As part of the release of OS X El Capitan, Apple has added a new layer named System Integrity Protection (SIP) to its security model. To understand how System Integrity Protection fits in, let’s first take a look at Apple’s security model as it existed as of OS X Yosemite.
Gatekeeper is one of the outer lines of defense. It allows users to restrict which sources they can install applications from, with the general idea being that malware will not be from an allowed source.
OS X also uses sandboxing extensively. A sandbox typically provides a tightly controlled set of resources for programs to run in. Network access, the ability to inspect the host system, or reading from input devices is usually disallowed or heavily restricted.
OS X uses the Unix permissions model as defined by POSIX, which governs which users and groups can access which files and directories. If a particular user account requests access to a particular file or directory and does not have the necessary rights, that account is refused access.
The innermost layer of defense are keychains. Keychains are very specialized databases which are designed for the storing of secrets, like passwords, private keys, PIN numbers, and then controlling access to those secrets. To help protect these secrets, keychains are encrypted.
There’s an issue with this model though and it’s been there for decades. It pre-exists OS X and even pre-exists Apple as a company. That issue is found in the POSIX permissions layer.
Root is the superuser for a Unix system and the Unix permissions model is designed around the assumption that root has access to everything. Apple has not ignored this issue and has put some controls in place to limit the actual root user. These controls include disabling the root user account, discouraging its use, and providing ways to access elevated or root privileges using other means.
However, the root user account is still present and still can do anything on the system.
System Integrity Protection
To limit what the superuser can do and add another layer to OS X’s security model, Apple has developed SIP and deployed it as part of OS X El Capitan. SIP is designed to limit the power of root and to protect the system even from the superuser. For more details, see below the jump.