Caddy Server Basic Auth and CORS Preflight: Resolving "Authorize Server is Not Responding Correctly"

Encountering issues with your Caddy server and CORS preflight requests, specifically seeing an error message indicating that the “Authorize Server Is Not Responding Correctly”? This article delves into a specific scenario where Caddy’s basic authentication interacts unexpectedly with CORS preflight requests, leading to a 401 error. We’ll break down the problem, illustrate it with a practical example, and clarify why this behavior might occur.

Understanding the Issue: 401 Error on CORS Preflight with Basic Authentication

The problem arises when you configure Caddy to serve an API that requires basic authentication and is accessed by a frontend application using CORS (Cross-Origin Resource Sharing). In a typical CORS setup, the browser sends a preflight request (using the OPTIONS method) to the server before the actual request. This preflight checks if the server allows requests from the frontend’s origin with specific headers and methods.

According to the CORS specification, preflight requests should not include authorization headers. However, if your Caddy server is configured with basic authentication, it seems to expect authorization even for these preflight OPTIONS requests. Consequently, when a preflight request is sent without the Authorization header, Caddy incorrectly responds with a 401 error, signaling that the “authorize server is not responding correctly” in the context of authentication requirements.

Let’s examine a minimal configuration and request examples to illustrate this behavior.

Caddy Configuration and Request Examples

Here’s a simple Caddyfile configuration used to demonstrate the issue:

localhost:8080 {
  log access.log errors errors.log
  basicauth / user pass
  cors
}

This Caddyfile sets up a server on localhost:8080 with:

Access and error logging.
Basic authentication for all paths (/).
Enables CORS.

Now, let’s look at the HTTP requests.

Correct Preflight Request (without Authorization – as per CORS spec):

curl -v 'http://localhost:8080' -X OPTIONS -H 'access-control-request-method: GET' -H 'origin: http://localhost:4200' -H 'access-control-request-headers: authorization'

This is the standard, correct preflight request. It specifies the intended request method (GET), the origin of the request (http://localhost:4200), and indicates that the actual request will include the authorization header.

Incorrect Preflight Request (with Authorization – for comparison):

curl -v 'http://localhost:8080' -X OPTIONS -H 'access-control-request-method: GET' -H 'origin: http://localhost:4200' -H 'access-control-request-headers: authorization' -H "Authorization: Basic dXNlcjpwYXNz"

This request is incorrect for a preflight because it includes the Authorization header. However, it helps to understand Caddy’s response in both scenarios.

Expected Behavior:

According to CORS and general web server behavior, the correct preflight request (without authorization) should receive a 200 OK response. This indicates that the server understands the CORS preflight and is ready to handle the actual request (assuming the CORS and basic auth configurations are compatible).

Actual Behavior:

In the scenario described, Caddy responds to the correct preflight request (without authorization) with a 401 error. This 401 error is the indication that the “authorize server is not responding correctly” to the preflight request as it should. Only the incorrect preflight request (with authorization) receives a 200 OK response.

This behavior suggests that Caddy’s basic authentication middleware is intercepting and requiring authentication even for CORS preflight OPTIONS requests, which deviates from the expected CORS handling.

Reproducing the Error

To reproduce this issue, you can follow these steps:

Install Caddy: Ensure you have Caddy version 0.9.5 (as reported in the original scenario) or a more recent version. You can download it from the official Caddy website.
Create a Caddyfile: Create a file named Caddyfile in your working directory with the configuration provided earlier:
```
localhost:8080 {
  log access.log errors errors.log
  basicauth / user pass
  cors
}
```
Create an index.html (optional): You can create a simple index.html file in the same directory, although it’s not strictly necessary for reproducing the 401 error for preflight requests.
Run Caddy: Open your terminal, navigate to the directory containing the Caddyfile, and run ./caddy.
Send Preflight Requests: Use curl or a similar tool to send the correct and incorrect preflight requests as shown above and observe the server responses. You should see a 401 error for the correct preflight request and a 200 OK for the incorrect one.

Conclusion

This behavior highlights a potential conflict between Caddy’s basic authentication and CORS preflight request handling. When basic authentication is enabled, Caddy might be incorrectly requiring authorization even for CORS preflight OPTIONS requests, leading to a 401 “unauthorized” error and the perception that the “authorize server is not responding correctly”.

If you are encountering this issue, it’s crucial to verify your Caddy configuration and understand how basic authentication and CORS are interacting. Further investigation and potentially reporting this behavior to the Caddy community might be necessary to find a proper resolution or workaround.