Deploy Lambda with code source from another accounts s3 bucket - amazon-web-services

Question

I store my Lambda zip files in an S3 bucket in Account A. In Account B I have my Lambda. I am trying to have my Lambda use the zip file in Account A's bucket but I keep getting:
Your access has been denied by S3, please make sure your request credentials have permission to GetObject for bucket/code.zip. S3 Error Code: AccessDenied. S3 Error Message: Access Denied
I have followed guides I have found online but I am still facing issues.
Here is my current config:
Account A's S3 Bucket Policy:
{
"Version": "2012-10-17",
"Id": "ExamplePolicy",
"Statement": [
{
"Sid": "ExampleStmt",
"Effect": "Allow",
"Principal": {
"AWS": "arn:aws:iam::AccountBID:role/MyLambdaRole"
},
"Action": [
"s3:GetObject",
"s3:ListBucket"
],
"Resource": [
"arn:aws:s3:::bucket",
"arn:aws:s3:::bucket/*"
]
}
]
}
Account B's Lambda Execution Role Policy:
{
"Version": "2012-10-17",
"Statement": [
{
"Effect": "Allow",
"Action": [
"s3:GetObject",
"s3:ListBucket"
],
"Resource": [
"arn:aws:s3:::bucket/*",
"arn:aws:s3:::bucket"
]
}
]
}

Answer 1

The principal in your bucket policy is the role that AWS Lambda uses during execution, which is not used when deploying your function. You could easily just allow the entire B account principal in the bucket policy and then use IAM policies in account B to allow access to the bucket that way.
A bucket policy allowing an entire account looks like this:
{
"Version": "2008-10-17",
"Statement": [
{
"Sid": "ProductAccountAccess",
"Effect": "Allow",
"Principal": {
"AWS": [
"arn:aws:iam::XXXX-account-number:root"
]
},
"Action": [
"s3:Get*",
"s3:List*"
],
"Resource": [
"arn:aws:s3:::bucket",
"arn:aws:s3:::bucket/*"
]
}
]
}
This means that the IAM policies in account B depend on how you do your deployment. Meaning that whatever credentials are used for the deployment need to have S3 permissions for that bucket.