CIS 12.10 Lambda Cross-Account Access

Ensure Lambda functions do not allow unknown cross account access via permission policies

Description

Ensure that all your Amazon Lambda functions are configured to allow access only to trusted AWS accounts in order to protect against unauthorized cross-account access.

Rationale

Allowing unknown (unauthorized) AWS accounts to invoke your Amazon Lambda functions can lead to data exposure and data loss. To prevent any unauthorized invocation requests for your Lambda functions, restrict access only to trusted AWS accounts.

Impact

Restricting cross-account access may break existing integrations with partner or trusted accounts. Ensure all legitimate cross-account relationships are documented before restricting access.

Audit Procedure

Using AWS Console

1. Login to the AWS Console using https://console.aws.amazon.com/lambda/.
2. In the left column, under AWS Lambda, click Functions.
3. Under Function name click on the name of the function that you want to review
4. Click the Configuration tab
5. In the left column, click Permissions.
6. In the Resource-based policy statements section, click View policy document
7. Review the Resource-based policy document box. Find the "Principal" element and check the element value (ARN).
8. Confirm that each AWS account ARN is an approved AWS account. If one or more of the ARNs is not an AWS account defined within your organization, refer to the remediation below.
9. Repeat steps no. 2-8 for each Lambda function available within the current AWS region.
10. Repeat this Audit for all the other AWS regions.

Using AWS CLI

1. Run aws lambda list-functions

aws lambda list-functions --output table --query "Functions[*].FunctionName"

2. This command will provide a table titled ListFunctions

3. Run aws lambda get-policy on the functions listed

aws lambda get-policy --function-name "name_of_function" --output text --query "Policy"

4. This will provide an output of the policy assigned to that function.
5. Identify the "Principal" element for each function for the ARN.
6. Confirm that each AWS account ARN is an approved AWS account. If one or more of the ARNs is not an AWS account defined within your organization, refer to the remediation below.
7. Repeat steps 2-5 for each Lambda function available.
8. Run the Audit in the other AWS cloud regions.

Expected Result

All Lambda function resource-based policies contain only Principal ARNs belonging to trusted and approved AWS accounts within the organization.

Remediation

Using AWS Console

1. Login to the AWS Console using https://console.aws.amazon.com/lambda/.
2. In the left column, under AWS Lambda, click Functions.
3. Under Function name click on the name of the function that you want to review
4. Click the Configuration tab
5. In the left column, click Permissions.
6. In the Resource-based policy statements section, select the policy statement that allows the unknown AWS Account cross-account access
7. Click Edit
8. On the Edit permissions page, replace or remove the AWS Account(s) ARN of the unauthorized principal in the Principal box
9. Click Save
10. Repeat steps for each Lambda function that failed the Audit

Using AWS CLI

N/A - This control is Console-based remediation only.

Default Value

Lambda functions do not allow cross-account access by default. Cross-account access requires explicit configuration of resource-based policies.

References

1. https://awscli.amazonaws.com/v2/documentation/api/latest/reference/lambda/index.html

CIS Controls

Profile

Level 1 | Manual