CVE-2022-4055 - Exploiting Thunderbird's XDG-Mail Configuration Vulnerability to Inject Unwanted Headers and Attachments

A critical vulnerability has been discovered in Thunderbird, a popular open-source email client, when it's being used with xdg-mail to handle mailto URLs. The vulnerability, titled CVE-2022-4055, allows an attacker to create a seemingly safe mailto URL that can be used to inject unwanted headers and attachments to emails when clicked. This post explores the details of how the vulnerability works, demonstrates how an attacker might exploit it, and provides links to the original security advisory and patches.

Exploit Details

The vulnerability lies in how xdg-mail handles mailto URLs when Thunderbird is configured as the default email client. According to RFC 2368, which defines the format of mailto URLs, certain headers should not be included when parsing the URL. However, xdg-mail improperly parses mailto URLs and passes additional headers to Thunderbird that should not be present.

When an attacker crafts a malicious mailto URL that includes these additional headers, anyone who clicks on the link will inadvertently attach files specified by the attacker to the composed email. This presents a potential security and privacy risk for users, as sensitive files could be unknowingly attached to emails.

The following snippet illustrates how an attacker could create a malicious mailto URL

mailto:test@example.com?Subject=Hello&Content-Type=multipart/mixed;&
boundary=xxx&body=This%20is%20a%20test%20email.%D%A--xxx%D%AContent-Type:%20
text/plain;%20name=attachment.txt%D%AContent-Transfer-Encoding:%20base64%D%
AContent-Disposition:%20attachment;%20filename=attachment.txt%D%A%D%AdGhl%20
cXVpY2sgYnJvd24gZm94JTIwJTIwIGp1bXBz\n%D%A--xxx--

When a user clicks on this seemingly benign mailto link, they end up inadvertently attaching a file named 'attachment.txt' into the email.

Links to Original References

- CVE-2022-4055 Official Advisory
- Mozilla Security Advisory for Thunderbird
- Mozilla Bugzilla Entry
- Thunderbird Release Notes
- RFC 2368 - The mailto URL scheme

Conclusion and Mitigation

Given the potential impact of CVE-2022-4055 on users' security and privacy, it's crucial to be aware of this vulnerability and make certain that your Thunderbird installation is up-to-date. Subsequent versions of Thunderbird have addressed the vulnerability. To mitigate the risk posed by the vulnerability, you should:

Exercise caution when clicking mailto links, especially those originating from unknown sources

Stay informed about security advisories related to the software you use, and always keep your system up-to-date to protect yourself from potential threats.

Timeline

Published on: 11/19/2022 00:15:00 UTC
Last modified on: 11/26/2022 03:18:00 UTC