Santa

Santa is a binary and file access authorization system for macOS. It consists of a system
extension that monitors for executions, a daemon that makes execution decisions
based on the contents of a local 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.

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

Docs

The Santa docs are stored in the
Docs directory and are published
at https://santa.dev.

The docs include deployment options, details on how parts of Santa work and
instructions for developing Santa itself.

Get Help

If you have questions or otherwise need help getting started,
the santa-dev group is a
great place.

If you believe you have a bug, feel free to report an
issue and we’ll respond as soon as we
can.

If you believe you’ve found a vulnerability, please read the
security policy for
disclosure reporting.

Features

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

• Event logging: When the system extension is loaded, all binary launches are logged. When in either mode, all unknown or denied binaries are stored in the database to enable later aggregation.

• Certificate-based rules, with override levels: Instead of relying on a
  binary’s hash (or ‘fingerprint’), executables can be allowed/blocked by their
  signing certificate. You can therefore allow/block all binaries by a
  given publisher that were signed with that cert across version updates. A
  binary can only be allowed by its certificate if its signature validates
  correctly but a rule for a binary’s fingerprint will override a decision for
  a certificate; i.e. you can allowlist a certificate while blocking a binary
  signed with that certificate, or vice-versa.

• Path-based rules (via NSRegularExpression/ICU): This allows a similar feature
  to that found in Managed Client (the precursor to configuration profiles,
  which used the same implementation mechanism), Application Launch
  Restrictions via the mcxalr binary. This implementation carries the added
  benefit of being configurable via regex, and not relying on LaunchServices.
  As detailed in the wiki, when evaluating rules this holds the lowest
  precedence.

• Failsafe cert rules: You cannot put in a deny rule that would block the
  certificate used to sign launchd, a.k.a. pid 1, and therefore all components
  used in macOS. The binaries in every OS update (and in some cases entire new
  versions) are therefore automatically allowed. This does not affect binaries
  from Apple’s App Store, which use various certs that change regularly for
  common apps. Likewise, you cannot block Santa itself, and Santa uses a
  distinct separate cert than other Google apps.

• 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.

• Caching: allowed binaries are cached so the processing required to make a
  request is only done if the binary isn’t already cached.

Intentions and Expectations

No single system or process will stop all attacks, or provide 100% security.
Santa is written with the intention of helping protect users from themselves.
People often download malware and trust it, giving the malware credentials, or
allowing unknown software to exfiltrate more data about your system. As a
centrally managed component, Santa can help stop the spread of malware among a
large fleet of machines. Independently, Santa can aid in analyzing what is
running on your computer.

Santa is part of a defense-in-depth strategy, and you should continue to
protect hosts in whatever other ways you see fit.

Security and Performance-Related Features

Known Issues

• Santa only blocks execution (execve and variants), it doesn’t protect against
  dynamic libraries loaded with dlopen, libraries on disk that have been
  replaced, or libraries loaded using DYLD_INSERT_LIBRARIES.

• 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
  allowlist 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.

Sync Servers

• The santactl command-line client includes a flag to synchronize with a
  management server, which uploads events that have occurred on the machine and
  downloads new rules. There are several open-source servers you can sync with:

  • Moroz - A simple golang server that
    serves hardcoded rules from simple configuration files.
  • Rudolph - An AWS-based serverless sync service
    primarily built on API GW, DynamoDB, and Lambda components to reduce operational burden.
    Rudolph is designed to be fast, easy-to-use, and cost-efficient.
  • Zentral - A
    centralized service that pulls data from multiple sources and deploy
    configurations to multiple services.
  • Zercurity - A dockerized service
    for managing and monitoring applications across a large fleet utilizing
    Santa + Osquery.

• Alternatively, santactl can configure rules locally (without a sync
  server).

Screenshots

A tool like Santa doesn’t really lend itself to screenshots, so here’s a video
instead.

Contributing

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

Disclaimer

This is not an official Google product.