macOS SIP

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Basic Information

System Integrity Protection (SIP) in macOS is a mechanism designed to prevent even the most privileged users from making unauthorized changes to key system folders. This feature plays a crucial role in maintaining the integrity of the system by restricting actions like adding, modifying, or deleting files in protected areas. The primary folders shielded by SIP include:

  • /System

  • /bin

  • /sbin

  • /usr

The rules that govern SIP's behavior are defined in the configuration file located at /System/Library/Sandbox/rootless.conf. Within this file, paths that are prefixed with an asterisk (*) are denoted as exceptions to the otherwise stringent SIP restrictions.

Consider the example below:

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

This snippet implies that while SIP generally secures the /usr directory, there are specific subdirectories (/usr/libexec/cups, /usr/local, and /usr/share/man) where modifications are permissible, as indicated by the asterisk (*) preceding their paths.

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

  • Disallows loading of unsigned kernel extensions (kexts), ensuring only verified extensions interact with the system kernel.

  • Prevents the debugging of macOS system processes, safeguarding core system components from unauthorized access and modification.

  • Inhibits tools like dtrace from inspecting system processes, further protecting the integrity of the system's operation.

Learn more about SIP info in this talk.

SIP Bypasses

Bypassing SIP enables an attacker to:

  • Access User Data: Read sensitive user data like mail, messages, and Safari history from all user accounts.

  • TCC Bypass: Directly manipulate the TCC (Transparency, Consent, and Control) database to grant unauthorized access to the webcam, microphone, and other resources.

  • Establish Persistence: Place malware in SIP-protected locations, making it resistant to removal, even by root privileges. This also includes the potential to tamper with the Malware Removal Tool (MRT).

  • Load Kernel Extensions: Although there are additional safeguards, bypassing SIP simplifies the process of loading unsigned kernel extensions.

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.

Inexistent 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

The entitlement com.apple.rootless.install.heritable allows to bypass SIP

Shrootless

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 behaviour 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.

A vulnerability was identified where fsck_cs was misled into corrupting a crucial file, due to its ability to follow symbolic links. Specifically, attackers crafted a link from /dev/diskX to the file /System/Library/Extensions/AppleKextExcludeList.kext/Contents/Info.plist. Executing fsck_cs on /dev/diskX led to the corruption of Info.plist. This file's integrity is vital for the operating system's SIP (System Integrity Protection), which controls the loading of kernel extensions. Once corrupted, SIP's ability to manage kernel exclusions is compromised.

The commands to exploit this vulnerability are:

ln -s /System/Library/Extensions/AppleKextExcludeList.kext/Contents/Info.plist /dev/diskX
fsck_cs /dev/diskX 1>&-
touch /Library/Extensions/
reboot

The exploitation of this vulnerability has severe implications. The Info.plist file, normally responsible for managing permissions for kernel extensions, becomes ineffective. This includes the inability to blacklist certain extensions, such as AppleHWAccess.kext. Consequently, with the SIP's control mechanism out of order, this extension can be loaded, granting unauthorized read and write access to the system's RAM.

It was possible to mount a new file system over SIP protected folders to bypass the protection.

mkdir evil
# Add contento to the folder
hdiutil create -srcfolder evil evil.dmg
hdiutil attach -mountpoint /System/Library/Snadbox/ evil.dmg

The system is set to boot from an embedded installer disk image within the Install macOS Sierra.app to upgrade the OS, utilizing the bless utility. The command used is as follows:

/usr/sbin/bless -setBoot -folder /Volumes/Macintosh HD/macOS Install Data -bootefi /Volumes/Macintosh HD/macOS Install Data/boot.efi -options config="\macOS Install Data\com.apple.Boot" -label macOS Installer

The security of this process can be compromised if an attacker alters the upgrade image (InstallESD.dmg) before booting. The strategy involves substituting a dynamic loader (dyld) with a malicious version (libBaseIA.dylib). This replacement results in the execution of the attacker's code when the installer is initiated.

The attacker's code gains control during the upgrade process, exploiting the system's trust in the installer. The attack proceeds by altering the InstallESD.dmg image via method swizzling, particularly targeting the extractBootBits method. This allows the injection of malicious code before the disk image is employed.

Moreover, within the InstallESD.dmg, there's a BaseSystem.dmg, which serves as the upgrade code's root file system. Injecting a dynamic library into this allows the malicious code to operate within a process capable of altering OS-level files, significantly increasing the potential for system compromise.

In this talk from DEF CON 31, it's shown how systemmigrationd (which can bypass SIP) executes a bash and a perl script, which can be abused via env variables BASH_ENV and PERL5OPT.

com.apple.rootless.install

The entitlement com.apple.rootless.install allows to bypass SIP

The entitlement com.apple.rootless.install is known to bypass System Integrity Protection (SIP) on macOS. This was notably mentioned in relation to CVE-2022-26712.

In this specific case, the system XPC service located at /System/Library/PrivateFrameworks/ShoveService.framework/Versions/A/XPCServices/SystemShoveService.xpc possesses this entitlement. This allows the related process to circumvent SIP constraints. Furthermore, this service notably presents a method that permits the movement of files without enforcing any security measures.

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 unauthorised 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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