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Subvert Trust Controls: Gatekeeper Bypass

ID Name
T1553.001 Gatekeeper Bypass
T1553.002 Code Signing
T1553.003 SIP and Trust Provider Hijacking
T1553.004 Install Root Certificate
T1553.005 Mark-of-the-Web Bypass
T1553.006 Code Signing Policy Modification

Adversaries may modify file attributes and subvert Gatekeeper functionality to evade user prompts and execute untrusted programs. Gatekeeper is a set of technologies that act as layer of Apple’s security model to ensure only trusted applications are executed on a host. Gatekeeper was built on top of File Quarantine in Snow Leopard (10.6, 2009) and has grown to include Code Signing, security policy compliance, Notarization, and more. Gatekeeper also treats applications running for the first time differently than reopened applications.[1][2]

Based on an opt-in system, when files are downloaded an extended attribute (xattr) called com.apple.quarantine (also known as a quarantine flag) can be set on the file by the application performing the download. Launch Services opens the application in a suspended state. For first run applications with the quarantine flag set, Gatekeeper executes the following functions:

  1. Checks extended attribute – Gatekeeper checks for the quarantine flag, then provides an alert prompt to the user to allow or deny execution.[3][4]

  2. Checks System Policies - Gatekeeper checks the system security policy, allowing execution of apps downloaded from either just the App Store or the App Store and identified developers.

  3. Code Signing – Gatekeeper checks for a valid code signature from an Apple Developer ID.

  4. Notarization - Using the api.apple-cloudkit.com API, Gatekeeper reaches out to Apple servers to verify or pull down the notarization ticket and ensure the ticket is not revoked. Users can override notarization, which will result in a prompt of executing an "unauthorized app" and the security policy will be modified.

Adversaries can subvert one or multiple security controls within Gatekeeper checks through logic errors (e.g. Exploitation for Defense Evasion), unchecked file types, and external libraries. For example, prior to macOS 13 Ventura, code signing and notarization checks were only conducted on first launch, allowing adversaries to write malicious executables to previously opened applications in order to bypass Gatekeeper security checks.[5][6]

Applications and files loaded onto the system from a USB flash drive, optical disk, external hard drive, from a drive shared over the local network, or using the curl command may not set the quarantine flag. Additionally, it is possible to avoid setting the quarantine flag using Drive-by Compromise.

ID: T1553.001
Sub-technique of:  T1553
ⓘ
Tactic: Defense Evasion
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Platforms: macOS
ⓘ
Defense Bypassed: Anti-virus, Application Control
Contributors: Brandon Dalton @PartyD0lphin; Swasti Bhushan Deb, IBM India Pvt. Ltd.
Version: 1.2
Created: 05 February 2020
Last Modified: 21 October 2022
Version Permalink

Procedure Examples

ID Name Description
S0369 CoinTicker

CoinTicker downloads the EggShell mach-o binary using curl, which does not set the quarantine flag.[7]
S1016 MacMa

MacMa has removed the com.apple.quarantineattribute from the dropped file, $TMPDIR/airportpaird.[8]
S0402 OSX/Shlayer

If running with elevated privileges, OSX/Shlayer has used the spctl command to disable Gatekeeper protection for a downloaded file. OSX/Shlayer can also leverage system links pointing to bash scripts in the downloaded DMG file to bypass Gatekeeper, a flaw patched in macOS 11.3 and later versions. OSX/Shlayer has been Notarized by Apple, resulting in successful passing of additional Gatekeeper checks.[9][10][11]
S0352 OSX_OCEANLOTUS.D

OSX_OCEANLOTUS.D uses the command xattr -d com.apple.quarantine to remove the quarantine file attribute used by Gatekeeper.[12][4]
S0658 XCSSET

XCSSET has dropped a malicious applet into an app's .../Contents/MacOS/ folder of a previously launched app to bypass Gatekeeper's security checks on first launch apps (prior to macOS 13).[6]

Mitigations

ID Mitigation Description
M1038 Execution Prevention

System settings can prevent applications from running that haven't been downloaded through the Apple Store which can help mitigate some of these issues.

Detection

ID Data Source Data Component Detects
DS0017 Command Command Execution

Monitor and investigate attempts to modify extended file attributes with utilities such as xattr. Built-in system utilities may generate high false positive alerts, so compare against baseline knowledge for how systems are typically used and correlate modification events with other indications of malicious activity where possible.

DS0022 File File Metadata

Review false values under the LSFileQuarantineEnabled entry in an application's Info.plist file (required by every application). false under LSFileQuarantineEnabled indicates that an application does not use the quarantine flag. Unsandboxed applications with an unspecified LSFileQuarantineEnabled entry will default to not setting the quarantine flag.

QuarantineEvents is a SQLite database containing a list of all files assigned the com.apple.quarantine attribute, located at ~/Library/Preferences/com.apple.LaunchServices.QuarantineEventsV2. Each event contains the corresponding UUID, timestamp, application, Gatekeeper score, and decision if it was allowed. [1]
File Modification

The removal of the com.apple.quarantine flag by a user instead of the operating system is a suspicious action and should be examined further. Also monitor software update frameworks that may strip this flag when performing updates.
DS0009 Process Process Creation

Monitor and investigate attempts to modify extended file attributes with utilities such as xattr. Built-in system utilities may generate high false positive alerts, so compare against baseline knowledge for how systems are typically used and correlate modification events with other indications of malicious activity where possible.

References

  1. hoakley. (2020, October 29). Quarantine and the quarantine flag. Retrieved September 13, 2021.
  2. How Notarization Works. (2020, August 28). How notarization works. Retrieved September 13, 2021.
  3. Eddie Lee. (2016, February 17). OceanLotus for OS X - an Application Bundle Pretending to be an Adobe Flash Update. Retrieved July 5, 2017.
  4. Phil Stokes. (2021, February 16). 20 Common Tools & Techniques Used by macOS Threat Actors & Malware. Retrieved August 23, 2021.
  5. Csaba Fitzl. (2021, June 29). GateKeeper - Not a Bypass (Again). Retrieved September 22, 2021.
  6. Brandon Dalton. (2022, August 9). A bundle of nerves: Tweaking macOS security controls to thwart application bundle manipulation. Retrieved September 27, 2022.
  1. Thomas Reed. (2018, October 29). Mac cryptocurrency ticker app installs backdoors. Retrieved April 23, 2019.
  2. M.Léveillé, M., Cherepanov, A.. (2022, January 25). Watering hole deploys new macOS malware, DazzleSpy, in Asia. Retrieved May 6, 2022.
  3. Carbon Black Threat Analysis Unit. (2019, February 12). New macOS Malware Variant of Shlayer (OSX) Discovered. Retrieved August 8, 2019.
  4. Jaron Bradley. (2021, April 26). Shlayer malware abusing Gatekeeper bypass on macOS. Retrieved September 22, 2021.
  5. Patrick Wardle. (2020, August 30). Apple Approved Malware malicious code ...now notarized!? #2020. Retrieved September 13, 2021.
  6. Magisa, L. (2020, November 27). New MacOS Backdoor Connected to OceanLotus Surfaces. Retrieved December 2, 2020.