CVE-2025-29822 - How a List Oversight in Microsoft OneNote Can Let Attackers Slip Through

Cybersecurity researchers are sounding the alarm over a newly disclosed vulnerability: CVE-2025-29822. This flaw, found in Microsoft Office OneNote, lets a local attacker bypass important security checks because a key feature’s "block list" (the set of inputs it denies) is incomplete. Here’s an exclusive, plain-English breakdown of the bug—including actual code snippets and an exploit example.

What’s Going on with CVE-2025-29822?

OneNote is used everywhere for notes, reminders, and pasted content. To keep users safe, OneNote tries to prevent dangerous files or scripts from being loaded—especially things like programs, macros, or certain types of links.
Microsoft set up a list: “if input looks like X, block it!”
But this list missed some cases. Hackers know how to get creative, and a local attacker (someone using the PC) can slip in an unlisted input, effectively dodging OneNote’s security gates.

🔗 NVD Entry for CVE-2025-29822
🔗 Microsoft Security Guidance Portal (CVE-2025-29822)

How Does the Exploit Work?

Let’s say a school issues laptops with OneNote locked down—certain files and links won’t open for students. But an attacker discovers a variant input (maybe a slightly different file extension or crafty URI syntax) to sneak past the deny-list.

Payload runs:

The attacker gets around security—possibly running a malicious command, launching a forbidden file, or dropping a virus.

Example Code: Bypassing Security with a Tweaked Extension

Suppose OneNote disallows links like file://C:/malware.bat or cmd.exe outright. But let’s say .cmd1 and a certain URL variety aren’t on the list.

# Attacker creates a file payload that's not on the block list
New-Item -Path "C:\Users\Public\malicious_script.cmd1" -ItemType "file"
Add-Content "C:\Users\Public\malicious_script.cmd1" 'Start-Process calc.exe'

# Now generate a OneNote link to it, using a protocol variant
$oneNoteHyperlink = 'file:///C:/Users/Public/malicious_script.cmd1'

If this .cmd1 extension or modified link syntax isn’t on OneNote’s disallowed inputs, clicking it in OneNote runs the script!
You could also use lesser-known protocols or Unicode tricks (like using file:///C:/Users/Public/%6d%61%6c%77%61%72%65.bat—URL-encoded input).

Exploit Scenario

Imagine you’re inside a restricted corporate network, where OneNote is meant to stop users from running scripts:

- You craft an “innocent” OneNote page with a hyperlink: file:///C:/Programs/Tools/mytool.cmd1

Victim clicks the link.

OneNote tries to block .bat, .exe—but doesn’t block .cmd1, so it launches the script.

This could be combined with other tricks, like

Why Is This Dangerous?

- Bypass Defense-In-Depth: Meant to stop lateral movement or script execution via OneNote, but doesn’t work due to the incomplete list.

Update Regularly: Microsoft will patch the bug—grab updates as soon as they come.

- Custom Blocklists: Use additional endpoint security tools that let you define your own allow/block lists.

More Technical References

- Microsoft’s Security Blog
- Microsoft OneNote Security Settings
- How To Harden Blocked File Types in Office

Final Notes

CVE-2025-29822 shows why security is never “set and forget.” Don’t assume the default block list will always protect you. Attackers are always hunting for those missing puzzle pieces. Tighten your policies, keep everything updated, and if you’re an IT admin—consider your own custom blocks for OneNote… just to be sure.

If you need more technical help or want to see live detection rules for this CVE, drop us a comment below!

Timeline

Published on: 04/08/2025 18:16:08 UTC
Last modified on: 04/29/2025 22:55:04 UTC