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Comment by codemonkey-zeta | Hacker News Reader

What XSS vulnerability? Any user can set the innerHTML of any element at any time.

XSS is when other users can write javascript that executes on your machine, like if they can set their forum signature to `<script>fetch('/send/@attacker/100usd')</script>` and the client uses innerHTML to render it on your machine in the context of your authenticated session.

> What XSS vulnerability? Any user can set the innerHTML of any element at any time.

This mindset right here is exactly why XSS is still an issue.

If you pull user generated content and put it in the DOM like this, you will open your users to XSS from other users. Basing your personal use DOM APIs on setting `el.innerHTML` will lead to a slip-up. Use `textContent` by default.

Often times you write code to defend against other developers who don't know any better.

Most of the time I write code to defend against future me, because I know that in a few days I'll have forgotten half of the code along with all the optimization hacks in it.

Oh my, of course. My assumption was that this function would not be used with dynamic, possibly user-defined input. This is small-scale thinking, and obviously if you intend for this code to be reused then this case must be accounted for. I'm pretty terrified that I was able to look at this code and have that assumption, even though I know better. I've even caught and resolved a couple XSS vulnerabilities at companies I've worked for. What does this say about me? Maybe another question to ask is, what does this say about the value of a web framework?

> I'm pretty terrified that I was able to look at this code and have that assumption, even though I know better. I've even caught and resolved a couple XSS vulnerabilities at companies I've worked for. What does this say about me? Maybe another question to ask is, what does this say about the value of a web framework?

I don't think it says anything about you, it's a very easy mistake to make. But it should say something to you, which is to stay vigilant and to try very hard to not dismiss security concerns without giving them some thought.

And of course always assume code will be misused if you let it, by others who don't know what you're going for and by yourself when you're trying to make a deadline. So always design your interfaces to be secure by default. Obviously easier said than done...

The problem is that the code $element("span", text) looks harmless, appears to work, and yet is dangerously wrong.

Dynamically setting text is very common. While dynamically setting innerHTML is a rare and dangerous operation which should be explicit in the code. The alternative syntax also supported by this function $element("span", { innerHTML: html }) is much better.

https://portswigger.net/web-security/cross-site-scripting/st...

You protect against someone else abusing it on other users.

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