iOS SelRef Obfuscation, Anti-Reversing in iOS Apps

Last updated March 2, 2024 by Appdome

Learn to Add SelRef Obfuscation in iOS apps, in mobile CI/CD with a Data-Driven DevSecOps™ build system.

What is Objective-C SelRef?

iOS applications run on Apple mobile devices within the Objective-C runtime. Because the runtime does not work with method names, when building iOS apps the class’s method name is converted into a selector that represents this method, and therefore acts as the link between the method name and its implementation in the app. This selector is referred to as a “SelRef” (Selector Reference) in the compiled binary. Using selectors has many benefits, such as reducing app size by storing only one ASCII representation of all methods that share the same name, as well as the ability to extend and override a specific method’s logic.

Why do developers need to obfuscate SelRefs in iOS apps?

Attackers use SelRef during static analysis and reverse engineering to gain a deep understanding of the inner workings of an iOS app. For example, determining which method is being called within a function is only possible by resolving the selRef being used to the method it is associated to. Furthermore, attackers can perform the opposite conversion, going from method names to it’s SelRef, to find all calls to method in question by finding all references to the SelRef. Obfuscating SelRef in the application binary prevents the attacker from performing these conversions, thereby significantly limiting the risk of reverse engineering.

Prerequisites for Using Obj-C SelRef Obfuscation:

To use Appdome’s mobile app security build system to Add SelRef Obfuscation , you’ll need:

Add SelRef Obfuscation on iOS apps using Appdome

On Appdome, follow these 3 simple steps to create self-defending iOS Apps that Add SelRef Obfuscation without an SDK or gateway:

  1. Upload the Mobile App to Appdome.

    1. Upload an app to Appdome’s Mobile App Security Build System

    2. Upload Method: Appdome Console or DEV-API
    3. iOS Formats: .ipa
    4. Obj-C SelRef Obfuscation Compatible With: Obj-C, Java, Swift, Flutter, React Native, Unity, Xamarin, Cordova and more
  2. Build the feature: Obj-C SelRef Obfuscation.

    1. Building Obj-C SelRef Obfuscation by using Appdome’s DEV-API:

      1. Create and name the Fusion Set (security template) that will contain the Obj-C SelRef Obfuscation feature as shown below:
      2. fusion set that contains Obj-C SelRef Obfuscation

        Figure 1: Fusion Set that will contain the Obj-C SelRef Obfuscation feature
        Note: Naming the Fusion Set to correspond to the protection(s) selected is for illustration purposes only (not required).

      3. Follow the steps in Sections 2.2.1-2.2.2 of this article, Building the Obj-C SelRef Obfuscation feature via Appdome Console, to add the Obj-C SelRef Obfuscation feature to this Fusion Set.

      4. Open the Fusion Set Detail Summary by clicking the “...” symbol on the far-right corner of the Fusion Set, as shown in Figure 1 above, and get the Fusion Set ID from the Fusion Set Detail Summary (as shown below): fusion Set Detail Summary image

        Figure 2: Fusion Set Detail Summary
        Note: Annotating the Fusion Set to identify the protection(s) selected is optional only (not mandatory).

      5. Follow the instructions below to use the Fusion Set ID inside any standard mobile DevOps or CI/CD toolkit like Bitrise, App Center, Jenkins, Travis, Team City, Circle CI or other system:
        1. Build an API for the app – for instructions, see the tasks under Appdome API Reference Guide
        2. Look for sample APIs in Appdome’s GitHub Repository
    2. Building the Obj-C SelRef Obfuscation feature via Appdome Console

      To build the Obj-C SelRef Obfuscation protection by using Appdome Console, follow the instructions below.

      1. Where: Inside the Appdome Console, go to Build > Security Tab > TOTALCode™ Obfuscation section.
      2. When you select the Obj-C SelRef Obfuscation you'll notice that your Fusion Set you created in step 2.1.1 now bears the icon of the protection category that contains Obj-C SelRef Obfuscation

        Fusion Set applied Obj-C SelRef Obfuscation

        Figure 4: Fusion Set that displays the newly added Obj-C SelRef Obfuscation protection

      3. Click Build My App at the bottom of the Build Workflow (shown in Figure 3).
    Congratulations!  The Obj-C SelRef Obfuscation protection is now added to the mobile app

Using Threat-Events™ for SelRef Obfuscation Intelligence and Control in iOS Apps

Appdome Threat-Events™ provides consumable in-app mobile app attack intelligence and defense control when SelRef Obfuscation is detected. To consume and use Threat-Events™ for SelRef Obfuscation in iOS Apps, use AddObserverForName in Notification Center, and the code samples for Threat-Events™ for SelRef Obfuscation shown below.

The specifications and options for Threat-Events™ for SelRef Obfuscation are:

Threat-Event™ Elements Add SelRef Obfuscation Method Detail
Appdome Feature Name Obj-C SelRef Obfuscation
Threat-Event Mode
OFF, IN-APP DEFENSE Appdome detects, defends and notifies user (standard OS dialog) using customizable messaging.
ON, IN-APP DETECTION Appdome detects the attack or threat and passes the event in a standard format to the app for processing (app chooses how and when to enforce).
ON, IN-APP DEFENSE Uses Appdome Enforce mode for any attack or threat and passes the event in a standard format to the app for processing (gather intel on attacks and threats without losing any protection).
Certified Secure™ Threat Event Check x
Visible in ThreatScope™ x
Developer Parameters for Adding SelRef Obfuscation Threat-Event™
Threat-Event NAME
Threat-Event DATA reasonData
Threat-Event CODE reasonCode
Threat-Event REF
Threat-Event SCORE
currentThreatEventScore Current Threat-Event score
threatEventsScore Total Threat-events score
Threat-Event Context Keys
message Message displayed for the user on event
externalID The external ID of the event which can be listened via Threat Events
osVersion OS version of the current device
deviceModel Current device model
deviceManufacturer The manufacturer of the current device
fusedAppToken The task ID of the Appdome fusion of the currently running app
kernelInfo Info about the kernel: system name, node name, release, version and machine.
carrierPlmn PLMN of the device
deviceID Current device ID
reasonCode Reason code of the occured event
buildDate Appdome fusion date of the current application
devicePlatform OS name of the current device
carrierName Carrier name of the current device
updatedOSVersion Is the OS version up to date
timeZone Time zone
deviceFaceDown Is the device face down
locationLong Location longitude conditioned by location permission
locationLat Location latitude conditioned by location permission
locationState Location state conditioned by location permission
wifiSsid Wifi SSID
wifiSsidPermissionStatus Wifi SSID permission status

With Threat-Events™ enabled (turned ON), iOS developers can get detailed attack intelligence and granular defense control in iOS applications and create amazing user experiences for all mobile end users when SelRef Obfuscation is detected.

The following is a code sample for native iOS apps, which uses all values in the specification above for Obj-C SelRef Obfuscation:


Using Appdome, there are no development or coding prerequisites to build secured iOS Apps by using Obj-C SelRef Obfuscation. There is no SDK and no library to code or implement in the app and no gateway to deploy in your network. All protections are built into each app and the resulting app is self-defending and self-protecting.

Releasing and Publishing Mobile Apps with Obj-C SelRef Obfuscation

After successfully securing your app by using Appdome, there are several available options to complete your project, depending on your app lifecycle or workflow. These include:

Related Articles:

If you have any questions, please send them our way at support.appdome.com or via the chat window on the Appdome platform.

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