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macOS MDM

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Basics

MDM (Mobile Device Management) Overview

Mobile Device Management (MDM) is utilized for overseeing various end-user devices like smartphones, laptops, and tablets. Particularly for Apple's platforms (iOS, macOS, tvOS), it involves a set of specialized features, APIs, and practices. The operation of MDM hinges on a compatible MDM server, which is either commercially available or open-source, and must support the MDM Protocol. Key points include:

  • Centralized control over devices.

  • Dependence on an MDM server that adheres to the MDM protocol.

  • Capability of the MDM server to dispatch various commands to devices, for instance, remote data erasure or configuration installation.

Basics of DEP (Device Enrollment Program)

The Device Enrollment Program (DEP) offered by Apple streamlines the integration of Mobile Device Management (MDM) by facilitating zero-touch configuration for iOS, macOS, and tvOS devices. DEP automates the enrollment process, allowing devices to be operational right out of the box, with minimal user or administrative intervention. Essential aspects include:

  • Enables devices to autonomously register with a pre-defined MDM server upon initial activation.

  • Primarily beneficial for brand-new devices, but also applicable for devices undergoing reconfiguration.

  • Facilitates a straightforward setup, making devices ready for organizational use swiftly.

Security Consideration

It's crucial to note that the ease of enrollment provided by DEP, while beneficial, can also pose security risks. If protective measures are not adequately enforced for MDM enrollment, attackers might exploit this streamlined process to register their device on the organization's MDM server, masquerading as a corporate device.

Security Alert: Simplified DEP enrollment could potentially allow unauthorized device registration on the organization's MDM server if proper safeguards are not in place.

Basics What is SCEP (Simple Certificate Enrolment Protocol)?

  • A relatively old protocol, created before TLS and HTTPS were widespread.

  • Gives clients a standardized way of sending a Certificate Signing Request (CSR) for the purpose of being granted a certificate. The client will ask the server to give him a signed certificate.

What are Configuration Profiles (aka mobileconfigs)?

  • Apple’s official way of setting/enforcing system configuration.

  • File format that can contain multiple payloads.

  • Based on property lists (the XML kind).

  • “can be signed and encrypted to validate their origin, ensure their integrity, and protect their contents.” Basics — Page 70, iOS Security Guide, January 2018.

Protocols

MDM

  • Combination of APNs (Apple servers) + RESTful API (MDM vendor servers)

  • Communication occurs between a device and a server associated with a device management product

  • Commands delivered from the MDM to the device in plist-encoded dictionaries

  • All over HTTPS. MDM servers can be (and are usually) pinned.

  • Apple grants the MDM vendor an APNs certificate for authentication

DEP

  • 3 APIs: 1 for resellers, 1 for MDM vendors, 1 for device identity (undocumented):

    • The so-called DEP "cloud service" API. This is used by MDM servers to associate DEP profiles with specific devices.

    • The DEP API used by Apple Authorized Resellers to enroll devices, check enrollment status, and check transaction status.

    • The undocumented private DEP API. This is used by Apple Devices to request their DEP profile. On macOS, the cloudconfigurationd binary is responsible for communicating over this API.

  • More modern and JSON based (vs. plist)

  • Apple grants an OAuth token to the MDM vendor

DEP "cloud service" API

  • RESTful

  • sync device records from Apple to the MDM server

  • sync “DEP profiles” to Apple from the MDM server (delivered by Apple to the device later on)

  • A DEP “profile” contains:

    • MDM vendor server URL

    • Additional trusted certificates for server URL (optional pinning)

    • Extra settings (e.g. which screens to skip in Setup Assistant)

Serial Number

Apple devices manufactured after 2010 generally have 12-character alphanumeric serial numbers, with the first three digits representing the manufacturing location, the following two indicating the year and week of manufacture, the next three digits providing a unique identifier, and the last four digits representing the model number.

pagemacOS Serial Number

Steps for enrolment and management

  1. Device record creation (Reseller, Apple): The record for the new device is created

  2. Device record assignment (Customer): The device is assigned to a MDM server

  3. Device record sync (MDM vendor): MDM sync the device records and push the DEP profiles to Apple

  4. DEP check-in (Device): Device gets his DEP profile

  5. Profile retrieval (Device)

  6. Profile installation (Device) a. incl. MDM, SCEP and root CA payloads

  7. MDM command issuance (Device)

The file /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/System/Library/PrivateFrameworks/ConfigurationProfiles.framework/ConfigurationProfiles.tbd exports functions that can be considered high-level "steps" of the enrolment process.

Step 4: DEP check-in - Getting the Activation Record

This part of the process occurs when a user boots a Mac for the first time (or after a complete wipe)

or when executing sudo profiles show -type enrollment

  • Determine whether device is DEP enabled

  • Activation Record is the internal name for DEP “profile”

  • Begins as soon as the device is connected to Internet

  • Driven by CPFetchActivationRecord

  • Implemented by cloudconfigurationd via XPC. The "Setup Assistant" (when the device is firstly booted) or the profiles command will contact this daemon to retrieve the activation record.

    • LaunchDaemon (always runs as root)

It follows a few steps to get the Activation Record performed by MCTeslaConfigurationFetcher. This process uses an encryption called Absinthe

  1. Retrieve certificate

    1. GET https://iprofiles.apple.com/resource/certificate.cer

  2. Initialize state from certificate (NACInit)

    1. Uses various device-specific data (i.e. Serial Number via IOKit)

  3. Retrieve session key

    1. POST https://iprofiles.apple.com/session

  4. Establish the session (NACKeyEstablishment)

  5. Make the request

    1. POST to https://iprofiles.apple.com/macProfile sending the data { "action": "RequestProfileConfiguration", "sn": "" }

    2. The JSON payload is encrypted using Absinthe (NACSign)

    3. All requests over HTTPs, built-in root certificates are used

The response is a JSON dictionary with some important data like:

  • url: URL of the MDM vendor host for the activation profile

  • anchor-certs: Array of DER certificates used as trusted anchors

Step 5: Profile Retrieval

  • Request sent to url provided in DEP profile.

  • Anchor certificates are used to evaluate trust if provided.

    • Reminder: the anchor_certs property of the DEP profile

  • Request is a simple .plist with device identification

    • Examples: UDID, OS version.

  • CMS-signed, DER-encoded

  • Signed using the device identity certificate (from APNS)

  • Certificate chain includes expired Apple iPhone Device CA

Step 6: Profile Installation

  • Once retrieved, profile is stored on the system

  • This step begins automatically (if in setup assistant)

  • Driven by CPInstallActivationProfile

  • Implemented by mdmclient over XPC

    • LaunchDaemon (as root) or LaunchAgent (as user), depending on context

  • Configuration profiles have multiple payloads to install

  • Framework has a plugin-based architecture for installing profiles

  • Each payload type is associated with a plugin

    • Can be XPC (in framework) or classic Cocoa (in ManagedClient.app)

  • Example:

    • Certificate Payloads use CertificateService.xpc

Typically, activation profile provided by an MDM vendor will include the following payloads:

  • com.apple.mdm: to enroll the device in MDM

  • com.apple.security.scep: to securely provide a client certificate to the device.

  • com.apple.security.pem: to install trusted CA certificates to the device’s System Keychain.

  • Installing the MDM payload equivalent to MDM check-in in the documentation

  • Payload contains key properties:

    • MDM Check-In URL (CheckInURL)

    • MDM Command Polling URL (ServerURL) + APNs topic to trigger it

  • To install MDM payload, request is sent to CheckInURL

  • Implemented in mdmclient

  • MDM payload can depend on other payloads

  • Allows requests to be pinned to specific certificates:

    • Property: CheckInURLPinningCertificateUUIDs

    • Property: ServerURLPinningCertificateUUIDs

    • Delivered via PEM payload

  • Allows device to be attributed with an identity certificate:

    • Property: IdentityCertificateUUID

    • Delivered via SCEP payload

Step 7: Listening for MDM commands

  • After MDM check-in is complete, vendor can issue push notifications using APNs

  • Upon receipt, handled by mdmclient

  • To poll for MDM commands, request is sent to ServerURL

  • Makes use of previously installed MDM payload:

    • ServerURLPinningCertificateUUIDs for pinning request

    • IdentityCertificateUUID for TLS client certificate

Attacks

Enrolling Devices in Other Organisations

As previously commented, in order to try to enrol a device into an organization only a Serial Number belonging to that Organization is needed. Once the device is enrolled, several organizations will install sensitive data on the new device: certificates, applications, WiFi passwords, VPN configurations and so on. Therefore, this could be a dangerous entrypoint for attackers if the enrolment process isn't correctly protected:

pageEnrolling Devices in Other Organisations
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