Ostiarius 1.2 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe. This free Mac application was originally produced by Objective-see LLC.
From the developer:
Ostiarius is tool for El Capitan that blocks unsigned internet binaries from executing. With the kernel extension loaded, any unsigned binary or application from the internet will be automatically blocked by Ostiarius. This will happen automatically and transparently.