Citrix products are used all over the world for secure remote access, but sometimes even trusted software has serious weaknesses. One such issue is CVE-2022-21827, an improper privilege vulnerability found in the Citrix Gateway Plug-in for Windows (now called Citrix Secure Access for Windows). Read on for a breakdown of what went wrong, who’s affected, and how attackers can exploit this flaw to gain SYSTEM-level power on a Windows machine.

What is CVE-2022-21827?

CVE-2022-21827 is an improper privilege management vulnerability identified in Citrix Gateway Plug-in for Windows versions prior to 21.9.1.2. In plain English, this bug lets a local attacker—meaning someone who already has access to your computer—corrupt or delete files with SYSTEM privileges. SYSTEM is the most powerful user on a Windows machine, even above Administrator, so this is a big deal.

- Product affected: Citrix Gateway Plug-in for Windows/Citrix Secure Access for Windows (before 21.9.1.2)

How Does the Vulnerability Work?

Citrix Gateway Plug-in runs certain background processes and services which have SYSTEM-level permissions. If those services interact badly with files or directories that normal users can replace or mess with (for example, via *insecure file permissions* or *symlink attacks*), smart attackers can trick those SYSTEM services into deleting or corrupting files that would otherwise be protected.

The classic trick: the attacker finds a folder or file created by Citrix with weak permissions or locations (such as C:\ProgramData\Citrix\...). If the plug-in isn’t careful, an attacker can replace a file with a symlink (a kind of Windows shortcut that tricks the system), pointing it to another file, like a crucial system DLL or config. When Citrix Gateway Plug-in then tries to access that file, it’s actually redirecting its action to the attacker’s chosen target—doing so as SYSTEM.

Local Access: The attacker must already have user access to the Windows machine.

2. Find Weak Permissions: The attacker identifies a file/folder that Citrix Gateway Plug-in’s SYSTEM service interacts with and that user accounts can modify.
3. Create a Symlink: The attacker deletes the original file and creates a Junction or Symbolic Link (symlink) pointing to a valuable system file (for example, C:\Windows\System32\config\SAM)
4. Trigger the Service: The attacker waits for—or triggers—the Citrix service to perform an action on the file (like cleaning up or installing).
5. Result: The SYSTEM service deletes or corrupts the target file, not realizing it’s been redirected.

Example Code Snippet

Here’s how an attacker could use the built-in mklink.exe to create a symlink. (Note: Administrative rights are needed to use mklink for some targets, but there are third-party tools and tricks for non-admin symlinks within specific user-writeable directories.)

:: Assume C:\ProgramData\Citrix\foo.log is deleted each time the Plug-in is started
del "C:\ProgramData\Citrix\foo.log"

:: Create a symlink to a critical target file
mklink "C:\ProgramData\Citrix\foo.log" "C:\Windows\System32\config\SAM"

:: When Citrix Plug-in tries to delete foo.log as SYSTEM, it deletes SAM instead!

Alternatively, attackers often use tools like Symlink Attack Tools (link) or James Forshaw's symboliclink-testing-tools to automate the process.

If successful

- Critical system files (like SAM, SECURITY, or important DLLs) can be corrupted or deleted, making the system unbootable or unusable.

Remediation and Patch

Citrix fixed this issue in version 21.9.1.2. All users should immediately upgrade to at least this version or newer.

Official Citrix security bulletin:  
Citrix Gateway Plug-in for Windows Privilege Escalation Vulnerability

Direct download:  
Citrix Secure Access Download Page

References & Further Reading

- NVD - CVE-2022-21827 entry
- Citrix Security Bulletin CTX339705
- Symlink Testing Tools (James Forshaw / Project Zero)
- What is SYSTEM on Windows? (Microsoft Documentation)

Periodically audit file permissions on installed software.

4. Remember: Local privilege escalation bugs like this are often chained with other vulnerabilities for devastating attacks.


Stay safe! Always keep your remote access software up to date and subscribe to vendor security alerts.


*This exclusive explanation breaks down CVE-2022-21827 in simple terms so everyone can understand the risks and fixes for this Citrix bug. Don’t let local attackers gain SYSTEM power on your Windows machines!*

Timeline

Published on: 05/26/2022 17:15:00 UTC
Last modified on: 06/08/2022 13:38:00 UTC