Exim4 in some variants is started as root but switches to uid/gid Debian-exim/Debian-exim. But as Exim might need to store received messages in user mailboxes, it has to have the ability to regain privileges. This is also true when Exim is started as “sendmail”. During internal operation, sendmail (Exim) will manipulate message spool files in directory structures owned by user “Debian-exim” without caring about symlink attacks. Thus execution of code as user “Debian-exim” can be used to gain root privileges by invoking “sendmail” as user “Debian-exim”.