SandBlaster: Reversing the Apple Sandbox

SandBlaster is a tool for reversing (decompiling) binary Apple sandbox profiles. Apple sandbox profiles are written in SBPL (Sandbox Profile Language), a Scheme-like language, and are then compiled into an undocumented binary format and shipped. Primarily used on iOS, sandbox profiles are present on macOS as well. SandBlaster is, to our knowledge, the first tool that reverses binary sandbox profiles to their original SBPL format. SandBlaster works on iOS from version 7 onwards including iOS 11.

The technical report SandBlaster: Reversing the Apple Sandbox presents extensive (though a bit outdated) information on SandBlaster internals.

SandBlaster relied on previous work by Dionysus Blazakis and Stefan Esser's code and slides.

The reverser (in the reverse-sandbox/ folder) and the helper tool (in the helpers/ folder) run on any Python running platform.

SandBlaster may be installed and run standalone, though we recommend installing and running it from within iExtractor. Check the iExtractor documentation for information.

iExtractor is open source software released under the 3-clause BSD license.

Installation

SandBlaster requires Python2 for the reverser (in reverse-sandbox/), Python3 with lief library for helper script (in helpers/).

After cloning the SandBlaster repository, you have to install lief for Python3:

pip3 install lief

If the installation of lief fails you need compile to it. More information about how to compile it can be found on the wiki page.

Usage

In order to use SandBlaster you need access to the binary sandbox profiles and the sandbox operations, a set of strings that define sandbox-specific actions. Sandbox operations and sandbox profiles are extracted using the helpers/extract_sandbox_data.py script. Sandbox profiles are extracted from the kernel sandbox extension (as a bundle for iOS 4 and 9-11) or from kernel cache (as a bundle for iOS 12) or from the sandboxd file in the iOS filesystem (for iOS 5-8). Sandbox operations are extracted either from kernel extension (for iOS 4-11) or from kernel cache (for iOS 12).

So, as input data, SandBlaster requires the kernelcache, the kernel sandbox extension and the sandboxd file. Information and scripts on extracting them from a publicly available IPSW (iPhone Software) file is presented by iExtractor.

Below are the steps and commands to reverse the sandbox profiles for iOS 8.4.1, assuming the sandbox kernel extension (com.apple.security.sandbox.kext) and the sandboxd file are available:

# Extract sandbox operations from kernelcache.
cd helpers/
./extract_sandbox_data.py -o iPad2,1_8.4.1_12H321.sb_ops iPad2,1_8.4.1_12H321.com.apple.security.sandox.kext 8.4.1
# Extract binary sandbox profile files from sandboxd.
mkdir iPad2,1_8.4.1_12H321.sandbox_profiles
./extract_sandbox_data.py -O iPad2,1_8.4.1_12H321.sandbox_profiles/ iPad2,1_8.4.1_12H321.sandboxd 8.4.1
# Reverse all binary sandbox profiles.
cd ../reverse-sandbox/
mkdir iPad2,1_8.4.1_12H321.reversed_profiles
for i in ../helpers/iPad2,1_8.4.1_12H321.sandbox_profiles/*; do python reverse_sandbox.py -r 8.4.1 -o ../helpers/iPad2,1_8.4.1_12H321.sb_ops -d iPad2,1_8.4.1_12H321.reversed_profiles/ "$i"; done

Below are the steps and commands to reverse the sandbox profiles for iOS 9.3, assuming the sandbox kernel extension (com.apple.security.sandbox.kext) is available:

# Extract sandbox operations from kernelcache.
cd helpers/
./extract_sandbox_data.py -o iPhone5,1_9.3_13E237.sb_ops iPhone5,1_9.3_13E237.com.apple.security.sandox.kext 9.3
# Extract sandbox profile bundle from kernel sandbox extension.
./extract_sandbox_data.py -O . iPhone5,1_9.3_13E237.com.apple.security.sandox.kext 9.3
cd ../reverse-sandbox/
# Reverse all binary sandbox profiles in sandbox bundle.
mkdir iPhone5,1_9.3_13E237.reversed_profiles
# Print all sandbox profiles in bundle.
python reverse_sandbox.py -r 9.3 -o ../helpers/iPhone5,1_9.3_13E237.sb_ops -d iPhone5,1_9.3_13E237.reversed_profiles/ ../helpers/sandbox_bundle -psb
# Do actual reversing.
python reverse_sandbox.py -r 9.3 -o ../helpers/iPhone5,1_9.3_13E237.sb_ops -d iPhone5,1_9.3_13E237.reversed_profiles/ ../helpers/sandbox_bundle

The extraction of the binary sandbox profiles differs between iOS <= 8 and iOS >= 9. Since iOS >= 9 the binary sandbox profiles are stored in a sandbox bundle in the kernel sandbox extension. The helpers/extract_sandbox_data.py script extracts them appropriately depending on the iOS version.

The -psb option for reverse_sandbox.py prints out the sandbox profiles part of a sandbox bundle without doing the actual reversing.

The reverse_sandbox.py script needs to be run in its directory (reverse-sandbox/) since it needs the other Python modules and the logger.config file.

Internals

The helpers/ subfolder contains helper scripts that provide a nicer interface for the external tools.

The actual reverser is part of the reverse-sandbox/ folder. Files here can be categorized as follows:

• The main script is reverse_sandbox.py. It parses the command line arguments, does basic parsing of the input binary file (extracts sections) and calls the appropriate functions from the other modules.
• The core of the implementation is operation_node.py. It provides functions to build the rules graph corresponding to the sandbox profile and to convert the graph to SBPL. It is called by reverse_sandbox.py.
• Sandbox filters (i.e. match rules inside sandbox profiles) are handled by the implementation in sandbox_filter.py and the configuration in filters.json, filter_list.py and filters.py. Filter specific functions are called by operation_node.py.
• Regular expression reversing is handled by sandbox_regex.py and regex_parse.py. regex_parse.py is the back end parser that converts the binary representation to a basic graph. sandbox_regex.py converts the graph representation (an automaton) to an actual regular expression (i.e. a string of characters and metacharacters). It is called by reverse_sandbox.py for parsing regular expressions, with the resulting regular expression list being passed to the functions exposed by operation_node.py; operation_node.py passes them on to sandbox filter handling files.
• The new format for storing strings since iOS 10 is handled by reverse_string.py. The primary SandboxString class in reverse_string.py is used in sandbox_filter.py.
• Logging is configured in the logger.config file. By default, INFO and higher level messages are printed to the console, while DEBUG and higher level messages are printed to the reverse.log file.

Supported iOS Versions

SandBlaster works for iOS version 4 onwards including iOS 12. Apple has been making updates to the binary format of the sandbox profiles: since iOS 9 sandbox profiles are stored in a bundle, since iOS 10 strings are aggregated together in a specialied binary format. iOS 11 didn't bring any change to the format.

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