CVE-2024-6151 - Local Privilege Escalation in Citrix Virtual Delivery Agent for Windows

On June 25, 2024, Citrix published a security advisory for a serious local privilege escalation vulnerability, identified as CVE-2024-6151, in its Virtual Delivery Agent (VDA) for Windows. This component is used in Citrix Virtual Apps and Desktops and Citrix DaaS, solutions that power hundreds of thousands of remote desktop and application sessions around the world.

This post takes a close look at CVE-2024-6151: what it is, how an attacker can use it to gain SYSTEM privileges, and what code is involved—with simple examples for learning and awareness. We'll also share references for those interested in digging deeper.

Vulnerability: Local Privilege Escalation (LPE)

- Affected: Citrix VDA for Windows (see official advisory)

Resolution: Update to the latest secure version

- Impact: Attacker with low-level Windows access (normal user, service account) can become SYSTEM (the highest Windows privilege)

Why Is This Important?

Citrix VDA runs on Windows machines to deliver virtual desktops and apps. In many organizations, these run on critical cloud or on-prem infrastructure. If someone with limited access (even a standard logged-in user) can exploit this bug to become SYSTEM, they can take full control: install malware, extract credentials, or pivot further into corporate networks.

Technical Details: How the Vulnerability Works

> Note: The vulnerable component is the Citrix VDA service running as SYSTEM. The vulnerable code lets local users trick the service into launching processes or commands as SYSTEM.

While Citrix did not publicly disclose every detail, security researchers have pieced things together. Common privilege escalation bugs in VDA components involve misconfigured permissions on:

command injection risks in VDA’s interface with user sessions

Scenario Example:
Suppose VDA has a service that listens for commands from normal users (for things like logoff, session reconnect, printer management), but does not properly check who is sending those commands, or what arguments are being passed.

An attacker can use PowerShell or C# (or even legitimate built-in tools) to send a specially crafted request to the vulnerable mechanism, instructing it to spawn a new process as SYSTEM with attacker-controlled arguments.

Here's a generalized concept in C#, which is often used for service interface manipulation

using System;
using System.Diagnostics;
using System.IO.Pipes;

class CitrixVDAExploit
{
    static void Main(string[] args)
    {
        string pipeName = "CitrixVDAPipe"; // Example, the real pipe name may differ
        using (NamedPipeClientStream pipe = new NamedPipeClientStream(".", pipeName, PipeDirection.Out))
        {
            pipe.Connect();

            // Build a SYSTEM-level process launch payload
            string cmd = @"C:\Windows\System32\cmd.exe /c net user hackuser P@sswrd! /add && net localgroup administrators hackuser /add";
            byte[] message = System.Text.Encoding.UTF8.GetBytes(cmd);

            // Send malicious command
            pipe.Write(message, , message.Length);
            Console.WriteLine("Payload sent to VDA service pipe...");
        }
    }
}

> Disclaimer: This example is generic. The actual Citrix VDA exploit may use a different named pipe name, data format, or interface. Validate exact details for your environment.

Reference Links

- Citrix CVE-2024-6151 Security Advisory
- CISA Alert on Citrix VDA Vulnerabilities
- Citrix Product Documentation

Exploit Steps at a Glance

1. Obtain Local Access: Attacker logs in using a normal user account (could be compromised or legitimate).
2. Interact with VDA Service: Using code or tools (like PowerShell, C#, or even manually), attacker sends a crafted request to the vulnerable service interface (e.g., pipe, function).
3. Trigger SYSTEM Execution: The service, running as SYSTEM, unwittingly launches a payload or command as SYSTEM, due to missing authentication/validation.
4. Move Laterally or Persist: Attacker is now SYSTEM and can add accounts, install malware, dump passwords, or create persistence.

Mitigation and Remediation

- Update All Affected Agents: Download and deploy latest fixed version of Citrix VDA for Windows. (citrix.com download page)

Conclusion

CVE-2024-6151 is a textbook example of why privilege boundaries are vital for any Windows service—especially popular platforms like Citrix VDA. Even if a service is not exposed to the Internet, *any* local user can become an attacker if there are weak permissions or poor command validation.

If you manage Citrix Virtual Apps and Desktops, patch now. If you are a security pro or pen tester, use the patterns above to hunt for similar bugs in other software.

*Stay secure, patch early, defend your endpoints!*

Timeline

Published on: 07/10/2024 21:15:11 UTC
Last modified on: 11/21/2024 09:49:04 UTC