AWS Lambda Function URL with CORS explained by example

This content originally appeared on DEV Community and was authored by Max

This post explores different configuration options for invoking AWS Lambda functions via a URL.

Overview

Why we need Cross-Origin Resource Sharing (CORS) for Lambda Function URLs

AWS Lambda Functions can be invoked via a public AWS Lambda Function URL with an HTTP call from a browser script, e.g.

const lambdaResponse = await fetch(
  "https://mq75dt64puwxk3u6gjw2rhak4m0bcmmi.lambda-url.us-east-1.on.aws/",
  {
    headers: {
      Authorization: `Bearer ${token}`,
    },
  },
);

Since the calling script and the lambda URL are running on different domains, the web browser would require the right CORS headers (also called CORS protocol) to be present for the call to succeed.

The CORS protocol for the above fetch() involves sending an HTTP OPTIONS request to the lambda with these three CORS headers to confirm that the lambda accepts requests from the script's domain:

Access-Control-Request-Method: GET
Access-Control-Request-Headers: authorization
Origin: https://localhost:8080

The lambda is expected to respond with this set of headers:

Access-Control-Allow-Methods: GET
Access-Control-Allow-Headers: authorization
Access-Control-Allow-Origin: https://localhost:8080

Option 1: Returning CORS from the lambda function handler

It is not hard to return a few headers from your lambda handler. For example, these 4 lines of Rust code do the job:

    let mut headers = HeaderMap::new();
    headers.append("Access-Control-Allow-Origin", HeaderValue::from_static("https://localhost:8080"));
    headers.append("Access-Control-Allow-Methods", HeaderValue::from_static("GET"));
    headers.append("Access-Control-Allow-Headers",HeaderValue::from_static("authorization"));

On the other hand, it is an extra coding, testing and maintenance effort. Any changes to the CORS settings would require code changes as well.

If your client app sends one OPTIONS request for every GET/POST it doubles the number of invocations.
See the Access-Control-Max-Age header to reduce repeat OPTIONS calls by the same client.

Returning CORS from the lambda function handler may be a good option if you need extra control over processing HTTP OPTIONS requests.

Option 2: Configuring CORS headers in AWS Lambda settings

Function URLs can be configured to let AWS handle CORS preflight requests (HTTP OPTIONS method) and add necessary headers to all the other HTTP methods after your lambda returns its response.

This is a simpler and more reliable option that does not require any changes to the function handler.

Custom domain names for Lambda URLs

It is not possible to use custom domain names with Lambda URLs because the web server that invokes the lambda relies on the Host HTTP header to identify which lambda to invoke, e.g. host: mq75dt64puwxk3u6gjw2rhak4m0bcmmi.lambda-url.us-east-1.on.aws.

CNAME example

• lambda URL: mq75dt64puwxk3u6gjw2rhak4m0bcmmi.lambda-url.us-east-1.on.aws
• CNAME record: lambda.example.com CNAME mq75dt64puwxk3u6gjw2rhak4m0bcmmi.lambda-url.us-east-1.on.aws
• host header sent to AWS: host: lambda.example.com

The web server handling that request at the AWS end would not know which lambda function to invoke by looking at host: lambda.example.com and would return an error or time out.

Use ApiGateway or CloudFront as the proxy if using lambda Function URLs is not an option.

Access control configuration for Lambda Function URL

Let's assume that our function is exposed to the internet and does not require AWS IAM authentication.

The following config allows public access to the function via its URL.

You need both the Auth type: NONE setting and a resource-based policy statement to allow public access.
Having one setting without the other results in 403 Permission Denied.

AWS automatically adds the required FunctionURLAllowPublicAccess access policy to the Lambda function when you choose Auth type: NONE in the console.

Request/response examples for different configuration options in detail

This section contains examples of HTTP headers exchanged between the web browser, AWS and the lambda function to help us understand how different configuration options affect the headers.

Example 1: Lambda URL request/response without CORS protocol

1. Enable public access to your lambda function URL as explained earlier
2. Do not enable CORS settings
3. Send a request to the lambda URL displayed in the config screen

Your Function URL config should look similar to this:

A sample lambda URL call that does not require the CORS protocol:

const lambdaResponse = await fetch("https://mq75dt64puwxk3u6gjw2rhak4m0bcmmi.lambda-url.us-east-1.on.aws/");

Your request may look similar to this example:

GET / HTTP/1.1
Host: mq75dt64puwxk3u6gjw2rhak4m0bcmmi.lambda-url.us-east-1.on.aws
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:128.0) Gecko/20100101 Firefox/128.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/png,image/svg+xml,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br, zstd
DNT: 1
Connection: keep-alive
Upgrade-Insecure-Requests: 1
Sec-Fetch-Dest: document
Sec-Fetch-Mode: navigate
Sec-Fetch-Site: same-origin
Sec-Fetch-User: ?1
Priority: u=4
Pragma: no-cache
Cache-Control: no-cache

The lambda function receives all the headers sent by the browser as part of the request payload and some additional AWS headers (see 6 headers starting with x- at the end of the list):

{
  "accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/png,image/svg+xml,*/*;q=0.8",
  "accept-encoding": "gzip, deflate, br, zstd",
  "accept-language": "en-US,en;q=0.5",
  "authorization": "foo-bar",
  "cache-control": "no-cache",
  "dnt": "1",
  "host": "mq75dt64puwxk3u6gjw2rhak4m0bcmmi.lambda-url.us-east-1.on.aws",
  "pragma": "no-cache",
  "priority": "u=4",
  "sec-fetch-dest": "document",
  "sec-fetch-mode": "navigate",
  "sec-fetch-site": "same-origin",
  "sec-fetch-user": "?1",
  "upgrade-insecure-requests": "1",
  "user-agent": "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:128.0) Gecko/20100101 Firefox/128.0",
  "x-amzn-tls-cipher-suite": "TLS_AES_128_GCM_SHA256",
  "x-amzn-tls-version": "TLSv1.3",
  "x-amzn-trace-id": "Root=1-66b44063-116ea8667f4b70e405b1b19a",
  "x-forwarded-for": "222.154.108.14",
  "x-forwarded-port": "443",
  "x-forwarded-proto": "https"
}

Function URL configuration with CORS: not enabled option invokes the lambda for all HTTP methods, including OPTIONS and passes all browser headers to the lambda.

Use this configuration if your lambda handles responses to the HTTP OPTIONS method with the CORS protocol.

Example 2: CORS headers added by lambda handler

This example has the same Lambda URL configuration as in the previous example:

This time we include an optional Authorization header to trigger the CORS protocol:

const lambdaResponse = await fetch("https://mq75dt64puwxk3u6gjw2rhak4m0bcmmi.lambda-url.us-east-1.on.aws/",
  { headers: { Authorization: `Bearer ${token}` } },
);

The new request/response flow:

• a browser script running on https://localhost:8080 attempts to fetch() from the lambda's URL
• the browser initiates the CORS protocol by sending an HTTP OPTIONS request to the lambda
• the lambda replies with the necessary CORS headers
• the browser sends the GET request

The HTTP OPTIONS request would be very similar to the previous example with the addition of a few CORS headers:

• origin: https://localhost:8080
• access-control-request-headers: authorization
• access-control-request-method: GET

This OPTIONS request can be translated as the browser asking our lambda: can I send you a GET request with the authorization header from a web page located at https://localhost:8080?

If the lambda replies Yes, the browser sends the GET request. If the lambda replies something other than Yes, the GET request is never sent and the script gets a CORS error.

Our lambda would see this list of headers for the above OPTIONS request (see 3 lines at the end for CORS):

{
  "accept": "*/*",
  "accept-encoding": "gzip, deflate, br, zstd",
  "accept-language": "en-US,en;q=0.5",
  "cache-control": "no-cache",
  "dnt": "1",
  "host": "mq75dt64puwxk3u6gjw2rhak4m0bcmmi.lambda-url.us-east-1.on.aws",
  "pragma": "no-cache",
  "priority": "u=4",
  "sec-fetch-dest": "empty",
  "sec-fetch-mode": "cors",
  "sec-fetch-site": "cross-site",
  "user-agent": "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:128.0) Gecko/20100101 Firefox/128.0",
  "x-amzn-tls-cipher-suite": "TLS_AES_128_GCM_SHA256",
  "x-amzn-tls-version": "TLSv1.3",
  "x-amzn-trace-id": "Root=1-66b42e1d-56f894f802ecd9bc345ef57a",
  "x-forwarded-for": "222.154.108.14",
  "x-forwarded-port": "443",
  "x-forwarded-proto": "https",
  "origin": "https://localhost:8080",
  "access-control-request-headers": "authorization",
  "access-control-request-method": "GET"
}

Assuming that our lambda function can handle CORS protocol and is happy with the request, the response would have the necessary CORS headers starting with access-control-allow-* to tell the browser that the lambda is happy to receive the GET request, as in this example (see 4 lines at the end for CORS):

HTTP/1.1 200 OK
Date: Thu, 08 Aug 2024 18:18:41 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 5
Connection: keep-alive
x-amzn-RequestId: 9f50a878-597b-4fd9-8a07-717e078a2ab0
X-Amzn-Trace-Id: root=1-66b50c01-207a612c605445d738495a39;parent=5ff8d4e2feb9297b;sampled=0;lineage=a964c7ca:0
access-control-allow-origin: https://localhost:8080
access-control-allow-headers: authorization
access-control-allow-methods: GET
access-control-allow-credentials: true

The browser receives the above response and follows with HTTP GET asking the lambda to do some work, e.g. return some data.
The GET request contains the Authorization header with a truncated JWT token (see the last line):

GET / HTTP/1.1
Host: mq75dt64puwxk3u6gjw2rhak4m0bcmmi.lambda-url.us-east-1.on.aws
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:128.0) Gecko/20100101 Firefox/128.0
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br, zstd
Origin: https://localhost:8080
DNT: 1
Connection: keep-alive
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: cross-site
Priority: u=0
Pragma: no-cache
Cache-Control: no-cache
Authorization: Bearer eyJhbGci...ba3mp4OQ

The lambda does its work and returns some payload with the following headers:

HTTP/1.1 200 OK
Date: Thu, 08 Aug 2024 22:44:38 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 22
Connection: keep-alive
x-amzn-RequestId: 7307ef70-161c-431c-a7ac-b6ef4838549e
Vary: Origin
X-Amzn-Trace-Id: root=1-66b54a56-2822789c217e2f0847d6b03f;parent=0e288bd87a583781;sampled=0;lineage=a964c7ca:0
Access-Control-Allow-Origin: https://localhost:8080
Access-Control-Allow-Credentials: true

The above response has to contain Access-Control-Allow-Origin: https://localhost:8080 and Access-Control-Allow-Credentials: true headers for the browser to accept it (2 lines at the end).

Example 3: CORS request/response headers added by AWS

This example shows how CORS headers can be configured out of the lambda handler:

• allow HTTP GET/POST requests containing Authorization and other headers from https://localhost:8080
• allow sending credentials

The client calls the lambda URL with the same fetch() as before:

const lambdaResponse = await fetch("https://mq75dt64puwxk3u6gjw2rhak4m0bcmmi.lambda-url.us-east-1.on.aws/",
  { headers: { Authorization: `Bearer ${token}` } },
);

The browser does the same CORS protocol as before with the same CORS headers:

Unlike the previous example where the OPTIONS request was handled by the lambda handler, no lambda invocation takes place for HTTP OPTIONS requests which are handled by AWS.

This response was generated by AWS and contains the settings configured in the CORS section of the Function URL:

HTTP/1.1 200 OK
Date: Thu, 08 Aug 2024 18:53:07 GMT
Content-Type: application/json
Content-Length: 0
Connection: keep-alive
x-amzn-RequestId: 215426b9-920c-4d1f-b994-54ccd29b2612
Vary: Origin
Access-Control-Allow-Origin: https://localhost:8080
Access-Control-Allow-Headers: authorization,content-type,x-books-authorization
Access-Control-Allow-Methods: GET,POST
Access-Control-Allow-Credentials: true

The last four lines of response tell the browser it may continue with more GET/POST requests.

Unlike the previous example, the lambda handler was not involved in handling the CORS protocol - it was processed by AWS outside of the function handler.
This is an easier way than handling CORS inside the lambda handler.

A few "gotchas"

This section lists a few minor things that can suck up a lot of your time.

Add one header per line in the CORS configuration form

Since the HTTP headers are sent as a comma-separated list it seems logical to enter the entire list in a single Allow headers box (red highlight).
AWS will let you save the invalid config and produce an incorrect response to the OPTIONS request later.

Enter one header per line (green highlight). Remember that header names are case-insensitive.

Don't allow localhost CORS in production

It is not trivial to exploit this, but it is possible if the site has XSS vulnerabilities or a reverse proxy is involved.

See https://stackoverflow.com/questions/39042799/cors-localhost-as-allowed-origin-in-production for detailed explanations.

Adding and deleting FunctionURLAllowPublicAccess access policy

FunctionURLAllowPublicAccess policy is added by AWS when you choose Auth type: NONE for the Function URL.

It is not removed if you change to Auth type: AWS_IAM, but the public access is no longer available. See AWS docs for more details.

Removing FunctionURLAllowPublicAccess policy while Auth type: NONE blocks public access to the lambda's URL.

Header double-up

AWS adds access-control-allow-origin and access-control-allow-credentials CORS headers regardless of their presence in the response from the lambda.

If our lambda returned CORS headers and the Function URL was configured to return CORS as well, the response would become invalid because access-control-allow-origin and access-control-allow-credentials would be included twice (see 4 lines at the end):

HTTP/1.1 200 OK
Date: Thu, 08 Aug 2024 18:53:09 GMT
Content-Type: text/html; charset=utf-8
Content-Length: 22
Connection: keep-alive
Vary: Origin
x-amzn-RequestId: f1d1d6ce-1208-450d-8c61-e5a51f878090
X-Amzn-Trace-Id: root=1-66b51414-3998ea28145bd0d8661c065f;parent=7df2ad2039925c82;sampled=0;lineage=a964c7ca:0
access-control-allow-headers: x-books-authorization,authorization,content-type
access-control-allow-methods: GET, OPTIONS, POST
access-control-allow-origin: https://localhost:8080
access-control-allow-origin: https://localhost:8080
access-control-allow-credentials: true
access-control-allow-credentials: true

References

• An awesome AWS Lambda debugging tool I used to experiment and capture requests and responses: Github / Crates.io
• AWS Lambda CORS docs: https://docs.aws.amazon.com/lambda/latest/dg/urls-configuration.html?icmpid=docs_lambda_help#urls-cors
• CORS overview on MDN: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS
• CORS protocol spec: https://fetch.spec.whatwg.org/#http-cors-protocol
• CORS request headers
  • Access-Control-Request-Method
  • Access-Control-Request-Headers
  • Origin
• Required CORS response headers
  • Access-Control-Allow-Methods
  • Access-Control-Allow-Headers
  • Access-Control-Allow-Origin
• Optional CORS response headers
  • Access-Control-Max-Age
  • Access-Control-Expose-Headers
  • Access-Control-Allow-Credentials

This content originally appeared on DEV Community and was authored by Max

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