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GCP - AppEngine Privesc

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App Engine

For more information about App Engine check:

{% content-ref url="../gcp-services/gcp-app-engine-enum.md" %} gcp-app-engine-enum.md {% endcontent-ref %}

appengine.applications.get, appengine.instances.get, appengine.instances.list, appengine.operations.get, appengine.operations.list, appengine.services.get, appengine.services.list, appengine.versions.create, appengine.versions.get, appengine.versions.list, cloudbuild.builds.get,iam.serviceAccounts.actAs, resourcemanager.projects.get, storage.objects.create, storage.objects.list

Those are the needed permissions to deploy an App using gcloud cli. Maybe the get and list ones could be avoided.

You can find python code examples in https://github.com/GoogleCloudPlatform/python-docs-samples/tree/main/appengine

By default, the name of the App service is going to be default, and there can be only 1 instance with the same name.
To change it and create a second App, in app.yaml, change the value of the root key to something like service: my-second-app

cd python-docs-samples/appengine/flexible/hello_world
gcloud app deploy #Upload and start application inside the folder

Give it at least 10-15min, if it doesn't work call deploy another of times and wait some minutes.

{% hint style="info" %} It's possible to indicate the Service Account to use but by default, the App Engine default SA is used. {% endhint %}

The URL of the application is something like https://<proj-name>.oa.r.appspot.com/ or https://<service_name>-dot-<proj-name>.oa.r.appspot.com

Update equivalent permissions

You might have enough permissions to update an AppEngine but not to create a new one. In that case this is how you could update the current App Engine:

# Find the code of the App Engine in the buckets
gsutil ls

# Download code
mkdir /tmp/appengine2
cd /tmp/appengine2
## In this case it was found in this custom bucket but you could also use the 
## buckets generated when the App Engine is created
gsutil cp gs://appengine-lab-1-gcp-labs-4t04m0i6-3a97003354979ef6/labs_appengine_1_premissions_privesc.zip .
unzip labs_appengine_1_premissions_privesc.zip

## Now modify the code..

## If you don't have an app.yaml, create one like:
cat >> app.yaml <<EOF
runtime: python312

entrypoint: gunicorn -b :\$PORT main:app

env_variables:
  A_VARIABLE: "value"
EOF

# Deploy the changes
gcloud app deploy

# Update the SA if you need it (and if you have actas permissions)
gcloud app update --service-account=<sa>@$PROJECT_ID.iam.gserviceaccount.com

If you have already compromised a AppEngine and you have the permission appengine.applications.update and actAs over the service account to use you could modify the service account used by AppEngine with:

gcloud app update --service-account=<sa>@$PROJECT_ID.iam.gserviceaccount.com

appengine.instances.enableDebug, appengine.instances.get, appengine.instances.list, appengine.operations.get, appengine.services.get, appengine.services.list, appengine.versions.get, appengine.versions.list, compute.projects.get

With these permissions, it's possible to login via ssh in App Engine instances of type flexible (not standard). Some of the list and get permissions could not be really needed.

gcloud app instances ssh --service <app-name> --version <version-id> <ID>

appengine.applications.update, appengine.operations.get

I think this just change the background SA google will use to setup the applications, so I don't think you can abuse this to steal the service account.

{% code overflow="wrap" %}

gcloud app update --service-account=<sa_email>

{% endcode %}

appengine.versions.getFileContents, appengine.versions.update

Not sure how to use these permissions or if they are useful (note that when you change the code a new version is created so I don't know if you can just update the code or the IAM role of one, but I guess you should be able to, maybe changing the code inside the bucket??).

Write Access over the buckets

As mentioned the appengine versions generate some data inside a bucket with the format name: staging.<project-id>.appspot.com. Note that it's not possible to pre-takeover this bucket because GCP users aren't authorized to generate buckets using the domain name appspot.com.

However, with read & write access over this bucket, it's possible to escalate privileges to the SA attached to the AppEngine version by monitoring the bucket and any time a change is performed, modify as fast as possible the code. This way, the container that gets created from this code will execute the backdoored code.

For more information and a PoC check the relevant information from this page:

{% content-ref url="gcp-storage-privesc.md" %} gcp-storage-privesc.md {% endcontent-ref %}

Write Access over the Artifact Registry

Even though App Engine creates docker images inside Artifact Registry. It was tested that even if you modify the image inside this service and removes the App Engine instance (so a new one is deployed) the code executed doesn't change.
It might be possible that performing a Race Condition attack like with the buckets it might be possible to overwrite the executed code, but this wasn't tested.

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