I'm a little wary of OwnCloud, and generally of any software written in PHP. PHP invites problems. That said, I find this page reassuring: https://owncloud.com/

I'm a little wary of OwnCloud, and generally of any software written in PHP. PHP invites problems. That said, I find this page reassuring: https://owncloud.com/security/ https://owncloud.com/security/. It does seem like they took security pretty seriously a few years ago, and only a few issues have been discovered since then.