There is 1 open security issue in buster.
1 issue left for the package maintainer to handle:
(postponed; to be fixed through a stable update)
A flaw was found in the xdg-email component of xdg-utils-1.1.0-rc1 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 a 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.
You can find information about how to handle this issue in the security team's documentation.