PutEvents request on EventBus times out - possible permissions issue?

Question

I have account A and account B. Account A has a lambda and execution role. Account B has a custom event bus and event bus rule and an IAM role that allows PutEvents on the custom event bus. The rule matches all events.

My goal is for the lambda in account A to make PutEvents requests on the custom event bus in account B. Then, the event bus rule in account B will match the event and I should see a metric for MatchedEvents in CloudWatch.

The IAM role in account B trusts the IAM role in account A. This statement is under trust relationships.
```
        {
            "Effect": "Allow",
            "Principal": {
                "AWS": "arn:aws:iam::ACCOUNT_A:role/AccountALambdaRole"
            },
            "Action": "sts:AssumeRole"
        }
```

And the IAM role in account A is allowed to assume role. This statement is under permissions:

```
    "Statement": [
        {
            "Action": [
                "sts:AssumeRole"
            ],
            "Resource": [
                "arn:aws:iam::ACCOUNT_B:role/AccountBIAMRole"
            ],
            "Effect": "Allow"
        }
    ]
```

The EventBus in account B also has resource-based policy to allow PutEvents on the EventBus from the IAM role in the same account. I'm not sure this is even necessary.

In Lambda code in account A, I build AWSCredentialsProvider with `STSAssumeRoleSessionCredentialsProvider` using the IAM role ARN from account B. The credentials are used in the EventBridge client. The `AWSSecurityTokenService` created uses `DefaultAWSCredentialsProviderChain.getInstance()` credentials and is passed to the  `STSAssumeRoleSessionCredentialsProvider`:

```
        final AWSSecurityTokenService sts = AWSSecurityTokenServiceClientBuilder.standard()
                .withCredentials(DefaultAWSCredentialsProviderChain.getInstance())
                .withRegion(eventBusAwsRegion)
                .build();
        return new STSAssumeRoleSessionCredentialsProvider.Builder(
                        AccountBIAMRoleArn,
                        "MySession")
                .withStsClient(sts)
                .build();
```

Regardless, I'm seeing a timeout when calling PutEvents:

```
com.amazonaws.http.timers.client.ClientExecutionTimeoutException: Client execution did not complete before the specified timeout configuration.
```

Also account B's "last activity" is never ("-"). So I'm assuming that account A role was never able to assume account B role?

We're using 5 second timeouts but the timeout looks almost immediate (<20ms). Any ideas?

Answer

Hello.

I tried searching for the error message, but couldn't find much useful information.  
Is it possible that the problem is caused by insufficient Lambda specifications?  
For example, will it improve if I slightly increase Lambda's memory usage?  
https://github.com/aws/aws-sdk-java/issues/1776

Also, I don't think it's very relevant, but will it be successful if I change the configuration to issue events from Account A's EventBridge Bus to Account B's EventBridge Bus instead of issuing events directly from Lambda to Account B's EventBridge Bus?  
https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-cross-account.html

Answer

Usually timeout errors means that you have network connectivity issues. Is your Lambda attached to a VPC? If so, do you have a NAT Gateway or an EventBridge VPC Endpoint in that VPC?

Also, based on [this](https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-event-bus-perms.html), you do not need to assume a role. You need to give your Lambda function in Account A the permissions to PutEvents in Account B's Bus and you need to give approval in account B to account A.

PutEvents request on EventBus times out - possible permissions issue?

Relevant content