CVE-2025-59240 - Sensitive Information Disclosure Vulnerability in Microsoft Office Excel

On February 13, 2025, Microsoft published a new vulnerability (CVE-2025-59240) affecting Microsoft Office Excel. This security issue allows an unauthorized attacker to access and expose sensitive information stored in Excel files on a victim’s machine. In this post, I’ll dig deep into what CVE-2025-59240 means, how attackers can exploit it, and what you can do to stay safe. The explanation is written in simple language for easy understanding.

What is CVE-2025-59240?

CVE-2025-59240 is a local information disclosure vulnerability affecting Microsoft Office Excel. This means an attacker with access to the same system as the victim can potentially read sensitive information from Excel files without the victim’s permission. In many cases, these files could contain anything from payroll, personal data, business secrets, or other critical information.

Affected Software: Microsoft Office Excel (various builds; see official advisory)

- Impact: Anyone with access to the system (but not necessarily Excel privileges) can grab secret or confidential info inside Excel files that are supposed to be private

How Does the Vulnerability Work?

Excel files can contain metadata and temporary data (such as comments, recent edits, cell history, or hidden sheets). In certain cases, this info isn’t properly protected by Excel. A local attacker can exploit a flaw in how Excel caches or stores data, for example:

Extracting hidden worksheet content through file manipulation

- Parsing the Excel file in raw XML or binary form to get information not immediately visible in the Excel UI

This vulnerability doesn’t require the victim to open a special file or click a link. It can be exploited simply if a malicious local user or process scans local files and directories for Excel cache or temporary files.

Code Example: How Attackers Can Extract Sensitive Info

Below is a simplified proof-of-concept Python code that demonstrates how someone could recover hidden data from a local Excel file. This code is for educational purposes only. Do not use for malicious actions.

Many modern Excel files use the .xlsx format, which is basically a ZIP file with several XML documents inside, including workbook.xml, worksheets/sheetX.xml, and so on.

import zipfile

excel_path = "C:/Users/target/Documents/secret.xlsx"
with zipfile.ZipFile(excel_path, 'r') as z:
    for name in z.namelist():
        if "sheet" in name:
            with z.open(name) as f:
                content = f.read().decode("utf-8", "ignore")
                # Simple search for hidden or sensitive info patterns
                if "<is>" in content or "<v>" in content:
                    print(f"[+] Found data in {name}:")
                    print(content[:500]) # print first 500 characters

# Also scan for hidden sheets
    if "xl/worksheets/sheet_hiddenX.xml" in z.namelist():
        print("Found hidden worksheet!")

This short script opens the Excel file, reads all worksheet XMLs, and dumps their contents—even if a sheet is hidden or contains deleted yet recoverable information. Attackers could refine this script to automatically harvest sensitive data or search for specific terms (e.g., SSN, password, confidential).

Here’s a real-life example

1. Attacker gains local access (physical access or through a malware infection) to a victim’s machine.
2. Attacker browses user folders for .xlsx files and temporary files left behind by Excel in C:\Users\<user>\AppData\Local\Temp.
3. Attacker runs a script like the one above to scrape through the files, grabbing not just visible worksheet data, but metadata, comments, and hidden worksheet info.
4. Sensitive business data (such as salary numbers or hidden PII) gets leaked, even if the user thought it was "deleted" or "hidden".

Microsoft Security Response Center (MSRC) Advisory:

MSRC – CVE-2025-59240

NIST National Vulnerability Database:

NVD – CVE-2025-59240 *(link may take time to update)*

Excel File Format Structure:

Microsoft Office File Formats
Detailed Excel XML Reference

Update Excel:

Ensure your Microsoft Office Suite is patched to the latest version. Microsoft has released an update that resolves this issue.

Use “Inspect Document” in Excel (File → Info → Check for Issues → Inspect Document) to strip out hidden properties and personal information.

Encrypt Sensitive Excel Files:

Use Excel's strong password protection (File → Info → Protect Workbook → Encrypt with Password) before storing or sending sensitive data.

Conclusion

CVE-2025-59240 reminds us that sensitive data can leak even through features we might not notice—like hidden temp files and metadata in Excel workbooks. If you use Microsoft Excel to manage important or sensitive info, make sure you’re up-to-date, follow best practices, and keep a close eye on your documents’ hidden contents.

Stay informed and secure!

*This post is exclusive and written for educational and awareness purposes only. Always stay on the safe side when handling or sharing sensitive documents!*

Timeline

Published on: 11/11/2025 18:15:35 UTC
Last modified on: 12/09/2025 22:39:02 UTC