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.
claws-mail.org/download.php?file=releases/claws-mail-3.18.0.tar.xz
git.claws-mail.org/?p=claws.git;a=commit;h=ac286a71ed78429e16c612161251b9ea90ccd431
lists.fedoraproject.org/archives/list/[email protected]/message/L2QNUIWASJLPUZZKWICGCEGYJZCQE7NH/
lists.fedoraproject.org/archives/list/[email protected]/message/RCJXHUSYHGVBSH2ULD7HNXLM7QNRECZ6/
sylpheed.sraoss.jp/sylpheed/v3.7/sylpheed-3.7.0.tar.xz