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hacktricks/macos-hardening/macos-security-and-privileg.../macos-security-protections/macos-sip.md

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macOS SIP

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Basic Information

System Integrity Protection (SIP) is a security technology in macOS that safeguards certain system directories from unauthorized access, even for the root user. It prevents modifications to these directories, including creation, alteration, or deletion of files. The main directories that SIP protects are:

  • /System
  • /bin
  • /sbin
  • /usr

The protection rules for these directories and their subdirectories are specified in the /System/Library/Sandbox/rootless.conf file. In this file, paths starting with an asterisk (*) represent exceptions to SIP's restrictions.

For instance, the following configuration:

/usr
* /usr/libexec/cups
* /usr/local
* /usr/share/man

indicates that the /usr directory is generally protected by SIP. However, modifications are allowed in the three subdirectories specified (/usr/libexec/cups, /usr/local, and /usr/share/man), as they are listed with a leading asterisk (*).

To verify whether a directory or file is protected by SIP, you can use the ls -lOd command to check for the presence of the restricted or sunlnk flag. For example:

ls -lOd /usr/libexec/cups
drwxr-xr-x  11 root  wheel  sunlnk 352 May 13 00:29 /usr/libexec/cups

In this case, the sunlnk flag signifies that the /usr/libexec/cups directory itself cannot be deleted, though files within it can be created, modified, or deleted.

On the other hand:

ls -lOd /usr/libexec
drwxr-xr-x  338 root  wheel  restricted 10816 May 13 00:29 /usr/libexec

Here, the restricted flag indicates that the /usr/libexec directory is protected by SIP. In a SIP-protected directory, files cannot be created, modified, or deleted.

Moreover, if a file contains the attribute com.apple.rootless extended attribute, that file will also be protected by SIP.

SIP also limits other root actions like:

  • Loading untrusted kernel extensions
  • Getting task-ports for Apple-signed processes
  • Modifying NVRAM variables
  • Allowing kernel debugging

Options are maintained in nvram variable as a bitflag (csr-active-config on Intel and lp-sip0 is read from the booted Device Tree for ARM). You can find the flags in the XNU source code in csr.sh:

SIP Status

You can check if SIP is enabled on your system with the following command:

csrutil status

If you need to disable SIP, you must restart your computer in recovery mode (by pressing Command+R during startup), then execute the following command:

csrutil disable

If you wish to keep SIP enabled but remove debugging protections, you can do so with:

csrutil enable --without debug

Other Restrictions

SIP also imposes several other restrictions. For instance, it disallows the loading of unsigned kernel extensions (kexts) and prevents the debugging of macOS system processes. It also inhibits tools like dtrace from inspecting system processes.

SIP Bypasses

Prices

If an attacker manages to bypass SIP this is what he will earn:

  • Read mail, messages, Safari history... of all users
  • Grant permissions for webcam, microphone or anything (by directly writing over the SIP protected TCC database)
  • Persistence: He could save a malware in a SIP protected location and not even toot will be able to delete it. Also he could tamper with MRT.
  • Easiness to load kernel extensions (still other hardcore protections in place for this).

Installer Packages

Installer packages signed with Apple's certificate can bypass its protections. This means that even packages signed by standard developers will be blocked if they attempt to modify SIP-protected directories.

Unexistent SIP file

One potential loophole is that if a file is specified in rootless.conf but does not currently exist, it can be created. Malware could exploit this to establish persistence on the system. For example, a malicious program could create a .plist file in /System/Library/LaunchDaemons if it is listed in rootless.conf but not present.

com.apple.rootless.install.heritable

{% hint style="danger" %} The entitlement com.apple.rootless.install.heritable allows to bypass SIP {% endhint %}

Researchers from this blog post discovered a vulnerability in macOS's System Integrity Protection (SIP) mechanism, dubbed the 'Shrootless' vulnerability. This vulnerability centers around the system_installd daemon, which has an entitlement, com.apple.rootless.install.heritable, that allows any of its child processes to bypass SIP's file system restrictions.

system_installd daemon will install packages that have been signed by Apple.

Researchers found that during the installation of an Apple-signed package (.pkg file), system_installd runs any post-install scripts included in the package. These scripts are executed by the default shell, zsh, which automatically runs commands from the /etc/zshenv file, if it exists, even in non-interactive mode. This behavior could be exploited by attackers: by creating a malicious /etc/zshenv file and waiting for system_installd to invoke zsh, they could perform arbitrary operations on the device.

Moreover, it was discovered that /etc/zshenv could be used as a general attack technique, not just for a SIP bypass. Each user profile has a ~/.zshenv file, which behaves the same way as /etc/zshenv but doesn't require root permissions. This file could be used as a persistence mechanism, triggering every time zsh starts, or as an elevation of privilege mechanism. If an admin user elevates to root using sudo -s or sudo <command>, the ~/.zshenv file would be triggered, effectively elevating to root.

In CVE-2022-22583 it was discovered that the same system_installd process could still be abused because it was putting the post-install script inside a random named folder protected by SIP inside /tmp. The thing is that /tmp itself isn't protected by SIP, so it was possible to mount a virtual image on it, then the installer would put in there the post-install script, unmount the virtual image, recreate all the folders and add the post installation script with the payload to execute.

com.apple.rootless.install

{% hint style="danger" %} The entitlement com.apple.rootless.install allows to bypass SIP {% endhint %}

From CVE-2022-26712 The system XPC service /System/Library/PrivateFrameworks/ShoveService.framework/Versions/A/XPCServices/SystemShoveService.xpc has the entitlement com.apple.rootless.install, which grants the process permission to bypass SIP restrictions. It also exposes a method to move files without any security check.

Sealed System Snapshots

Sealed System Snapshots are a feature introduced by Apple in macOS Big Sur (macOS 11) as a part of its System Integrity Protection (SIP) mechanism to provide an additional layer of security and system stability. They are essentially read-only versions of the system volume.

Here's a more detailed look:

  1. Immutable System: Sealed System Snapshots make the macOS system volume "immutable", meaning that it cannot be modified. This prevents any unauthorized or accidental changes to the system that could compromise security or system stability.
  2. System Software Updates: When you install macOS updates or upgrades, macOS creates a new system snapshot. The macOS startup volume then uses APFS (Apple File System) to switch to this new snapshot. The entire process of applying updates becomes safer and more reliable as the system can always revert to the previous snapshot if something goes wrong during the update.
  3. Data Separation: In conjunction with the concept of Data and System volume separation introduced in macOS Catalina, the Sealed System Snapshot feature makes sure that all your data and settings are stored on a separate "Data" volume. This separation makes your data independent from the system, which simplifies the process of system updates and enhances system security.

Remember that these snapshots are automatically managed by macOS and don't take up additional space on your disk, thanks to the space sharing capabilities of APFS. It’s also important to note that these snapshots are different from Time Machine snapshots, which are user-accessible backups of the entire system.

Check Snapshots

The command diskutil apfs list lists the details of the APFS volumes and their layout:

+-- Container disk3 966B902E-EDBA-4775-B743-CF97A0556A13
|   ====================================================
|   APFS Container Reference:     disk3
|   Size (Capacity Ceiling):      494384795648 B (494.4 GB)
|   Capacity In Use By Volumes:   219214536704 B (219.2 GB) (44.3% used)
|   Capacity Not Allocated:       275170258944 B (275.2 GB) (55.7% free)
|   |
|   +-< Physical Store disk0s2 86D4B7EC-6FA5-4042-93A7-D3766A222EBE
|   |   -----------------------------------------------------------
|   |   APFS Physical Store Disk:   disk0s2
|   |   Size:                       494384795648 B (494.4 GB)
|   |
|   +-> Volume disk3s1 7A27E734-880F-4D91-A703-FB55861D49B7
|   |   ---------------------------------------------------
|   |   APFS Volume Disk (Role):   disk3s1 (System)
|   |   Name:                      Macintosh HD (Case-insensitive)
|   |   Mount Point:               /System/Volumes/Update/mnt1
|   |   Capacity Consumed:         12819210240 B (12.8 GB)
|   |   Sealed:                    Broken
|   |   FileVault:                 Yes (Unlocked)
|   |   Encrypted:                 No
|   |   |
|   |   Snapshot:                  FAA23E0C-791C-43FF-B0E7-0E1C0810AC61
|   |   Snapshot Disk:             disk3s1s1
|   |   Snapshot Mount Point:      /
|   |   Snapshot Sealed:           Yes
[...]
+-> Volume disk3s5 281959B7-07A1-4940-BDDF-6419360F3327
    |   ---------------------------------------------------
    |   APFS Volume Disk (Role):   disk3s5 (Data)
    |   Name:                      Macintosh HD - Data (Case-insensitive)
    |   Mount Point:               /System/Volumes/Data
    |   Capacity Consumed:         412071784448 B (412.1 GB)
    |   Sealed:                    No
    |   FileVault:                 Yes (Unlocked)

In the previous output it's possible to see that user-accessible locations are mounted under /System/Volumes/Data.

Moreover, macOS System volume snapshot is mounted in / and it's sealed (cryptographically signed by the OS). So, if SIP is bypassed and modifies it, the OS won't boot anymore.

It's also possible to verify that seal is enabled by running:

csrutil authenticated-root status
Authenticated Root status: enabled

Moreover, the snapshot disk is also mounted as read-only:

mount
/dev/disk3s1s1 on / (apfs, sealed, local, read-only, journaled)
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