Cross account/region access to SNS topic

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As the title says, im trying to publish to an SNS topic that is in account B which is in eu-west-1, using a lambda function in account A which is in us-east-1. I am getting permissions errors and cannot quite figure out the correct setup to accomplish this. I have a role in account B which has publish permissions to the topic and a trust relationship setup with account A, I also have permissions on the lambdas role to allow for the publish but still it does not work. Any advice would be appreciated.

1 Answer
2
Accepted Answer

To troubleshoot and set up cross-account and cross-region access to an SNS topic, follow these steps:

1. IAM Role in Account B: Ensure that the IAM role in Account B, which has permissions to publish to the SNS topic, has a trust relationship allowing the Lambda function's role in Account A to assume it. The trust policy should look something like this:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::<Account-A-ID>:role/<Lambda-Role-Name>"
            },
            "Action": "sts:AssumeRole"
        }
    ]
}

2. Permissions Policy for the Role in Account B: Ensure that the role in Account B has the necessary permissions to publish to the SNS topic:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "sns:Publish",
            "Resource": "arn:aws:sns:eu-west-1:<Account-B-ID>:<SNS-Topic-Name>"
        }
    ]
}

3. Lambda Function's IAM Role in Account A: Ensure that the Lambda function's role in Account A has the necessary permissions to assume the role in Account B and to publish to the SNS topic:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": "sts:AssumeRole",
            "Resource": "arn:aws:iam::<Account-B-ID>:role/<Role-Name-in-Account-B>"
        },
        {
            "Effect": "Allow",
            "Action": "sns:Publish",
            "Resource": "arn:aws:sns:eu-west-1:<Account-B-ID>:<SNS-Topic-Name>"
        }
    ]
}

4. SNS Topic Policy: Ensure that the SNS topic in Account B has a policy allowing the role from Account A to publish to it. The SNS topic policy should look something like this:

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::<Account-B-ID>:role/<Role-Name-in-Account-B>"
            },
            "Action": "sns:Publish",
            "Resource": "arn:aws:sns:eu-west-1:<Account-B-ID>:<SNS-Topic-Name>"
        }
    ]
}

5. Lambda Code to Assume the Role: In your Lambda function in Account A, you need to assume the role in Account B before publishing to the SNS topic. Here's an example of how you might do this in Python using Boto3:

import boto3

def lambda_handler(event, context):
    # Assume the role in Account B
    sts_client = boto3.client('sts')
    assumed_role = sts_client.assume_role(
        RoleArn='arn:aws:iam::<Account-B-ID>:role/<Role-Name-in-Account-B>',
        RoleSessionName='AssumeRoleSession'
    )

    # Use the temporary credentials to create an SNS client
    credentials = assumed_role['Credentials']
    sns_client = boto3.client(
        'sns',
        region_name='eu-west-1',
        aws_access_key_id=credentials['AccessKeyId'],
        aws_secret_access_key=credentials['SecretAccessKey'],
        aws_session_token=credentials['SessionToken']
    )

    # Publish to the SNS topic
    response = sns_client.publish(
        TopicArn='arn:aws:sns:eu-west-1:<Account-B-ID>:<SNS-Topic-Name>',
        Message='Your message'
    )

    return response

Ensure you replace placeholders (<Account-A-ID>, <Account-B-ID>, <Lambda-Role-Name>, <Role-Name-in-Account-B>, <SNS-Topic-Name>, etc.) with your actual values. This setup should help you resolve the permissions errors and successfully publish to the SNS topic in Account B from the Lambda function in Account A.

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EXPERT
answered 9 days ago
  • Thank you for your answer, it was clear and helpful! It's working for me now, I did not realise I had to assume the role within the lambda using STS. It is surprising that the trust relationship and roles are not enough.

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