Are OS X installers up-to-date with all the security updates?
Update After Installing OS X

The OS X installer from the Mac App Store does not always include the latest security updates; be sure to run Software Update after installing.

Best Defence

It is not unreasonable to assume the latest version of OS X and any third party products contain bugs and security vulnerabilities. Ideally, the latest version is more secure than the previous versions.

As a user, your best defence is to update your software as updates become available and to favour third party developers who actively address security concerns.

Apple offers the following security advice:


  Technical Support
  
  Keeping your software up-to-date is one of the most important things you can do to maintain your Apple product's security. Instructions on how to update your Apple products can be found below:
  
  
  Update OS X and App Store apps on your Mac
  Update your iPhone, iPad, or iPod touch iOS software
  …
  


Apple provides a security updates page:


  Apple security updates
  
  This document outlines security updates for Apple products.
  
  For the protection of our customers, Apple does not disclose, discuss or confirm security issues until a full investigation has occurred and any necessary patches or releases are available. To learn more about Apple Product Security, see the Apple Product Security website.
  
  This document describes recent updates and releases.


Found a problem? Contact Apple!

If you have specific security concerns, you can report these to Apple directly. Apple's security page explains how to contact them privately.