Lambda Invoking Lambda Causes InvalidSignatureException

2 min read 05-10-2024

Lambda Invoking Lambda Causes InvalidSignatureException

Lambda Calling Lambda: Unraveling the "InvalidSignatureException" Mystery

You've built a fantastic serverless application. Your Lambda functions are humming along, performing their tasks with efficiency and grace. But then, you decide to add a layer of complexity by having one Lambda function invoke another. Suddenly, you hit a wall – the dreaded "InvalidSignatureException" appears, leaving you scratching your head.

Let's demystify this common error and equip you with the knowledge to conquer it.

The Scenario: Lambda Calling Lambda

Imagine you have two Lambda functions:

LambdaA: This function is responsible for triggering some event, such as processing a file upload.
LambdaB: This function is called by LambdaA to perform a specific task, like sending a notification.

You've implemented the call using the invoke function within your LambdaA code:

import boto3
lambda_client = boto3.client('lambda')

def lambda_handler(event, context):
    response = lambda_client.invoke(
        FunctionName='your-lambda-b-function-name',
        InvocationType='RequestResponse',
        Payload=json.dumps({'key': 'value'}),
    )
    # Process the response from LambdaB

Everything seems fine until you encounter the "InvalidSignatureException."

Why is the Signature Invalid?

The error arises when Lambda cannot verify the authenticity of the request being made by LambdaA. Here's why:

Authentication: Lambda functions use temporary security credentials to access AWS services. When LambdaA calls LambdaB, it uses its own credentials, which might not be authorized to access LambdaB.
IAM Role: The InvalidSignatureException often indicates a misconfiguration of the IAM role assigned to LambdaA. It might lack the necessary permissions to invoke other Lambda functions.

Troubleshooting the "InvalidSignatureException"

Here's a breakdown of common solutions and how to implement them:

Ensure Proper IAM Permissions:
- Create or Update IAM Role for LambdaA: The role assigned to LambdaA should have the lambda:InvokeFunction permission for LambdaB.
- Attach Policy: You can use a managed policy like AWSLambdaBasicExecutionRole or create a custom policy that grants the necessary permissions.
Example Policy:
```
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "lambda:InvokeFunction",
      "Resource": "arn:aws:lambda:your-region:your-account-id:function:your-lambda-b-function-name" 
    }
  ]
}
```
Verify Credentials:
- Check for Expired Credentials: Ensure that the temporary credentials used by LambdaA are valid and have not expired.
- Review Role Assumptions: If your Lambda functions are assuming roles, double-check that the role assumptions are correctly configured and have the required permissions.
Account Id Considerations:
- Cross-Account Calls: If you're trying to call a Lambda function in a different AWS account, ensure that the IAM role has the necessary permissions for cross-account invocation. You'll need to set up a trust relationship between the accounts.
Code Review:
- Lambda Client: Check that you are using the correct Lambda client and the invoke function is properly implemented.
- Payload Format: Make sure the data you're sending to LambdaB in the Payload is correctly formatted as JSON.

Additional Tips

Error Logging: Enable logging for both LambdaA and LambdaB to capture relevant error messages that can provide clues about the issue.
CloudTrail: Utilize CloudTrail to monitor Lambda invocations and review events related to the error.
Monitoring: Use AWS CloudWatch metrics to monitor Lambda invocation errors and troubleshoot performance bottlenecks.

Conclusion

The "InvalidSignatureException" can be a frustrating obstacle, but with careful analysis and the right steps, you can overcome it. By understanding the cause of the error and following these troubleshooting tips, you can ensure seamless communication between your Lambda functions and build robust, scalable serverless applications.