There are 4 open security issues in buster.
1 important issue:
textview_uri_security_check in textview.c in Claws Mail before 3.18.0, and Sylpheed through 3.7.0, does not have sufficient link checks before accepting a click.
3 issues left for the package maintainer to handle:
(postponed; to be fixed through a stable update)
In Claws Mail 3.14.1, an attacker in possession of S/MIME or PGP encrypted emails can wrap them as sub-parts within a crafted multipart email. The encrypted part(s) can further be hidden using HTML/CSS or ASCII newline characters. This modified multipart email can be re-sent by the attacker to the intended receiver. If the receiver replies to this (benign looking) email, they unknowingly leak the plaintext of the encrypted message part(s) back to the attacker.
common/session.c in Claws Mail before 3.17.6 has a protocol violation because suffix data after STARTTLS is mishandled.
In imap_scan_tree_recursive in Claws Mail through 3.17.6, a malicious IMAP server can trigger stack consumption because of unlimited recursion into subdirectories during a rebuild of the folder tree.
You can find information about how to handle these issues in the security team's documentation.