Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response when creating API through SAM

Question

I am creating Rest APIs with SAM template.

I am testing these APIs through Postman and everything is working as expected, but as soon I integrate these APIs in my application, I am getting this error:

from origin 'http://localhost:3033' has been blocked by CORS policy: Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response.

This is happening only for PUT, POST and PATCH method. GET method is working fine.

I have added the below in the code return part:

return {
    "statusCode": 200,
    "headers": {"Access-Control-Allow-Origin": "*", "Access-Control-Allow-Headers": "*"},
    "body": json.dumps(item)
}

However, the error still shows up.

I created an API and lambda to serve the same function with the console and it was working fine with the Application.

Here is my SAM code for creating an API:

ApiGatewaySignUpFlowUserApi:
  Type: AWS::Serverless::Api
  Properties:
    Name: loadeo_signup
    StageName: Stage
    Cors: "'*'"

CreateUserSignup:
  Type: AWS::Serverless::Function
  Properties:
    FunctionName: loadeo_create_user_signup
    CodeUri: createUserSignup/
    Handler: create_user_signup.lambda_handler
    Timeout: 5
    Runtime: python3.8
    Role: arn:aws:iam::272075499248:role/loadeo_lambda_execution
    MemorySize: 128
    Events:
      GetAllUser:
        Type: Api
        Properties:
          Path: /create-user
          Method: post
          RestApiId:
            Ref: ApiGatewaySignUpFlowUserApi

I can't really understand what might be wrong here. Could anyone please help me with this?

Answer 1

Changing the API definition to below helped me solve the issue:

ApiGatewaySignUpFlowUserApi:
  Type: AWS::Serverless::Api
  Properties:
    Name: loadeo_signup
    StageName: Stage
    Cors: 
      AllowMethods: "'*'"
      AllowHeaders: "'*'"
      AllowOrigin: "'*'"  

Explicitly specifying the CORS rule helped.