AWS API Gateway custom Authorizer strange showing error

Question

Here is the context:

• I set up a resource in the API gateway. /user/company
• This resource have 2 methods. Get and POST.
• I have configured a custom Authorizer for this resource.

The problem:

• I can call the GET method by sending right authorization information and I get the results as expected.
• I try to send a POST request and I get the following error:

{    "message": "User is not authorized to access this resource"  }

• If I wait for few minutes, then call the POST method, it will work.
• If after calling the POST method and getting the results I call GET method, it will show the same error as mentioned above.

In addition, I have disabled cache for the authorizer.

What might have caused this issue?

Answer 1

This could be fixed in two ways that are described in buggy's answer: https://forum.serverless.com/t/rest-api-with-custom-authorizer-how-are-you-dealing-with-authorization-and-policy-cache/3310

Short version:

1. Set TTL for customer authorizer to 0
2. Set custom authorizer policy resource as "*"

I've tried each solution and they both solved the issue with "User is not authorized to access this resource" for me.

Answer 2

This error will occur if you use event.methodArn as a resource for generated policy and share an authorizer between different functions, because of how policy caching works. For provided token it caches a policy across an entire API, it will be the same cache entry for all methods and resources within the same API and stage (if they share the same authorizer).

For example, when making a request to GET /users, ARN will look something like this:

arn:aws:execute-api:us-1:abc:123/prod/GET/users 

Next call to any endpoint with the same authentication token will use a cached policy, which was created on the first call to GET /users. The problem with that cached policy is that it's resource only allows a single particular resource arn: ... /prod/GET/users, any other resource will be rejected.

Depending on how much do you want to limit policy permissions, you can either mention every possible resource when creating a policy

{   "principalId": "user",   "policyDocument": {     "Statement": [       {         "Action": "execute-api:Invoke",         "Effect": "Allow",         "Resource": [           "arn:aws:execute-api:us-1:abc:123/prod/GET/v1/users",           "arn:aws:execute-api:us-1:abc:123/prod/POST/v1/users",           "arn:aws:execute-api:us-1:abc:123/prod/GET/v1/orders"         ]       }     ],     "Version": "2012-10-17"   } } 

or use wildcards

"Resource": "arn:aws:execute-api:us-1:abc:123/prod/*/v?/*" 

or even

"Resource": "*" 

You can use policy variables for some advanced templates.

It is also possible to use a blacklist approach by allowing everything using wildcards and then denying specific resources in another policy statement.

Sources:

• AWS forums: API Gateway issue about custom authorizers
• AWS docs: IAM Policy resource field