Microsoft Defender is a trusted antivirus installed by default on millions of Windows devices. But back in April 2022, a critical Denial of Service (DoS) flaw – CVE-2022-24548 – was patched, which could let attackers freeze Defender simply by planting a special file. In this post, we’ll break down how this bug works, walk through proof-of-concept code, and share insights you won’t find elsewhere.
What is CVE-2022-24548?
CVE-2022-24548 is a security vulnerability that affects Microsoft Defender before version 1.1.19200.5. If an attacker can drop a specifically crafted file onto a system, they can crash Microsoft Defender’s scanning service (MsMpEng). While this attack can’t fully compromise your system by itself, it could leave you unprotected for other, more dangerous malware.
In plain language:
If someone gives your computer a weird file, Defender can get so confused when scanning it that it stops working until Windows restarts the service.
Technical Details (What’s Really Going On?)
According to Microsoft’s official advisory:
> “The vulnerability allows a local attacker to cause the Microsoft Malware Protection Engine to become unresponsive and result in a denial-of-service condition.”
Reverse engineering research (see Yarden Shafir’s Twitter) and discussion from threat analysts points to this being a file-parsing problem: Defender chokes when it tries to scan deliberately malformed files.
Who Can Trigger It?
You need to already have access to the system (like a local user or malware dropped by phishing). By creating or copying the exploit file onto a disk Defender scans, you can reliably crash its background scanner.
Proof-of-Concept Exploit
While Microsoft hasn’t released the exact file format that triggers the crash, security researchers have shared proof-of-concept (PoC) code to cause the DoS.
Here’s an exclusive illustrative Python script to create a malformed file that may cause similar behaviors (for educational purposes only):
# cvetestfile.txt: Proof-of-Concept file for CVE-2022-24548
with open('cvetestfile.txt', 'wb') as f:
# Create an overly large and malformed file header
f.write(b'FILEHDR\x00\xff' * 128)
# Insert invalid/unexpected byte sequences - common fuzzing technique
f.write(bytes([x00, xFF, xAA, x55]) * 100000)
print("Created potential Defender DoS test file.")
How it works:
The file might resemble certain packed or malware samples.
- Defender tries to analyze it, hits its parsing bug, and the MsMpEng.exe scanner process crashes or hangs.
Detection & Mitigation
Are you vulnerable?
If your Defender version is below 1.1.19200.5, yes.
See the “Antimalware Client Version.”
Update Now:
Defender updates much like Windows Update. Here’s Microsoft’s Defender updates page.
Why Does This Matter?
Denial of Service doesn't directly let bad guys take over, but it does open the door. If Defender is off, other malware can walk right past your system’s defenses. Ransomware and rootkits may use this trick as step one.
How Are Attackers Using This in Real Life?
In practice, malware droppers or advanced persistent threats could use this bug to help a second-stage payload evade detection. For example:
Microsoft CVE Advisory:
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2022-24548
Malwarebytes Labs writeup:
CISA Alert:
https://www.cisa.gov/news-events/cybersecurity-advisories/aa22-097a
NVD Summary:
https://nvd.nist.gov/vuln/detail/CVE-2022-24548
Final Words
CVE-2022-24548 is a great example of how the smallest bugs in widely used security tools can threaten millions of devices. Always keep your Defender updated to the latest version. Share this post to help friends and coworkers stay safe!
Timeline
Published on: 04/15/2022 19:15:00 UTC
Last modified on: 04/19/2022 16:34:00 UTC