Cross-Account IAM Access Denied with GUI Client, but permitted via CLI - amazon-web-services

Question

I am stuck with provisioning end-user access into a cross account shared bucket, and need help figuring out if there are specific policy requirements for using clients to access the bucket, vs straight CLI.
IAM User Accounts are managed in our "Core" AWS Account.
S3 Bucket is provisioned in our "Dev" AWS Account.
S3 Bucket in Dev account is encrypted with KMS key in Dev Account.
We have configured our Bucket Policy to permit the user access.
We have configured user policies to permit access to the S3 bucket.
We have configured user policies to permit use of the KMS key.
When using the CLI our user account can succesfully access and use the S3 bucket. When attempting to connect with a GUI Client (Win-SCP, CyberDuck, MAC ForkLift) we receive permission denied errors.
BUCKET POLICY
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "ListObjectsInBucket",
"Effect": "Allow",
"Principal": {
"AWS": [
"arn:aws:iam::[DEVACCOUNT#]:role/EC2-ROLE-FOR-APP-ACCESS",
"arn:aws:iam::[COREACCOUNT#]:user/end.user"
]
},
"Action": "s3:List*",
"Resource": [
"arn:aws:s3:::dev-mybucket",
"arn:aws:s3:::dev-mybucket/*"
]
},
{
"Sid": "AllObjectActions",
"Effect": "Allow",
"Principal": {
"AWS": [
"arn:aws:iam::[DEVACCOUNT#]:role/EC2-ROLE-FOR-APP-ACCESS",
"arn:aws:iam::[COREACCOUNT#]:user/end.user"
]
},
"Action": [
"s3:GetObject",
"s3:Put*"
],
"Resource": "arn:aws:s3:::dev-mybucket/*"
}
]
}
User Policy - access KMS
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "AllowUseOfDevAPPSKey",
"Effect": "Allow",
"Action": [
"kms:Encrypt",
"kms:Decrypt",
"kms:ReEncrypt*",
"kms:GenerateDataKey*",
"kms:Describe*"
],
"Resource": [
"arn:aws:kms:ca-central-1:[DEVACCOUNT#]:key/[redacted-key-number]"
]
},
{
"Sid": "AllowAttachmentOfPersistentResources",
"Effect": "Allow",
"Action": [
"kms:CreateGrant",
"kms:List*",
"kms:RevokeGrant"
],
"Resource": [
"arn:aws:kms:ca-central-1:[DEVACCOUNT#]:key/[redacted-key-number]"
],
"Condition": {
"Bool": {
"kms:GrantIsForAWSResource": true
}
}
}
]
}
User policy - Access S3 Bucket
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "AllowAccessToMyBucket",
"Effect": "Allow",
"Action": [
"s3:*"
],
"Resource": [
"arn:aws:s3:::dev-mybucket/",
"arn:aws:s3:::dev-mybucket/*"
]
}
]
}
From aws s3 commands we can 'ls' content and 'cp' content from local to remote and from remote to local.
When configuring access with the GUI Clients we always receive somewhat generic 'permission denied' or 'access denied' type errors.

Answer 1

The GUI client is probably making a call that is not List*, Put* or GetObject.
For example, it might be calling GetObjectVersion, GetObjectAcl or GetBucketAcl.
Try adding Get* permissions in addition to List*.
You might also be able to look at the events in your AWS CloudTrail trail to see what specific API calls were denied.
For details, see: Specifying Permissions in a Policy - Amazon Simple Storage Service

Answer 2

Access to an S3 bucket via a GUI such as the AWS web console or SFTP clients with s3 functionality(FileZilla, Cyberduck, ForkLift, etc.) requires the s3:ListAllMyBuckets action in a policy attached to that IAM user. This is very unfortunate as the user will now have access to see ALL your bucket names in that account even if they just have read, write, and or List access to just one bucket in that account.
https://docs.aws.amazon.com/AmazonS3/latest/API/API_Operations.html
https://docs.aws.amazon.com/AmazonS3/latest/API/API_ListBuckets.html
One other option is to go to the bucket URL directly. The user/role will require access via that bucket's Bucket policy.
https://s3.console.aws.amazon.com/s3/buckets/dev-mybucket