Using Rust Cloudflare Workers: Serverless hCaptcha

Using Netlify Functions with SvelteKit

Using Rust Cloudflare Workers: Serverless hCaptcha


☁️ What are Cloudflare Workers and Why use Them?

We'll start this post on using Cloudflare Workers in Rust by taking a quick look at what workers are. Next we'll see why you might want to write Cloudflare Workers in Rust. Then we will build out a full serverless function which you can use for verifying front-end web users with hCaptcha. If that sounds exciting, then let's crack on!

Using Rust Cloudflare Workers: Test Route: Free plan details: 100k requests per day, up to 10 ms CPU time per request, lowest latency after the first request
Screenshot: Using Rust Cloudflare Workers: Free Plan

Cloudflare Workers provide a way to run serverless code with a free tier. Serverless means you do not have to provision and maintain the server yourself. The provider scales up capacity as and when neeed. Cloudflare workers are akin to Netlify serverless functions but work differently. They start up quicker than other serverless solutions. Have you been to a quiet terminal in a small airport which has escalators that only start up when they detect a user wants to step on? This is kind of how most serverless environments work. You have to wait a moment as the server spins up and is ready to process your request, just like it takes the escalator a moment to start moving. Cloudflare workers are like escalators at a busy airport, they are always running, ready for someone to step on.

What Makes Workers Fast?

Workers are intrinsically faster because there is no startup time. You just drop your code (as an isolate ) into a WebAssembly context capable of running hundreds of other isolates simultaneously and it runs straight away. They run almost three times faster than Lambda functions . The Cloudflare network itself helps in reducing running time, with the request arriving quick at your endpoint. Cloudflare Workers run on the V8 WebAssembly engine.

On a free tier account you can run the worker up to 10ms. Although that might sound quite stingy, I found no issues running hCaptcha requests in this time. Here's some output from the dashboard showing CPU time per execution. I was running three Supabase queries on each request as well as the hCaptcha verification on the most intensive requests here:

Using Rust Cloudflare Workers: Test Route: C P U time. Chart shwowing C P U time, 50th percentile is 2.3 ms 75th percentile is 3.2 ms, 99th percentile is 7.3 ms, 99.9th percentile is 7.4ms
Screenshot: Using Rust Cloudflare Workers: CPU Time

πŸ”₯ Why Write Cloudflare Workers in Rust instead of JavaScript?

Native Rust support has only just been introduced into Cloudflare Workers , which is pretty exciting! Rust is a modern language which offers best-in-class speed with low resource usage . Typically it runs faster that JavaScript (partly) because it is compiled ahead of time so does not need to be interpreted at runtime. One drawback is that this comes with the trade-off of development being slightly slower as each change has to be compiled before it can be tested. Rust also assists in writing more secure code than many other languages. This security also comes at a cost as the compiler has to perform some extra checks.

Finally any crates you use with Rust Workers need to be able to compile to Web Assembly Language (WASM). It's not all doom and gloom as the speed and security offer significant potential over JavaScript. You can also compile C or C++ into WASM so might consider using those as alternatives to Rust and JavaScript in Workers.

🧱 What we're Building: Serverless hCaptcha Verification

hCaptcha offers a similar service to Google reCAPTCHA. Both of these services are designed to give you some confidence that the user interacting with your site is actually a person and not a bot. As an example you might use hCaptcha on a contact form to filter responses submitted by bots. When the user submits the form, code running in the background will ask the user to complete a challenge. On successful completion of the challenge, we can let the user complete the form submission.

Using Rust Cloudflare Workers: Serverless hCaptcha

There are two parts to the verification process. In the browser, your code will submit the challenge response to hCaptcha with your site key. hCaptcha replies with a response key. That response key is needed in the second part. The second part is usually performed on a server. We will be using Rust Cloudflare Workers though to verify serverlessly (if that's a word)! We'll focus on that second part here. So we will need to listen for a verification request from our site, on a route we provide. Then we contact hCaptcha with our secret key and the user response key. If hCaptcha than verifies the user is a not a bot, we can proceed with processing the requested action. This might be sending an email to a site admin with the submitted contact form details.

Now we know what we're doing, why don't we set up out worker environment?

πŸ—³οΈ Poll

What language do you use most in WebAssembly (WASM) code?
Voting reveals latest results.

πŸ§‘πŸ½β€πŸŽ“ How to Get Started Using Rust Cloudflare Workers


We'll start by setting up our dev environment, before creating, deploying and testing our worker. You can follow along even if you are new to Rust, I will provide a little extra explanation to help here. If you don't already have Rust dev tools set up, just head over to the official Rust site for the recommended one-line terminal command to get that up and running. Also include ~/.cargo/bin in your PATH environment variable (see previous link).

wrangler installation

Cloudflare have developed wrangler; a command line interface (CLI) tool written in Rust to assist in worker development. Official documentation suggests installing wrangler with npm. I got on better using cargo from the Rust toolchain. I'll show you that way, but keen to hear your feedback on which works best for you.

OK lets install wrangler with cargo (the Rust package manager):

cargo install wrangler

this will take a few minutes to download all the dependencies needed and then build wrangler. Alternatively use npm as per the official documentation .

That's the environment set up. Next we'll create our project.

βš™οΈ Creating a new Project for Using Rust Cloudflare Workers

Firing up a new project is as simple as using the command below.

wrangler generate --type=rust hcaptcha-serverless-rust-worker
cd hcaptcha-serverless-rust-worker

Ignore any errors about the recommended type. The command creates all the boilerplate we need. Let's have a quick look through the generated files.

Project Structure

β”œβ”€β”€ Cargo.lock
β”œβ”€β”€ Cargo.toml
β”œβ”€β”€ src
β”‚Β Β  β”œβ”€β”€
β”‚Β Β  └──
└── wrangler.toml
Boilerplate code structure
  • Cargo.toml - the usual Cargo configuration file. Cargo is Rust's package manager and we list any crates (dependencies) our project needs in here.

  • src/ this is the file that does our heavy lifting. We will define the hCaptcha verification function in here and create a route we can send verification requests to from our client front end.

  • wrangler.toml - a worker configuration file, we can list environment variables in here (like you might do in a .env file in a JavaScript project). There is a different mechanism for handling secret variables which we will come to later.

Initial Test

Let's test the example code out before continuing and adding our own code. The first time we build will take a little bit longer as we have to compile all the crates from scratch. Let's do it now:

wrangler build

You should get a message saying ✨ Build completed successfully! once complete. Next we will link the worker to our Cloudflare account. If you don't yet have a Cloudflare account, you can set one up for free .

Now open the Cloudflare dashboard , logging in if necessary.

Linking Local Environment to your Cloudflare Account

Once you have logged in, in the terminal in our project folder type the command:

wrangler login

Answer β€œyes” to open the link in your browser. You will need to accept the prompt to be able to use wrangler to create your worker. Back in the terminal you will see a message telling you wrangler is successfully configured (if everything went well). The command creates a .wrangler folder. Be sure to add this to you .gitignore file so it is not committed if you push your project to GitHub.

We are all ready to fire up the example code now! The final command in the launch sequence is

wrangler dev

This is pretty much like the dev environment when working on a Next, Node or SvelteKit app. When you make changes the code will automatically compile and create an updated version of the binary. By default wrangler dev serves the worker at .

Environment Variables

Let's see what we can do! Jump to src/

49.get("/worker-version", |_, ctx| {
50 let version = ctx.var("WORKERS_RS_VERSION")?.to_string();
51 Response::ok(version)
52 })

This block defines a route /worker-version which responds to GET requests. In line 50 we reference the WORKERS_RS_VERSION variable. You can see this is defined in line 8 of wrangler.toml. You can create your own varaibles there and use them in your code in the same way. Let's test our worker. Go to in your browser. This won't be the most exciting web page you look at today! Nonetheless, it contains the defined variable.

Using Rust Cloudflare Workers: Test Route: browser screenshot with the address in the navigation bar.  Only 0.0.6 is displayed in the main window
Screenshot: Using Rust Cloudflare Workers: Test Route

You have successfully tested your first Rust Worker! Next, let's code up our hCaptcha endpoint.

πŸ–₯ Coding up our Rust Cloudflare Worker

First we will need a function to process the request sent from our client website and interact with hCaptcha, verifying the user for us. Add the following to src/

16async fn verify_captcha(client_response: &str, secret: &str, sitekey: &str) -> Option<bool> {
17 let mut map = HashMap::new();
18 map.insert("response", client_response);
19 map.insert("secret", secret);
20 map.insert("sitekey", sitekey);
21 let client = reqwest::Client::new();
22 let response = match client
23 .post("")
24 .form(&map)
25 .send()
26 .await
27 {
28 Ok(res) => res,
29 Err(_) => return None,
30 };
31 match response.json::<HcaptchaResponse>().await {
32 Ok(res) => Some(res.success),
33 Err(_) => None,
34 }

Here we get the client response key and our secret site key as inputs to the function and return a boolean wrapped in an Option. Options are a Rust feature which allow us to return a β€œnothing” variable in the case something went wrong β€” so we can return true or false (Some results) if we get a regular response from hCaptcha and None if we get an error or can't reach the server for some reason. Lines 17–20 build up the query string needed for our hCaptcha request. We send that in lines 21–25.

hCaptcha JSON Response Struct

hCaptcha will respond with a JSON object. Because Rust is a strongly typed language we need to let the compiler know what types the hCaptcha response variables are. We can do that using the HCaptchaResponse struct used in line 31. Let's define the struct now:

16// Expected response
17// {
18// "success": true|false, // is the passcode valid, and does it meet security criteria you specified, e.g. sitekey?
19// "challenge_ts": timestamp, // timestamp of the challenge (ISO format yyyy-MM-dd'T'HH:mm:ssZZ)
20// "hostname": string, // the hostname of the site where the challenge was solved
21// "credit": true|false, // optional: whether the response will be credited
22// "error-codes": [...] // optional: any error codes
23// "score": float, // ENTERPRISE feature: a score denoting malicious activity.
24// "score_reason": [...] // ENTERPRISE feature: reason(s) for score.
25// }
27struct HcaptchaResponse {
28 success: bool,

We only use the success field from the response, though we have the other variables listed in a comment here for reference.

See the hCaptcha docs for more on the verification processes and additional data sent in the response .

Setting up Crates

You might notice we used the reqwest crate here to contact hCaptcha (equivalent functionality to fetch or axios in the JavaScript world). We need to include the crate in our Cargo.toml file for our code to work:

14cfg-if = "0.1.2"
15reqwest = { version = "0.11.4", features = ["json"]}
16worker = "0.0.6"
17serde = "1.0.117"
18serde_json = "1.0.67"

Also add the serde crate which we need too. Now we are missing a route to receive hCaptcha requests from our client front end on. Update the main function, replacing the routes we no longer need:

53pub async fn main(req: Request, env: Env) -> Result<Response> {
54 log_request(&req);
56 // Optionally, get more helpful error messages written to the console in the case of a panic.
57 utils::set_panic_hook();
59 // Optionally, use the Router to handle matching endpoints, use ":name" placeholders, or "*name"
60 // catch-alls to match on specific patterns. Alternatively, use `Router::with_data(D)` to
61 // provide arbitrary data that will be accessible in each route via the `` method.
62 let router = Router::new();
64 // Add as many routes as your Worker needs! Each route will get a `Request` for handling HTTP
65 // functionality and a `RouteContext` which you can use to and get route parameters and
66 // Environment bindings like KV Stores, Durable Objects, Secrets, and Variables.
67 router
68 .options("/verify", |req, ctx| {
69 preflight_response(req.headers(), &ctx.var("CORS_ORIGIN")?.to_string())
70 })
71 .post_async("/verify", |mut req, ctx| async move {
72 let data: CaptchaRequest;
73 match req.json().await {
74 Ok(res) => data = res,
75 Err(_) => return Response::error("Bad request", 400),
76 }
77 let hcaptcha_sitekey = ctx.var("HCAPTCHA_SITEKEY")?.to_string();
78 let hcaptcha_secretkey = ctx.var("HCAPTCHA_SECRETKEY")?.to_string();
79 match verify_captcha(&data.response, &hcaptcha_secretkey, &hcaptcha_sitekey).await {
80 Some(value) => {
81 if value {
82 // you would proceed with request here
83 console_log!("User verified");
84 }
85 // We don't let the user know we think they are a bot if verify failed
86 Response::ok("Have a great day!")
87 }
88 // something went wrong - we don't know if the user is a bot or not
89 None => Response::error("Error verifying user", 400),
90 }
91 })
92 .run(req, env)
93 .await

Here you see we created a route with code for handling a POST request (we will look at the OPTIONS part in a moment). We expect to receive a JSON object from our client, which looks like this:

"response": "10000000-aaaa-bbbb-cccc-000000000001"

CaptchaRequest in line 72 is a struct analogous to the one we created for the hCaptcha response earlier. For a real-world app, we might receive form data along with the response field from the client. In lines 77–78, we access secret variables which we need to identify and authenticate ourselves with hCatpcha. Let's define these now. You can set up a free hCaptcha site to get valid credentials .

Handling Secrets

At the command line type this command to store a secret variable in our worker's environment:

wrangler secret put HCAPTCHA_SITEKEY

You will get a prompt to type in the value. Repeat for HCAPTCHA_SECRETKEY. Finally add a CORS_ORIGIN string. This will be a comma separated list of allowed origins. If you are testing and your client dev server is at and you want to test staging environment at, enter, We will add the code for this shortly.

Now if you jump to the Cloudflare dashboard , you should see the Cloudflare logo top left. Click the dropdown just beside it and select Workers. Next find the hcaptcha-serverless-rust-worker, click it and open the Settings tab, you will see the secrets have been saved.

Using Rust Cloudflare Workers: Test Route: Cloud flare dashboard screenshot with the names of the environment variable just set up listed within the Environment Variable section.  The values are not shown, in their place is the text Value encrypted
Screenshot: Using Rust Cloudflare Workers: Secrets

CORS Preflight Response

Because we will receive JSON on our endpoint, the client browser will likely run a preflight CORS check . This will contact the same endpoint as the verify request itself, albeit using the OPTIONS method rather than POST. We just need to respond back, with the CORS headers so that the browser will proceed. Let's add the code for this now and also add missing parts so we can wrap up.

Add use statements for the crates we use (these are the Rust equivalent of JavaScript import statements):

1use serde::Deserialize;
2use std::collections::HashMap;
3use worker::*;

Define the CaptchaRequest struct (include client data, for example form field entries in a real-world app):

18struct CaptchaRequest {
19 response: String,

Finally, define the preflight_response function:

37fn preflight_response(headers: &worker::Headers, cors_origin: &str) -> Result<Response> {
38 let origin = match headers.get("Origin").unwrap() {
39 Some(value) => value,
40 None => return Response::empty(),
41 };
42 let mut headers = worker::Headers::new();
43 headers.set("Access-Control-Allow-Headers", "Content-Type")?;
44 headers.set("Access-Control-Allow-Methods", "POST")?;
46 for origin_element in cors_origin.split(',') {
47 if origin.eq(origin_element) {
48 headers.set("Access-Control-Allow-Origin", &origin)?;
49 break;
50 }
51 }
52 headers.set("Access-Control-Max-Age", "86400")?;
53 Ok(Response::empty()
54 .unwrap()
55 .with_headers(headers)
56 .with_status(204))

This just lets the browser know which types of requests we are happy to receive.

πŸ’― Testing

When you are happy with dev performance you can run the preview and publish commands to get your worker live:

wrangler preview
wrangler publish
Using Rust Cloudflare Workers: Test client - h captcha where user must identify pictures with a train. 9 images are shown
Screenshot: Using Rust Cloudflare Workers: Client Test

To test this all out, you will need to knock up a client app which sends the verify request to our worker. It sends this once it has a client response token from hCaptcha. In JavaScript, using fetch, the request to our worker might look something like this:

async function handleVerify() {
try {
if (browser) {
const { response } = await hcaptcha.execute(hcaptchaWidgetID, {
async: true,
const responsePromise = fetch(`${workerUrl}/verify`, {
method: 'POST',
credentials: 'omit',
headers: {
'Content-Type': 'application/json',
body: JSON.stringify({
} catch (error) {
console.error(`Error in handleVerify: ${error}`);

See a full SvelteKit hCapthca example with a contact form.

πŸ™ŒπŸ½ Wrapup

In this post we saw:

  • why you should consider using Rust Cloudflare Workers,

  • creating the Rust worker from scratch,

  • how you might handle server user verification using hCaptcha, including responding to preflight CORS requests.

I hope you have found this useful. If it is your first time looking at Rust code, I hope you like it! Sorry there wasn't time/space to explain more of the Rust code. I highly recommend you take a look at the Rust book . It is really well written and arguably the best way to get started in Rust. Let me know if you would like to see more posts on Rust generally or serverless Rust in particular.

You can use Rust in Netlify functions and in the AWS serverless sphere. I'm keen to hear how you are using Rust Cloudflare Workers in your own projects. Drop a comment below or mention @askRodney on Twitter .

The full code is on the Rodney Lab GitHub page .

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