Скачать или смотреть How to Correctly Call a Lambda Function in Another AWS Account from Your Lambda

Learn how to solve the issue of invoking a lambda function in another AWS account from your own lambda, including IAM roles and permissions setup.
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This video is based on the question https://stackoverflow.com/q/63019007/ asked by the user 'Chris Chevalier' ( https://stackoverflow.com/u/8644368/ ) and on the answer https://stackoverflow.com/a/63024420/ provided by the user 'Marcin' ( https://stackoverflow.com/u/248823/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding Cross-Account Lambda Invocation in AWS

When working with AWS Lambda functions, you may find yourself needing to invoke a Lambda function located in a different AWS account. This cross-account interaction can be tricky due to the strict security policies AWS employs. This post will guide you through the steps and potential pitfalls of calling a Lambda function in another account from your own Lambda function.

The Problem at Hand

You might have already experienced the following issues when attempting to invoke a Lambda function in Account B from a Lambda function in Account A. Here’s a quick recap of the situation:

You have a role in Account A that is supposed to invoke a Lambda function in Account B.

You receive AccessDenied errors when trying to assume the necessary role or invoke the function.

Let’s examine how to fix these issues step by step.

Step 1: Double-Check Your Role ARNs

One of the first places to check is the role ARNs for your AWS Lambda functions in both accounts. Based on the setup you provided, ensure that your Principal matches the correct role ARN format. For reference, here’s what that looks like:

[[See Video to Reveal this Text or Code Snippet]]

Make sure you are not mistakenly using arn:aws:sts or any other incorrect service prefix. The ARNs should consistently use arn:aws:iam for IAM roles.

Step 2: Adjust The Permissions for Lambda Invocation

The next potential error lies in how you're granting permissions to invoke the Lambda function. The Lambda invocation permissions should be applied directly to the function, not the IAM role. The following snippet of a policy incorrectly refers to a role instead of the function's ARN:

[[See Video to Reveal this Text or Code Snippet]]

Instead, you should update it to point to the function's ARN like this:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Correctly Assume Role with STS

To assume a role in Account B, you would want to leverage AWS Security Token Service (STS). Here’s what a corrected example might look like:

[[See Video to Reveal this Text or Code Snippet]]

Ensure that the ARN of the role from which you are trying to assume also allows for sts:AssumeRole.

Additional Considerations

IAM Policy Accessibility: Ensure that the permissions assigned to Account A’s role include the sts:AssumeRole action allowing for the correct role in Account B.

Role Trust Relationships: Verify that Account B allows Account A's role to assume it. Check the Trust Relationships tab in the IAM console for correct configurations.

Conclusion

Invoking a Lambda function in a different AWS account requires diligent setup of IAM roles and permissions. Always double-check ARNs, ensure the correct resources are being applied, and verify that trusts are effectively established between accounts. Having correct permissions and a clear understanding of AWS's policies will go a long way in ensuring smooth cross-account Lambda function invocations.

If you encounter any further issues or have unique scenarios to discuss, feel free to reach out or leave a comment below!