Macos Sandbox

macOS Sandbox Security

A comprehensive guide to understanding, working with, and debugging macOS Sandbox security mechanisms.

What is macOS Sandbox?

macOS Sandbox (originally called Seatbelt) limits applications to only the allowed actions specified in their Sandbox profile. This ensures applications access only expected resources.

Key facts:

• Any app with entitlement com.apple.security.app-sandbox runs inside the sandbox
• All App Store apps have this entitlement
• Apple binaries are usually sandboxed
• Uses MACF (Mandatory Access Control Framework) hooks for most syscalls

Core Components

Understanding Containers

Every sandboxed application has its own container:

# List all containers
ls -l ~/Library/Containers

# Inspect a specific container
cd ~/Library/Containers/{CFBundleIdentifier}
ls -la

Container structure:

• .com.apple.containermanagerd.metadata.plist - Container configuration
• Data/ - App data directory (mimics home folder structure)

Important: Even with symlinks to Desktop, Downloads, etc., the app needs explicit permissions in the .plist file's RedirectablePaths to access them.

Sandbox Profiles (SBPL)

Sandbox profiles use Sandbox Profile Language (SBPL), based on Scheme:

(version 1)                    ; Profile version
(deny default)                 ; Default action when no rule matches
(allow network*)               ; Allow all network operations
(allow file-read*              ; Allow file reads from:
    (subpath "/Users/username/")
    (literal "/tmp/afile")
    (regex #"^/private/etc/.*")
)
(allow mach-lookup             ; Allow specific Mach port
    (global-name "com.apple.analyticsd")
)

Common operations:

• file* - All file operations
• file-read* - File read operations
• file-write* - File write operations
• process* - Process execution
• network* - Network operations
• mach-lookup - Mach port access

Creating and Testing Sandbox Profiles

Basic Profile Creation

# Create a profile file
cat > /tmp/test.sb << 'EOF'
(version 1)
(deny default)
(allow file* (literal "/tmp/test.txt"))
(allow process* (literal "/usr/bin/touch"))
(allow file-read-data (literal "/"))
EOF

# Run command with profile
sandbox-exec -f /tmp/test.sb /usr/bin/touch /tmp/test.txt

Common Profile Patterns

Minimal network access:

(version 1)
(deny default)
(allow network-client-connect)
(allow file-read* (subpath "~/"))

File server (read-only):

(version 1)
(deny default)
(allow network-server)
(allow file-read* (subpath "/var/www/"))

Development with temp access:

(version 1)
(deny default)
(allow file* (subpath "/tmp/"))
(allow file* (subpath "~/Downloads/"))
(allow process-exec (subpath "/usr/bin/"))

Inspecting Sandboxed Processes

Using sbtool

# Check if PID can access a file
sbtool <pid> file /tmp

# Check Mach port access
sbtool <pid> mach

# Full inspection of sandbox state
sbtool <pid> inspect

# Check all permissions
sbtool <pid> all

Using log show

# View recent sandbox denials
log show --style syslog --predicate 'eventMessage contains[c] "sandbox"' --last 30s

# Filter for specific process
log show --predicate 'processImagePath contains "AppName" and eventMessage contains "sandbox"' --last 1h

Container Configuration

# Get container metadata (requires FDA)
plutil -convert xml1 ~/Library/Containers/{bundle-id}/.com.apple.containermanagerd.metadata.plist -o -

# Look for:
# - SandboxProfileData (compiled profile)
# - Entitlements (granted permissions)
# - RedirectablePaths (accessible paths)

Sandbox Tracing

Via Profile

# Create trace profile
cat > /tmp/trace.sb << 'EOF'
(version 1)
(trace /tmp/trace.out)
EOF

# Run with tracing
sandbox-exec -f /tmp/trace.sb /bin/ls

# View trace output
cat /tmp/trace.out

Via Command Line

# Inline trace
sandbox-exec -t /tmp/trace.out -p "(version 1)" /bin/ls

System Sandbox Profiles

Locations:

• /usr/share/sandbox/ - System profiles
• /System/Library/Sandbox/Profiles/ - Application profiles
• /System/Library/Sandbox/rootless.conf - SIP platform profile

Common profiles:

• application.sb - Default App Store app profile
• platform.sb - Platform policy (SIP)
• *.sb - Various system daemon profiles

Sandbox Extensions

Extensions grant additional privileges to sandboxed processes:

Extension types:

• sandbox_extension_issue_file - File access
• sandbox_extension_issue_mach - Mach port access
• sandbox_extension_issue_iokit_user_client_class - I/O Kit access
• sandbox_extension_issue_generic - Generic extension

Key points:

• Extensions are usually granted by allowed processes (e.g., tccd for photos)
• Extension tokens are long hex strings encoding permissions
• Tokens can be consumed by multiple processes
• Related to entitlements - some entitlements auto-grant extensions

Debugging Sandbox Issues

Common Problems and Solutions

Problem: Command fails with "deny(1) process-exec*" Solution: Add (allow process* (literal "/path/to/command"))

Problem: File access denied Solution: Add appropriate file permission:

(allow file-read-data (literal "/path/to/file"))
(allow file-write-data (literal "/path/to/file"))

Problem: Dynamic library loading fails Solution: Add dyld and library paths:

(allow file-read-data (literal "/usr/lib/dyld"))
(allow file-read-data (literal "/usr/lib/"))

Debug Workflow

1. Check the denial:

   log show --predicate 'eventMessage contains "sandbox"' --last 1m
   

2. Identify the operation:

   • process-exec* - Need process permission
   • file-read-metadata - Need file read permission
   • file-read-data - Need file data permission

3. Add minimal permission: Start with (deny default) and add only what's needed

4. Test incrementally: Add one permission at a time to identify what's required

Security Considerations

Quarantine Attribute

Files created/modified by sandboxed apps get the quarantine attribute, preventing Gatekeeper bypass:

# Check quarantine attribute
xattr -l /path/to/file | grep com.apple.quarantine

# Remove quarantine (if needed)
xattr -d com.apple.quarantine /path/to/file

Entitlements

Key entitlements:

• com.apple.security.app-sandbox - Enables sandboxing
• com.apple.security.network.server - Network server access
• com.apple.security.temporary-exception.sbpl - Custom SBPL (Apple-approved only)
• keychain-access-groups - Keychain access

SIP Integration

System Integrity Protection uses sandbox:

• Platform profile in /System/Library/Sandbox/rootless.conf
• Protects system directories
• Cannot be disabled without boot flags

Quick Reference

Best Practices

1. Start restrictive: Begin with (deny default) and add permissions as needed
2. Use literals over wildcards: More specific = more secure
3. Test incrementally: Add one permission at a time
4. Check logs: Always verify what's being denied
5. Document profiles: Comment your SBPL files
6. Review extensions: Understand what extensions your app needs

References

• OS Internals Volume III
• Apple Sandbox Guide
• OSX Sandbox Profiles
• Sandbox Escape Research