog posts that reach the HN front page" is not terribly interesting as an anecdotal source on blog traffic patterns

[tobr]

> It's not hard

It’s also not hard to look around for a few seconds to find that information, is my point.

[rerdavies]

What's hard in this case is that you end up making it 80% of the way through the article before you start to wonder what the heck this guy is talking about. So you have to click away to another page to figure out who the heck this guy is, then start again at the top of the article, reading it with that context in mind.

One word would have fixed the problem. "Why does the Mozilla API blah blah blah.". Perhaps "The Mozilla implementation used to...". Something like that.

THAT is not hard.

[LegionMammal978]

They had a link in their post [0]: it seems like most of the examples are with HTML elements with wacky contextual parsing semantics such as <svg> or <noscript>. Their recommendation for server-side sanitization is "don't, lol", and they don't offer much advice regarding it.

Personally, my recommendation in most cases would be "maintain a strict list of common elements/attributes to allow in the serialized form, and don't put anything weird in that list: if a serialize-parse roundtrip has the remote possibility of breaking something, then you're allowing too much". Also, "if you want to mutate something, then do it in the object tree, not in the serialized version".

[0] https://www.sonarsource.com/blog/mxss-the-vulnerability-hidi...

[tlb]

setHTML needs to support just about every element if it's going to be the standard way of rendering dynamic content. Certainly <svg> has to work or the API isn't useful.

SanitizeHTML functions in JS have had big security holes before, around edge cases like null bytes in values, or what counts as a space in Unicode. Browsers decided to be lenient in what they accept, so that means any serialize-parse chain creates some risk.

[LegionMammal978]

If you're rendering dynamic HTML, then either the source is authorized to insert arbitrary dynamic content onto the domain, or it isn't. And if it isn't, then you'll always have a hard time unless you're as strict as possible with your sanitization, given how many nonlocal effects can be embedded into an HTML snippet.

The more you allow, the less you know about what might happen. E.g., <svg> styling can very easily create clickjacking attacks. (If I wanted to allow SVGs at all, I'd consider shunting them into <img> tags with data URLs.) So anyone who does want to use these more 'advanced' features in the first place had better know what they're doing.

[bffjjfjf]

That overly reductive thinking can go back to the 80s before we had learned any lessons. There are degrees of trust. Binary thinking invites dramatic all or nothing failures.

[LegionMammal978]

And my point is that with HTML, there's always an extremely fine line between allowing "almost nothing" and "almost all of it" when it comes to sanitization. I'd love to live in a world where there are natural delineations of features that can safely be flipped on or off depending on how much control you want to give the source over the content, but in practice, there are dozens of HTML/CSS features (including everything in the linked article) that do wacky stuff that can cross over the lines.

[_jzlw]

Ah, I see what they're talking about. That's a good article; my brain totally skipped over that link. Thanks.

[rebane2001]

> Because it's pretty much the only thing you can do when sanitizing server-side

I'd suggest not sanitizing user-provided HTML on the server. It's totally fine to do if you're fully sanitizing it, but gets a little sketchy when you want to keep certain elements and attributes.

[masklinn]

> Are there any examples where the first approach (sanitize to string and set inner html) is actually dangerous?

The term to look for is “mutation xss” (or mxss).