This document discusses the security risks of Google Cloud Shell. Cloud Shell provides a free Linux container with 5GB of storage and tools that can be accessed through a web browser. It is invisible to most security controls and logging within an organization. The document outlines how an attacker could use Cloud Shell to exfiltrate data by uploading files through the web interface or command line to external sites without leaving any traces in standard cloud logs. It recommends limiting access to Cloud Shell and monitoring its usage through a proxy or by inspecting traffic to port 6000 to detect any malicious activity.

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Gray Cover: The dangers of cloud shells
Colin Estep
Principal Security Researcher
Netskope Threat Labs

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• Introduction to Google Cloud Shell
• Common Controls and Logging
• How to abuse Cloud Shell
• Mitigating your risk
Agenda

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Google Cloud Shell can be an avenue for data exfiltration
Why is it a problem?
• Invisible to most organizations
• Easy to exploit
• Evades robust security controls
The Problem

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Introduction to Google Cloud Shell

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What is Google Cloud Shell?
• A free linux-based container providing some compute services
• Can provide a shell in the browser
• Deleted an hour after the session is closed

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Does anything persist?
• 5 GB of persistent storage (unless running in Ephemeral mode)
• It can hold your shell customization (like .bashrc) across sessions
• In order to make use of the storage, Cloud Shell allows you to upload files to the
container.

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Tools included

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Options for launching Cloud Shell
Via Web Browser
Via Command Line

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Where is it running?
Google Managed
Customer Managed
Cloud Shell
Compute
Services
App
Engine
Kubernetes
Engine
Compute
Engine
Google runs the container for you
Corporate Laptops /
Desktops

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Common Controls and Logging

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What GCP provides - Basic Controls
Enable / Disable services
Control group/user access
Firewall policies

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What GCP provides - VPC Service Controls

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Google Ops Agent
Collects telemetry from Compute Engine instances

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What GCP provides - Logging

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Common Controls and Logging
Customer Managed Environment
Corporate Laptops /
Desktops
Cloud
Firewall Rules
Cloud
APIs
Cloud
IAM
VPC Service
Controls
Ops Agent
Cloud
Logging

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How to abuse Cloud Shell

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Exploitation Scenario: Data Exfiltration
Cloud Shell
Internet
Corporate Laptops /
Desktops
File.io
Pastebin.com
[malicious domain]
Upload Upload

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Controls and Logging
Cloud Shell
Internet
Corporate Laptops /
Desktops
File.io
Pastebin.com
[malicious domain]
Upload Upload

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Controls and Logging
Cloud Shell
Internet
Corporate Laptops /
Desktops
File.io
Pastebin.com
[malicious domain]
Upload Upload
No IAM controls to limit access
No audit logs for Cloud Shell API

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Controls and Logging
Cloud Shell
Internet
Corporate Laptops /
Desktops
File.io
Pastebin.com
[malicious domain]
Upload Upload
No visibility into the traffic
No limiting controls on the traffic

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Controls and Logging
Cloud Shell
Internet
Corporate Laptops /
Desktops
File.io
Pastebin.com
[malicious domain]
Upload Upload
No logs for uploads
No shell history
No agent support

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Controls and Logging
Cloud Shell
Internet
Corporate Laptops /
Desktops
File.io
Pastebin.com
[malicious domain]
Upload Upload
No traffic visibility
No Internet access restrictions

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Uploads to Cloud Shell
Cloud Shell
Internet
Corporate Laptops /
Desktops
File.io
Pastebin.com
[malicious domain]
Upload Upload

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Uploads to Cloud Shell
Cloud Shell
Internet
Corporate Laptops /
Desktops
File.io
Pastebin.com
[malicious domain]
Upload Upload
HTTPS via Web Interface
SSH via Command Line

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Uploads via Browser

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Uploading through the web interface looks like this:
Uploads via Browser (metadata)

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Uploading through the web interface looks like this:
Uploads via Browser (metadata)
Destination directory Host domain

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Uploads via Browser (file content)
File contents

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• You must install the google command line utility: gcloud
• Use gcloud to start a shell or an upload
A file upload command example:
gcloud cloud-shell scp localhost:~/sensitive_file cloudshell:~/sensitive_file
Uploads via Command Line

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Uploads via Command Line
Below is the output of debugging from gcloud (--verbosity="debug"):
Key pair: “google_compute_engine”
Sends SCP to port 6000

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Mitigating your risk

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Limit Usage
• Cloud Shell can be disabled for Google Workspace or Cloud Identity Users
• Allow a subset of your users that need it (least privilege)

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Finding Exploitation - Web Interface
Cloud Shell
Internet
Corporate Laptops /
Desktops
File.io
Pastebin.com
[malicious domain]
Upload Upload
Web Proxy with Decryption
Data Loss Prevention

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Finding Exploitation - Command Line
Cloud Shell
Internet
Corporate Laptops /
Desktops
File.io
Pastebin.com
[malicious domain]
Upload Upload
Monitor calls to cloud shell
Monitor traffic volume to port 6000

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• Cloud Shell is not included in commonly used controls and logging provided by GCP
• It would be very easy for a malicious insider or malware to exploit it
• We need monitoring in place from our environment to address:
– Web-based uploads
– SCP-based uploads
• We should take a least privilege approach, and limit access to Cloud Shell
Take-Aways

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Colin Estep
Netskope Threat Labs
LinkedIn: https://www.linkedin.com/in/colinestep/
Twitter: @colinsecure
Thank you!