Santa

Santa is a binary whitelisting/blacklisting system for Mac OS X. It consists of a kernel extension that monitors for executions, a userland daemon that makes execution decisions based on the contents of a SQLite database, a GUI agent that notifies the user in case of a block decision and a command-line utility for managing the system and synchronizing the database with a server.

Santa is not yet a 1.0. We're writing more tests, fixing bugs, working on TODOs and finishing up a security audit.

Santa is named because it keeps track of binaries that are naughty and nice.

Santa is a project of Google's Macintosh Operations Team.

Features

• Multiple modes: MONITOR and LOCKDOWN. In MONITOR mode all binaries except those marked as blacklisted will be allowed to run, whilst being logged and recorded in the database. In LOCKDOWN mode, only whitelisted binaries are allowed to run.

• Codesign listing: Binaries can be whitelisted/blacklisted by their signing certificate, so you can trust/block all binaries by a given publisher. The binary will only be whitelisted by certificate if its signature validates correctly. However, a decision for a binary will override a decision for a certificate; i.e. you can whitelist a certificate while blacklisting a binary signed by that certificate or vice-versa.

• In-kernel caching: whitelisted binaries are cached in the kernel so the processing required to make a request is only done if the binary isn't already cached.

• Userland components validate each other: each of the userland components (the daemon, the GUI agent and the command-line utility) communicate with each other using XPC and check that their signing certificates are identical before any communication is accepted.

• Event logging: all executions processed by the userland agent are logged and all unknown or denied binaries are also stored in the database for upload to a server.

• Kext uses only KPIs: the kernel extension only uses provided kernel programming interfaces to do its job. This means that the kext code should continue to work across OS versions.

Known Issues

Santa is not yet a 1.0 and we have some known issues to be aware of:

• Potential race-condition: we currently have a single TODO in the kext code to investigate a potential race condition where a binary is executed and then very quickly modified between the kext getting the SHA-1 and the decision being made.

• Kext communication security: the kext will only accept a connection from a single client at a time and said client must be running as root. We haven't yet found a good way to ensure the kext only accepts connections from a valid client, short of hardcoding the SHA-1 in the kext. This shouldn't present a huge problem as the daemon is loaded on boot-up by launchd, so any later attempts to connect will be blocked.

• Database protection: the SQLite database is installed with permissions so that only the root user can read/write it. We're considering approaches to secure this further.

• Sync client: the command-line client includes a command to synchronize with a management server, including the uploading of events that have occurred on the machine and to download new rules. We're still very heavily working on this server (which is AppEngine-based and will be open-sourced in the future), so the sync client code is unfinished. It does show the 'API' that we're expecting to use so if you'd like to write your own management server, feel free to look at how the client currently works (and suggest changes!)

• Scripts: Santa is currently written to ignore any execution that isn't a binary. This is because after weighing the administration cost vs the benefit, we found it wasn't worthwhile. Additionally, a number of applications make use of temporary generated scripts, which we can't possibly whitelist and not doing so would cause problems. We're happy to revisit this (or at least make it an option) if it would be useful to others.

• Documentation: There currently isn't any.

• Tests: There aren't enough of them.

Building

git clone https://github.com/google/santa
cd santa

# Build a debug build. This will install any necessary CocoaPods, create the
# workspace and build, outputting the full log only if an error occurred.
# If CocoaPods is not installed, you'll be prompted to install it.
#
# For other build/install/run options, run rake without any arguments
rake build:debug

Note: the Xcode project is setup to use any installed "Mac Developer" certificate and for security-reasons parts of Santa will not operate properly if not signed.

Kext Signing

10.9 requires a special Developer ID certificate to sign kernel extensions and if the kext is not signed with one of these special certificates a warning will be shown when loading the kext for the first time. In 10.10 this is a hard error and the kext will not load at all unless the machine is booted with a debug boot-arg.

There are two possible solutions for this, for distribution purposes:

1. Use a pre-built, pre-signed version of the kext that we supply. Each time changes are made to the kext code we will update the pre-built version that you can make use of. This doesn't prevent you from making changes to the non-kext parts of Santa and distributing those. If you make changes to the kext and make a pull request, we can merge them in and distribute a new version of the pre-signed kext.

2. Apply for your own kext signing certificate. Apple will only grant this for broad distribution within an organization, they won't issue them just for testing purposes.

If you just want to locally test changes to the kext code, you should enable kext-dev mode, instructions for which can be found on the Apple developer site.

Contributing

Patches to this project are very much welcome. Please see the CONTRIBUTING file.

Disclaimer

This is not an official Google product.