security bug by Horde (need popen-Function) Plesk 8.2

[rethus]

i've removed the popen-Function in php.ini and webmail with Horde now works. (Plesk 8.2)

But i'm not happy with it.
For me, this is a security problem, cause with popen - user could run shell-commands.

How can i configure horde not to use popen?