this post was submitted on 03 Feb 2024
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HTMX is great and is the only frontend development tool I don't absolutely loathe. It enables lightweight SPA development, and provides a very simple and efficient mechanism for doing HTML over the wire.
Not sure I would call HTMX a SPA framework though? Like it all's easy async content fetching for sure, but it's usually done across a MPA?
Where did I call it a framework?
Sure, wrong term. I think my point still stands though. A SPA is *generally *"rehydrated" DOM elements from JSON data pulled from an API though. Where as HTMX is more akin to classic AJAX style page dynamism.
You'll forgive me if I say this is an instance of splitting hairs and having a particular definition for something that includes extra qualities separate from what those terms are actually describing for most people. Also, things like, I dunno, React, are going to extensively use ajax to accomplish what they do. It's literally just asynchronous javascript. It's like someone saying "my vehicle of choice is a motorcycle" and then someone else saying "A motorcycle isn't really a vehicle. It's a transportation device with wheels. A car is a vehicle." They are both vehicles. They both have wheels. The wheels are ajax. A page made with htmx and a page made with React are both SPAs.
My point is just that I think to most people, HTMX doesn't create what is classically considered to be a SPA any more than older MPA's that called utilized AJAX would be. There's a reason those old-style pages aren't really considered SPA's. Is it splitting hairs? Absolutely.
I think we're gonna have to agree to disagree on definitions. To me, and I believe, to most people, an SPA refers to a UI/UX design pattern that can be implemented with any number of underlying techniques. I would also say that the Wikipedia page for SPAs (on the assumption that wikipedia is a valid tool for establishing consensus for definitions) supports my definition:
And it goes on to list frameworks, AJAX, Websockets, etc.
Unfortunately it also kicks Content Security Policy square in the nuts and shoots a giant hole right through your website security, so if anyone on my team brings up using it I inform them it's an instant security fail if we so much as touch it.
It's a cute idea but horribly implemented. If your website has any security requirements, do not use htmx
Edit: the fact so many people have no idea about this and are downvoting is sad. People need to learn how CSP headers work, and why inherently HTMX completely bypasses this as it currently is designed.
Can you elaborate on that? I haven't used it, but just assume if you host it on your own domain you can have it play nicely with csp, there are docs in their site about it. Where did it fall short for your use case?
CSP allows you to whitelist/blacklist arbitrary Javascript, and ideally you completely blacklist online js from being executed at all, such that only .js files of same domain can be invoked by your website.
This serves the role of locking down injection attacks, only your explicitly approved Javascript can be invoked.
HTMX enables invoking of logic via HTML attributes on HTML elements... which CSP can't cover
Which means you re-open yourself to injection attacks via HTML. Attackers can inject an HTML element with HTMX attributes and no amount of CSP will stop HTMX from going "Okey doke!" And invoking whatever the attributes say to do.
This effectively shoots even a completely locked down CSP config square in the nuts, totally defeating the entire point of using it.
It's a cute idea but what is needed is a way to pre-emptively treat HTMX as a template file that transpiles everything out so the ajax happens in a separate .js file
If we had that, then it'd be safe and secure, as the whole "htmx attributes on elements" thing would just be a templating syntax, but when transpiled it wouldn't be supported anymore so attackers can no longer inject html as an attack vector
This demonstrates a profound misunderstanding of HTMX, and how websites in general operate. So much so that I would not hesitate to describe this as somewhere between a baldfaced lie and just malicious incompetence. You can't "invoke logic via HTML attributes," but you can describe it. HTMX is a client side javascript library that parses custom elements you define in your HTML and uses the data described by them to initiate AJAX calls via the fetch() or XMLHttpRequest browser APIs, which CSP explicitly covers via the connect-src directive: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/connect-src. It's literally just a javascript library that parses HTML and uses it to parameterize AJAX calls. If HTMX were somehow able to bypass CSP, then every single piece of clientside JavaScript in the world could violate it.
Oh boy a semantic argument
Whatever you want to call it, trigger, invoke, whatever.
You can leverage HTML attributes to automatically cause arbitrary Javascript ajax calls to happen by extension if those attributes being present.
Trying to argue the semantics of this is stupid.
You put HTML attributes on shit, and the presence of those attributes in turn causes arbitrary Javascript client side logic to fire off purely due to the presence of those attributes.
That's like, literally it's entire shtick.
And any web dev who remotely understands the point of CSP and why it was created, should instantly have alarm bells going off at the concept of triggering arbitrary ajax via html attributes.
"HTMX doesn't bypass CSP! It just (proceeds to describe the exact mechanism by which it bypasses CSP)"
It's bonkers how many people don't grok this, SMH.
I felt like I had a good understanding of both htmx and csp, but after this discussion I'm going to have to read up on both because both of you are making a logically sound argument to my mind.
I'm struggling to see how htmx is more vulnerable than say react or vue or angular, because with csp as far as I can tell I can explicitly lock down what htmx can do, despite any maliciously injected html that might try to do otherwise.
Thanks for this discussion 🙂
CSP works on the browser API level - all HTMX does is what you could do yourself with any AJAX: send an HTTP request to an endpoint. If the CSP disallows that endpoint, it will fail.
It turns out the language you use can be semantically ambiguous or misleading if you phrase it incorrectly. Today you learned.
Oh, did you finally manage to fucking Google how HTMX works so you could fish for more reasons to say it's unsafe? What you're describing is not a particular concern to HTMX. If an attacker can inject HTML into your page (for example, through an XSS vulnerability), they could potentially set up HTMX attributes to make requests to any endpoint, including endpoints designed to collect sensitive information. But, and this is very important, this is not a unique issue to HTMX; it's a general security concern related to XSS vulnerabilities and improper CSP configurations.
Do you know what the correct cure for that is?
PROPER CSP CONFIGURATION.
Do you genuinely not understand that CSP works on the browser API level? It doesn't check to see if your JavaScript contains reference to disallowed endpoints and then prevents it from running. I don't know how you "think" CSP operates, but what happens is this: The browser exposes an API to allow JavaScript to make HTTP requests - specifically XMLHttpRequest and fetch(). What CSP does is tell the browser "Hey, if you get an API request via XMLHttpRequest or fetch to a disallowed endpoint, don't fucking issue it." That's it. HTMX does not magically bypass the underlying CSP mechanism, because those directives operate on a level beyond HTMX's (or any JS library's) influence BY DESIGN. You cannot bypass if it if's properly configured. Two very serious questions: what part of this is confusing to you? And, have you ever tested this yourself in any capacity to even see if what you're claiming is even true? Because I have tested it and CSP will block ANY HTMX issued request that is not allowed by CSP's connect-src directive, assuming that's set.
HTMX comes with a variety of CSP options, though...
Doesn't matter, the entire implementation principle of how HTMX works and what it does inherently bypasses CSP. There's no getting around that.
You fundamentally are invoking logic via HTML attributes, which bypasses CSP
Well, no, not really. All HTMX really does are AJAX requests to remote resources, which are performed by interpreting attributes in HTML. You specify the type of request and the target for updating. Those requests can sometimes contain parameters, of course, but any API that accepts any kind of conditional or user generated input has to sanitize that input before doing anything meaningful with it. This requirement isn't something particular to HTMX.
This is not true, though. You are manipulating the DOM via HTMX, but CSP has nothing to do with dynamic content manipulation. CSP is more concerned with preventing the injection of malicious code. If what you're referring to, however, is the possibility of someone maliciously injecting HTML with HTMX that performs some nefarious action, then I have to ask (again) why you didn't properly sanitize user input or limit the possible connection sources in your CSP.
If you have a specific example, however, of a way in which HTMX by design violates CSP that can't be dismissed with "you coded your website poorly," I would love to know.
This is like someone pointing out that blowing a giant hole in the hull of your ship causes it to take on water, and you respond by asking "well why aren't you bailing out the water with a bucket?"
You do understand why Content Security Policy exists, and what it is for... right?
"We don't need a watertight ship hull for the voyage, just reinvent and implement a bunch of strapping young lads that 24/7 bail water out of the ship as it sails, it's faster and more efficient than doing something crazy like building your ship to be secure and water tight."
"Wow, these screen doors really suck. I've stuck them on my submarine, but they just don't keep the water out at all. Some people are going to say that I'm a fucking moron and don't understand the technology I use or that I'm too goddamn lazy to actually take the necessary steps to keep water out of my submarine, but I know they're wrong and it's the technology's fault."
In all seriousness, HTMX is a tool designed for a specific job. If you have an API that has either non-parameterized endpoints to hit or an endpoint that accepts a single integer value or UUID or....whatever to perform a database lookup and return stored values to be interpolated into the HTML that endpoint returns, then, great, you've got a lightweight tool to help do that in an SPA. If you're using it to send complex data that will be immediately and unsafely exposed to other users, then...that's not really what it's for. So, I think the core issue here is that you don't really understand the use case and are opposed to it because to use it in a way that is beyond or outside the scope of its established convention is unsafe without extra work involved to guarantee said safety. It also implies you are running a website with a content security policy that either explicitly allows the execution of unsafe inline scripts or which does not care about the sources to which a script connects, which is the only way you could realistically leverage HTMX for malicious ends. So, ultimately, the choice to not adopt comprehensive security measures is one you are free to make, but I wouldn't exactly go around telling people about it.
That's not broad enough.
If you in any way have functionality that handles anything remotely requiring security, do not use HTMX.
This goes way beyond "parameterized endpoints".
Listen extremely closely and pray to God anyone dev with more than 2 brain cells groks how serious th8s vulnerability is:
HTMX enables arbitrary invocation of ANY api endpoint with cookies included, through html attributes, which inherently can't be covered by Content Security Policy
This is deeply important for any web dev worth their salt to understand.
Sanitizing User input should be your LAST layer of defence against attack vectors. Not, NOT, your first and only
It's supposed to be your "break in case of emergency" system, not your primary (and only remaining) defense layer.
I want you to please explain how HTMX bypasses the Content Security Policy connect-src directive, or any -src directive, for that matter, assuming it is specified (which it should be). Because I'm genuinely curious why the HTMX dev team would include a section on CSP in their docs if it did literally nothing, as you say.
Actually, as an even more basic question...you do know that HTMX is literally just an AJAX library, right? It doesn't actually "do" anything via HTML attributes. The additional HTMX attributes, like hx-get, hx-post, etc. just tells HTMX where and how to make the API requests. These requests are executed by the browser's native fetch or XMLHttpRequest APIs, depending on compatibility and implementation. Therefore, HTMX is subject to the same security constraints and policies as any other JavaScript-based operation that makes HTTP requests. Which also, by definition, means that it adheres to the Content Security Policy directives configured for that website.
In other words, an HTML button element with hx-get="https://www.some-endpoint.com/" on it would eventually translate into
on click.
You do understand that, right?
Just to be clear, are you talking about some kind of templating library that literally transpiles all the htmx logic and instead packs it into individual ajax logic in js files "per element", such that you don't need to serve htmx client side and instead you pre-transpile all the ajax logic out to separate files?
Cause the very start of my statements was that if we had something like that then HTMX would be fine, as a templating lib that transpiled out to html+js.
That you can CSP lockdown, because now you no longer are able to invoke arbitrary logic with html attributes, only the explicitly transpiled ajax can and all concepts of htmx have been actually removed from the final html+js you actually serve to the client.
If that is what you are talking about above, then please link me because that sounds awesome and is what HTMX outta be, and would remove all of its security issues.
If that's not what you are talking about, and you truly dont understand the fact that you can't compare an html element that triggers logic (which you can't CSP block), to a script chunk that performs logic (which you can CSP block), then I think you do indeed need to go read up on and understand what the point if CSP is and why it was implemented in browsers.
The two are apples and oranges. HTML elements should not be capable of invoking logic arbitrarily, that violates a core principle of html.
My brother in Christ, what the fuck are you talking about "transpiling HTMX" and "serving HTMX client side?" You don't "serve" HTMX and there's nothing to "transpile into JavaScript." It is JavaScript. That's like saying you "serve React client side" and "transpile JavaScript into more JavaScript." Jesus, I feel like I'm taking crazy pills.
Oh, okay, so you don't actually know what HTMX is or how it works, then? Because HTMX (https://htmx.org/) is a JavaScript library. Like, literally just a JavaScript library. It's like....4000 lines of JavaScript. In fact you can read the source code for it here: https://github.com/bigskysoftware/htmx/blob/master/src/htmx.js. For some...insane reason you seem to think HTMX is its own language. It's not. It's...just a JavaScript library. There is no other language called HTMX. There is no other mechanism or tool called HTMX. No implementation or protocol or ANYTHING else. It's just a small JavaScript library.
Once again, HTMX enhances HTML with various attributes declaratively. It utilizes custom data attributes in HTML (like hx-get, hx-post) to specify how elements on the page should behave - essentially, how and where to fetch data or submit forms without a full page reload. This is a form of declarative programming that tells the htmx.js library (which is just doing fucking AJAX) what to do when certain events occur (e.g., a click or a form submission). The actions (like the actual requesting of data from an endpoint) are performed by the code in htmx.js.
This is a fancy way of saying "if you stick an hx-get attribute on a button, then you can just say where you want a GET request to go to and what element you want updated with the HTML returned from it and htmx.js will parse that out on page load and set an event listener for the button click to know when to initiate an AJAX request to the defined endpoint." If you had an hx-get attribute in an element in a page and that page didn't have the htmx.js library loaded it would do literally nothing.
And, once again, HTMX, being a JavaScript library, operates under the same security constraints as any JavaScript executed in the browser. This means that:
I see you don't understand what the word "if" means, and you also don't understand modern js practices.
You don't serve react client side, any junior dev is familiar with transpiling framework code to produce their website. Yes, you 100% transpile react code before serving it, the fact you dont understand what I am talking about speaks volumes. It's clear this whole time I've been having a discussion with someone who doesn't even know the absolute bare minimum of day 1 front end dev. If you don't understand how literally normal and industry standard something as basic as transpiling js is, you have literally zero business spreading info about something far more serious as HTMX.
You are in zero way qualified to be recommending anyone expose their websites to the security nightmare that is HTMX, stop spreading misinfo, stop encouraging devs to do so.ething stupid, and go learn the basics of FE dev practices.
If you don't understand the tools of the trade, stop spreading terrible info about them online.
Everything you have written in this entire thread has made everyone who has read it stupider and you have actively made the internet a worse place. You are a prime example of the exact thing that is wrong with web devs nowadays.
Go back to the drawing board, you have a LOT to learn still it sounds like.