EventBus Should Not Allow Cross Account Access

Using Console

To remediate the “EventBus Should Not Allow Cross Account Access” misconfiguration in AWS using the AWS console, follow these steps:

Login to the AWS Management Console.
Navigate to the Amazon EventBridge service.
Select the Event Bus that is allowing cross-account access.
Click on the “Permissions” tab.
In the “Event bus policies” section, click on the “Edit” button.
Remove any statements that grant cross-account access to the Event Bus.
Add a new statement that only allows access from the AWS account(s) that require access to the Event Bus.
Click on the “Save” button to apply the changes.

After following these steps, the Event Bus will no longer allow cross-account access and will only allow access from the specified AWS account(s).

Using CLI

To remediate the “EventBus Should Not Allow Cross Account Access” misconfiguration in AWS using AWS CLI, follow these steps:

Open your terminal or command prompt and ensure that you have the AWS CLI installed and configured with the appropriate credentials.
Run the following command to list all the Amazon EventBridge event buses in your AWS account:
aws events list-event-buses
Identify the event bus that is allowing cross-account access.
Run the following command to modify the event bus policy and restrict cross-account access:
aws events put-policy --event-bus-name <event-bus-name> --policy '{"Statement":[{"Sid":"RestrictCrossAccountAccess","Effect":"Deny","Principal":"*","Action":"events:PutEvents","Resource":"arn:aws:events:<region>:<account-id>:event-bus/<event-bus-name>"}]}'
Replace <event-bus-name> with the name of the event bus that is allowing cross-account access, <region> with the AWS region where the event bus is located, and <account-id> with your AWS account ID.
Verify that the policy has been updated by running the following command:
aws events describe-event-bus --name <event-bus-name>
The output should show the updated policy with the Deny statement.
Repeat steps 3-5 for any other event buses that are allowing cross-account access.

By following these steps, you have successfully remediated the “EventBus Should Not Allow Cross Account Access” misconfiguration in AWS using AWS CLI.

Using Python

To remediate the misconfiguration “EventBus Should Not Allow Cross Account Access” in AWS, you can follow the below steps in Python:

Create a new EventBridge event bus policy that allows access only to the specific AWS account that should have access to the event bus.

import boto3
import json

eventbridge = boto3.client('events')

account_id = boto3.client('sts').get_caller_identity().get('Account')

policy = {
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "AllowAccountAccess",
            "Effect": "Allow",
            "Principal": {
                "AWS": f"arn:aws:iam::{account_id}:root"
            },
            "Action": "events:*",
            "Resource": "arn:aws:events:us-east-1:<account_id>:event-bus/default"
        }
    ]
}

policy_json = json.dumps(policy)

eventbridge.put_permission(
    Action='events:PutEvents',
    Principal='*',
    StatementId='AllowAccountAccess',
    Condition={
        'ArnEquals': {
            'aws:SourceArn': f'arn:aws:events:us-east-1:<account_id>:event-bus/default'
        }
    }
)

eventbridge.put_event_bus_policy(
    EventBusName='default',
    Policy=policy_json
)

Remove any existing event bus policies that allow cross-account access.

policy = {
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "DenyCrossAccountAccess",
            "Effect": "Deny",
            "Principal": "*",
            "Action": "events:*",
            "Resource": "arn:aws:events:us-east-1:<account_id>:event-bus/default",
            "Condition": {
                "StringNotEquals": {
                    "aws:PrincipalAccount": "<account_id>"
                }
            }
        }
    ]
}

policy_json = json.dumps(policy)

eventbridge.put_event_bus_policy(
    EventBusName='default',
    Policy=policy_json
)

Verify that the event bus policy now only allows access to the specific AWS account that should have access to the event bus.

policy = eventbridge.describe_event_bus(
    Name='default'
)['Policy']

print(policy)

This should remediate the “EventBus Should Not Allow Cross Account Access” misconfiguration in AWS.

Using Terraform

resource "aws_cloudwatch_event_bus" "this" {
  name = "EVENT_BUS_NAME" # replace with your event bus name
}

# Secure example: only allow a specific trusted AWS account (or remove this block entirely
# if you do not want any cross-account access).
resource "aws_cloudwatch_event_permission" "trusted_account" {
  statement_id   = "ALLOW_TRUSTED_ACCOUNT"          # replace with a unique SID
  principal      = "TRUSTED_AWS_ACCOUNT_ID"         # replace with the allowed AWS account ID
  action         = "events:PutEvents"
  event_bus_name = aws_cloudwatch_event_bus.this.name
}

To match the CLI remediation (aws events remove-permission), remove from your Terraform configuration any aws_cloudwatch_event_permission resources whose principal represents unintended or unknown accounts (including "*"); on terraform apply Terraform will delete those permissions from the event bus policy.This change is destructive to those permissions and may break existing cross-account event delivery workflows—review carefully before applying.Verification: terraform plan should show the unwanted aws_cloudwatch_event_permission resources as - destroy (or their principal narrowed to only the trusted IDs/ARNs), with no changes to the aws_cloudwatch_event_bus itself.

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EventBus Should Not Allow Cross Account Access

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