Joomla! 3.4.5 is now available. This is a security release for the 3.x series of Joomla which addresses a critical security vulnerability. Because of the nature of this release, it is not expected to break anything so upgrading should should be quite stress-free.

We strongly recommend that you update your sites immediately. This release only contains the security fixes; no other changes have been made compared to the Joomla 3.4.4 release.

What's in 3.4.5
Version 3.4.5 is released to address three reported security vulnerabilities and includes additional security hardening of the UploadShield system.
Security Issues Fixed
High Priority - Core - SQL Injection (affecting Joomla 3.2 through 3.4.4).
Medium Priority - Core - ACL Violations (affecting Joomla 3.2 through 3.4.4).
Medium Priority - Core - ACL Violations (affecting Joomla 3.0 through 3.4.4).

source: https://www.todhost.com/announcements/29/-Joomla-345-Released-Addresses-Major-Security-Issues.html