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Lambda Function Fails with CORS Error Despite Perfect Configuration - Suspected Platform Issue

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Hello AWS Support Team and Community,

We are experiencing a critical and persistent CORS/Internal Server Error with a Lambda function that we believe points to a platform-level issue. We have exhausted all standard and advanced debugging procedures.

Function ARN:

  • arn:aws:lambda:us-east-1:954212210973:function:GetStudentDetails

Function URL:

  • https://ilc7n2nf5xharcinet22j33umm0pyizf.lambda-url.us-east-1.on.aws/

The Problem: When calling the Function URL from a web browser (origin: http://127.0.0.1:5500 ), the request fails with a CORS error: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

Debugging Steps Already Taken (This is not a standard configuration issue):

  1. Code Verification: The Lambda function code (Python 3.12) includes a robust try...except block and a centralized response function that guarantees Access-Control-Allow-Origin headers are added to ALL responses, including all error conditions (4xx and 5xx). The code correctly handles OPTIONS preflight requests.

  2. IAM Permissions: The function's execution role (GetStudentDetails-role-l7wplv9e) currently has the AdministratorAccess policy attached. This was done as a final diagnostic step to completely rule out any IAM permission issues. The error persists even with full admin rights.

  3. Custom Lambda Layer: We encountered and resolved an initial No module named 'jwt' error by creating and deploying our own custom Lambda Layer (MyPyJWTLayer) containing the PyJWT library. The import error is now resolved.

  4. Environment Variables: We have verified that all environment variables (DYNAMODB_TABLE_NAME, JWT_SECRET_ARN, CORS_ALLOWED_ORIGINS) are correctly configured and accessible by the function.

  5. Function URL Configuration: The Function URL's built-in CORS configuration is DISABLED, as CORS is handled entirely within the Lambda code to prevent conflicts.

  6. VPC Configuration: The function is NOT connected to a VPC, ruling out any network timeout issues related to VPC endpoints or NAT Gateways.

  7. Recreation of Resources: We have even tried creating a brand new Lambda function (GetStudentDetailsV2) from scratch, but the issue persists, suggesting the problem is not tied to a single "corrupt" function state.

Conclusion: Given that the function fails to return a proper CORS header even with AdministratorAccess and a code structure designed to prevent this, we strongly suspect a platform-level bug or a silent failure that is occurring before our try...except block can catch it.

Please escalate this case to a Lambda specialist to investigate the internal execution logs and platform metrics for this function. We need to understand why the function is terminating without returning the headers we have programmed it to.

Thank you.

Topics
Front-End Web & MobileComputeNetworking & Content DeliverySecurity, Identity, & Compliance
Tags
Amazon API GatewayAWS IAM Access AnalyzerAWS Lambda
Language
English

asked a year ago308 views

1 Answer
0

The CORS error you're experiencing with your Lambda function URL is likely not a platform-level issue but rather a configuration or implementation problem. Let me explain some potential causes and solutions:

  1. Lambda Function URL CORS Configuration: When you say the Function URL's built-in CORS configuration is disabled, this could be part of the problem. For Lambda Function URLs, you should actually enable the built-in CORS configuration rather than handling it entirely in your code. The Function URL needs to respond properly to the preflight OPTIONS request that browsers send.

  2. Preflight OPTIONS Request Handling: Browsers send an OPTIONS request before the actual request when making cross-origin requests. Your Lambda function needs to properly respond to this preflight request with the appropriate CORS headers. Make sure your function explicitly handles OPTIONS requests and returns a 200 status code with the necessary CORS headers.

  3. CORS Headers in Error Responses: Even though you mentioned your code has a centralized response function that adds CORS headers to all responses, there might be scenarios where your function is terminating before reaching that code. For example, if there's a syntax error or an unhandled exception, the Lambda runtime might return an error response without your CORS headers.

  4. Headers Configuration: Ensure your CORS headers include:

  • Access-Control-Allow-Origin: Set to your origin (http://127.0.0.1:5500) or * for testing
  • Access-Control-Allow-Methods: Include all HTTP methods your API supports
  • Access-Control-Allow-Headers: Include any custom headers your application uses

  1. Response Structure: For Lambda Function URLs, make sure your response structure includes the headers at the top level, not nested within another object.

  2. Early Returns: Check if there are any code paths that might return a response before your CORS headers are added.

  3. Lambda Execution Environment: Sometimes issues can occur due to the Lambda execution environment. Try increasing the function timeout and memory allocation to rule out resource constraints.

I recommend enabling the built-in CORS configuration for your Lambda Function URL as a first step, as this is specifically designed to handle preflight requests correctly. If you must handle CORS in your code, ensure your OPTIONS handler is properly implemented and that all error paths also include the necessary CORS headers.
Sources
CORS error while accessing an endpoint in API gateway | AWS re:Post
CORS Preflight fails on HTTP API Gateway | AWS re:Post
Blocked by CORS policy: Response to preflight request doesn't pass access control check: It does not have HTTP ok status | AWS re:Post

answered a year ago

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