A flaw was found in the xdg-email component of xdg-utils 1.1.0rc1 and newer. When handling mailto: URIs, xdg-email allows attachments to be discreetly added via the URI when being passed to Thunderbird. An attacker could potentially send a victim an URI that automatically attaches a sensitive file to a new email. If a victim user does not notice that an attachment was added and sends the email, this could result in sensitive information disclosure. It has been confirmed that the code behind this issue is in xdg-email and not in Thunderbird.
Workaround ========== To mitigate this flaw, either do not use mailto links at all, or always double-check in the user interface that there are no unwanted attachments before sending emails, especially when the email originates from clicking on a mailto link.