CVE-2023-36721 is a critical security vulnerability affecting the Windows Error Reporting (WER) Service. Disclosed and patched by Microsoft in September 2023, attackers can exploit this flaw to gain higher privileges (SYSTEM) on targeted Windows machines. In this exclusive, beginner-friendly overview, we’ll explain how this bug works, look at code examples, outline exploitation techniques, and provide helpful original references.

What is CVE-2023-36721?

CVE-2023-36721 is an *Elevation of Privilege* (EoP) vulnerability. It exists because of improper validation within the Windows Error Reporting Service. When exploited, it enables a normal user to run code as the SYSTEM user — the highest level of access in Windows.

Why Does It Matter?

If attackers are able to “escape” their low-privileged account, they can entirely compromise the system. This allows them to:

How Does the Vulnerability Work?

Windows Error Reporting is responsible for handling system error reports and memory dumps. CVE-2023-36721 appears because this service improperly handles certain file or directory permissions and interactions, specifically allowing symbolic link (symlink) attacks or misused file writes.

In short: By tricking the WER Service into interacting with a file path controlled by a low-privileged user, an attacker could overwrite critical system files or execute code with SYSTEM privileges.

Let’s break down a generic exploitation flow for similar bugs

1. Monitor WER Service Behavior: Figure out where it reads/writes files as SYSTEM.
2. Find Overwritable Paths: Spot any file or folder where a user can manipulate permissions or make symlinks.
3. Create Malicious Symlink or File: Replace a watched file with a symbolic link to a high-value target (like SYSTEM32).

5. Privilege Escalation: Because the service trusts the path, it unwittingly executes or modifies protected locations as SYSTEM.

Below is a simplified PowerShell snippet demonstrating a technique where a user abuses NTFS junctions (symlinks):

# WARNING: For educational demo only. Don't use on live systems!

# 1. Identify a temporary directory WER will use
$targetDir = "C:\Users\Public\WerDemo"
$privFile = "C:\Windows\System32\evil.dll"

# 2. Create a directory junction from the WER temp dir to System32
cmd.exe /c "mklink /J $targetDir $privFile"

# 3. Cause a crash or error to invoke WER
[System.Environment]::FailFast("Boom!")

# 4. WER tries to write to the temp dir, actually overwrites evil.dll as SYSTEM

*Note: Pseudo-code for illustration. Actual exploit code is more complex, and Microsoft’s patch closes these paths!*

Real-World Exploit

After CVE-2023-36721’s disclosure, proof-of-concept (PoC) exploit code and write-ups started appearing online. Notably, Github user hackerhouse-opensource published a detailed PoC repo showcasing how local users can easily trigger this flaw.

Gains SYSTEM shell through manipulated file execution.

_For security, using your own systems for testing is unsafe unless they are isolated and fully patched!_

Running unpatched Windows versions released before September 2023 Patch Tuesday

- Third-party security scanners (like Nessus) can check for the vulnerable code path

How to Fix or Prevent Exploitation

- Patch Immediately: Microsoft fixed the bug in September 2023. Update your Windows.
- Lock Down Temp Permissions: Ensure regular users can’t create symlinks or manipulate temp folders outside their own profiles.
- Monitor for Symlink Abuse: Watch for suspicious junction/symlink activity using Sysinternals tools like Sysmon.

References and Further Reading

- Microsoft Security Update Guide (CVE-2023-36721)
- hackerhouse-opensource/CVE-2023-36721 PoC on GitHub
- Wiz Research: The Mystery of Windows Error Reporting EoP
- NIST NVD Record for CVE-2023-36721

Final Thoughts

CVE-2023-36721 highlights the ongoing risks posed by classic Windows privilege escalation bugs. Modern attackers routinely search for any service or application running as SYSTEM that mishandles file paths, symlinks, or permissions. If you’re responsible for Windows devices — at home or work — patching right away is a must. Stay curious, stay secure!


Disclaimer: This post is for educational awareness only. Attempting unauthorized exploitation is illegal and unethical — always use these insights responsibly!

Timeline

Published on: 10/10/2023 18:15:16 UTC
Last modified on: 10/13/2023 20:07:31 UTC