macOS XPC Analysis Skill

A comprehensive skill for analyzing macOS XPC (XNU inter-Process Communication) services for security research and privilege escalation assessment.

When to Use This Skill

Use this skill when you need to:

• Enumerate XPC services on a macOS system
• Analyze XPC plist configurations for security issues
• Understand XPC communication patterns and message flows
• Create test XPC clients or servers for research
• Investigate potential XPC-based privilege escalation vectors
• Audit privilege separation in macOS applications
• Sniff or monitor XPC communications
• Analyze Remote XPC configurations

Core Concepts

What is XPC?

XPC (XNU inter-Process Communication) is Apple's framework for safe, asynchronous method calls between processes on macOS and iOS. It enables:

1. Security: Privilege-separated applications where each component runs with only necessary permissions
2. Stability: Isolated crashes that don't affect the entire system
3. Performance: Easy concurrency through process-level parallelism

XPC Service Types

Application-Specific XPC Services

• Located inside applications:

  /Applications/<App>.app/Contents/XPCServices/

• Extension:

  .xpc

  (bundle format)
• Only accessible by the parent application
• Example:

  /Applications/Safari.app/Contents/XPCServices/com.apple.Safari.SandboxBroker.xpc/

System-Wide XPC Services

• Accessible to all users
• Defined in plist files in:

  • /System/Library/LaunchDaemons/

  • /Library/LaunchDaemons/

  • /System/Library/LaunchAgents/

  • /Library/LaunchAgents/

• Critical: LaunchDaemons run as root - unprivileged processes communicating with these could escalate privileges

Key Plist Configuration

<dict>
    <key>Label</key>
    <string>com.example.service</string>
    <key>MachServices</key>
    <dict>
        <key>com.example.service</key>
        <true/>
    </dict>
    <key>Program</key>
    <string>/path/to/binary</string>
    <key>ServiceType</key>
    <string>System</string>
    <key>_AllowedClients</key>
    <array>
        <string>com.example.client</string>
    </array>
</dict>

Important Keys:

• MachServices

  : Service name for XPC connections

• ServiceType

  : Application, User, System, or _SandboxProfile

• _AllowedClients

  : Entitlements or IDs required to connect

• JoinExistingSession

  : If true, runs in same security session as caller

Security Analysis Workflow

Step 1: Enumerate XPC Services

Use the enumeration script to find all XPC services:

# Run from skill directory
./scripts/enumerate_xpc_services.sh

This searches:

• Application XPCServices directories
• LaunchDaemons and LaunchAgents plists
• System and user-level locations

Step 2: Analyze Service Configuration

For each service of interest:

# Analyze a specific plist
./scripts/analyze_xpc_plist.sh /path/to/service.plist

The analyzer checks for:

• Missing client restrictions (

  _AllowedClients

  )
• Root-owned services accessible to unprivileged users

• JoinExistingSession

  configuration
• Program path security (world-writable, symlink risks)
• Sandbox profile presence

Step 3: Test XPC Communication

Create test clients to probe service behavior:

# C-based client
./scripts/create_xpc_client.sh <service-name>

# Objective-C based client
./scripts/create_xpc_client_oc.sh <service-name>

Step 4: Monitor XPC Traffic

Use XPC sniffing tools:

# Using xpcspy (requires Frida)
xpcspy -U -r -W <bundle-id>
xpcspy -U <prog-name> -t 'i:com.apple.*' -t 'o:com.apple.*' -r

# Using XPoCe2
# Download from: https://newosxbook.com/tools/XPoCe2.html

Common Vulnerability Patterns

1. Missing Client Authorization

Issue: System-wide XPC service without

_AllowedClients

Impact: Any process can connect, potentially executing privileged operations

Detection:

grep -r "MachServices" /Library/LaunchDaemons/*.plist | grep -v "_AllowedClients"

2. JoinExistingSession Misconfiguration

Issue: XPC service with

JoinExistingSession

Impact: Service runs in caller's security context, bypassing privilege separation

Detection:

grep -r "JoinExistingSession" /Applications/*/Contents/XPCServices/*.xpc/Contents/Info.plist

3. World-Writable Program Path

Issue: XPC service binary in world-writable location

Impact: Attacker can replace binary and execute arbitrary code with service privileges

Detection:

# Check program paths from plists
find /Library/LaunchDaemons -name "*.plist" -exec grep -H "Program" {} \;

4. Input Validation Issues

Issue: XPC service doesn't validate incoming message types or content

Impact: Type confusion, buffer overflows, or logic bypass

Detection: Requires code review or fuzzing

XPC Object Types

xpc_object_t

xpc_pipe

NSXPC*

xpc_dictionary

Message Structure

// Every XPC message is a dictionary
xpc_object_t message = xpc_dictionary_create(NULL, NULL, 0);
xpc_dictionary_set_string(message, "key", "value");

// Type checking
xpc_type_t type = xpc_get_type(object);
if (type == XPC_TYPE_DICTIONARY) {
    // Process dictionary
}

Remote XPC Analysis

Remote XPC allows cross-host communication via

RemoteXPC.framework

Detection

# Find services using Remote XPC
grep -r "UsesRemoteXPC" /System/Library/LaunchDaemons/*.plist

# List remote services
/usr/libexec/remotectl list
/usr/libexec/remotectl show <device>

Network Analysis

# Monitor XPC-related network traffic
netstat -an | grep -E "(60000|60001|60002)"
nettop -m 1 -d 1000

Practical Examples

Example 1: Analyze a Suspicious Service

# 1. Find the service
./scripts/enumerate_xpc_services.sh | grep "suspicious"

# 2. Analyze its configuration
./scripts/analyze_xpc_plist.sh /Library/LaunchDaemons/com.suspicious.service.plist

# 3. Create a test client
./scripts/create_xpc_client.sh com.suspicious.service

# 4. Test communication
./xpc_test_client

Example 2: Audit All System XPC Services

# Full enumeration
./scripts/enumerate_xpc_services.sh > xpc_inventory.txt

# Analyze all LaunchDaemon plists
for plist in /Library/LaunchDaemons/*.plist; do
    ./scripts/analyze_xpc_plist.sh "$plist"
done > xpc_audit_report.txt

Tools Reference

xpcspy

pip3 install xpcspy

XPoCe2

supraudit

remotectl

Best Practices

1. Always check

   _AllowedClients

   - System services should restrict who can connect
2. Verify program paths - Ensure binaries aren't in writable locations
3. Review

   JoinExistingSession

   - Understand privilege implications
4. Test with unprivileged accounts - Verify actual access restrictions
5. Monitor with xpcspy - Understand message flows before exploitation
6. Check for sandbox profiles -

   _SandboxProfile

   provides additional isolation

Limitations

• Requires macOS system access for enumeration
• Some services may require elevated privileges to analyze
• XPC message formats are application-specific
• Remote XPC requires network access to target devices

References

• Apple XPC Documentation
• XPC Security Best Practices
• xpcspy GitHub
• XPoCe2 Tool