Allowing CloudWatch Alarm to send to SNS in other account - amazon-sns

Question

I have a SNS topic in account "A", which is a trigger of a Lambda function in that same account. This Lambda function sends a message to a private Slack channel.
This works fine, as long as the CloudWatch alarm is in the same account (Account A).
But I also want to do this from "Account B", but there I get:
{
"error": "Resource: arn:aws:cloudwatch:REGION:ACCOUNT_B:alarm:ALARM is not authorized to perform: SNS:Publish on resource: arn:aws:sns:REGION:ACCOUNT_A:TOPIC",
"actionState": "Failed",
"notificationResource": "arn:aws:sns:REGION:ACCOUNT_A:TOPIC",
"stateUpdateTimestamp": 1495732611020,
"publishedMessage": null
}
So how do I allow the CloudWatch Alarm ARN access to publish to the topic?
Trying to add the policy fails with:
Invalid parameter: Policy Error: PrincipalNotFound (Service: AmazonSNS; Status Code: 400; Error Code: InvalidParameter; Request ID: 7f5c202e-4784-5386-8dc5-718f5cc55725)
I see that someone else have/had the same problem (years ago!) at https://forums.aws.amazon.com/thread.jspa?threadID=143607, but it was never answered.
Update:
Trying to solve this, I'm now trying to use a local SNS topic, which then sends this to the remove account. However, I'm still getting:
"error": "Resource: arn:aws:cloudwatch:REGION:LOCAL_ACCOUNT:alarm:ALARM is not authorized to perform: SNS:Publish on resource: arn:aws:sns:REGION:LOCAL_ACCOUNT:TOPIC"
This, with this SNS policy:
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "AllowLambdaAccountToSubscribe",
"Effect": "Allow",
"Principal": {
"AWS": "arn:aws:iam::REMOTE_ACCOUNT:root"
},
"Action": [
"sns:Subscribe",
"sns:Receive"
],
"Resource": "arn:aws:sns:REGION:LOCAL_ACCOUNT:TOPIC"
},
{
"Sid": "AllowLocalAccountToPublish",
"Effect": "Allow",
"Principal": "*",
"Action": "sns:Publish",
"Resource": "arn:aws:sns:REGION:LOCAL_ACCOUNT:TOPIC",
"Condition": {
"StringEquals": {
"AWS:SourceAccount": "LOCAL_ACCOUNT"
}
}
}
]
}
If I manually send a message to the topic with the Publish to topic, I can see that it reaches the Lambda function, so everything except the CloudWatch access rights.

Answer 1

By means of trial-and-error, I discovered it was the Condition that didn't work. For some reason. Not sure why it didn't see the source account...
A more extensive policy made it work:
{
"Version": "2012-10-17",
"Statement": [
{
"Sid": "AllowLambdaAccountToSubscribe",
"Effect": "Allow",
"Principal": {
"AWS": "arn:aws:iam::REMOTE_ACCOUNT:root"
},
"Action": [
"sns:Subscribe",
"sns:Receive"
],
"Resource": "arn:aws:sns:REGION:LOCAL_ACCOUNT:TOPIC"
},
{
"Sid": "AllowLocalAccountToPublish",
"Effect": "Allow",
"Principal": {
"AWS": "*"
},
"Action": "sns:Publish",
"Resource": "arn:aws:sns:REGION:LOCAL_ACCOUNT:TOPIC",
"Condition": {
"StringEquals": {
"AWS:SourceAccount": "LOCAL_ACCOUNT"
}
}
},
{
"Sid": "AllowCloudWatchAlarmsToPublish",
"Effect": "Allow",
"Principal": {
"AWS": "*"
},
"Action": "sns:Publish",
"Resource": "arn:aws:sns:REGION:LOCAL_ACCOUNT:TOPIC",
"Condition": {
"ArnLike": {
"AWS:SourceArn": "arn:aws:cloudwatch:REGION:LOCAL_ACCOUNT:alarm:*"
}
}
}
]
}