Logstash S3 output requires bucket object permissions at the root of the bucket, regardless of prefix config

Logstash requires write access to the root of the bucket, regardless of the prefix defined in config.

There are a few issues to note with this behavior:

  • Logstash should be isolating its work into the defined prefix path in config, but it's not.

  • I'm forced to give logstash write access to the root of the bucket! ("Resource": "arn:aws:s3:::my-log-bucket/*") regardless of the prefix setting.

  • After starting logstash it creates test-write files in the root of the bucket, not in the defined prefix path. The logs themselves go in the prefix, these test files should too.

  • Even though the "root" check can be disabled, (S3 output plugin | Logstash Reference [7.16] | Elastic) however I don't want to disable permission checking overall.

Bucket contents:

logstash-programmatic-access-test-object-2022-01-10 21:44:46 +0000
logstash-programmatic-access-test-object-2022-01-10 21:58:05 +0000
logstash-programmatic-access-test-object-2022-01-10 21:59:34 +0000
logstash-programmatic-access-test-object-2022-01-10 22:01:27 +0000
logstash-programmatic-access-test-object-2022-01-10 22:04:12 +0000


Error thrown:

LogStash::ConfigurationError: Logstash must have the privileges to write to root bucket `my-log-bucket`, check your credentials or your permissions.>

Below is the config, that "should" work as-is, but since Logstash isn't respecting the prefix, the above error is shown and logstash doesn't start.

Output config looks like this:

output {
  s3 {
    region => "us-east-1"
    bucket => "my-log-bucket"
    prefix => "logstash/"
    encoding => "gzip"
    server_side_encryption => true
    server_side_encryption_algorithm => "aws:kms"
    size_file => 5242880
    time_file => 30

An access policy such as this should suffice, but it doesn't. The entire root object needs to be granted, not just the prefix.

    "Version": "2012-10-17",
    "Statement": [
            "Sid": "VisualEditor0",
            "Effect": "Allow",
            "Action": "s3:ListBucket",
            "Resource": "arn:aws:s3:::my-log-bucket"
            "Sid": "VisualEditor1",
            "Effect": "Allow",
            "Action": [
            "Resource": "arn:aws:s3:::my-log-bucket/logstash/*"

Granting it write access to "arn:aws:s3:::my-log-bucket/*" puts a band-aid on the problem but exposes more permissions than should be needed for any given service.

This setting S3 output plugin | Logstash Reference [7.16] | Elastic appears to just disable the check completely which is not ideal.

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