Swift and Objective-C

Because Objective-C and Swift are fundamentally different, the programming language in which the app is written affects the possibilities for reverse engineering it. For example, Objective-C allows method invocations to be changed at run time. This makes hooking into other app functions (a technique heavily used by Cycript and other reverse engineering tools) easy. This "method swizzling" is not implemented the same way in Swift, and the difference makes the technique harder to execute with Swift than with Objective-C.

The majority of this chapter applies to applications written in Objective-C or having bridged types, which are types compatible with both Swift and Objective-C. The Swift compatibility of most tools that work well with Objective-C is being improved. For example, Frida supports Swift bindings.

Utilities

• ​Class-dump by Steve Nygard "is a command line utility for examining the Objective-C runtime information stored in Mach-O (Mach object) files. It generates declarations for the classes, categories, and protocols."

• ​Class-dump-z is class-dump re-written from scratch in C++, avoiding the use of dynamic calls. Removing these unnecessary calls makes class-dump-z nearly 10 times faster than its predecessor.

• ​Class-dump-dyld by Elias Limneos allows symbols to be dumped and retrieved directly from the shared cache, eliminating the necessity of extracting the files first. It can generate header files from app binaries, libraries, frameworks, bundles, or the whole dyld_shared_cache. Directories or the entirety of dyld_shared_cache can be recursively mass-dumped.

• ​MachoOView is a useful visual Mach-O file browser that also allows in-file editing of ARM binaries.

• otool is a tool for displaying specific parts of object files or libraries. It works with Mach-O files and universal file formats.

Reversing Frameworks

​Radare2 is a complete framework for reverse engineering and analyzing. It is built with the Capstone disassembler engine, Keystone assembler, and Unicorn CPU emulation engine. Radare2 supports iOS binaries and many useful iOS-specific features, such as a native Objective-C parser and an iOS debugger.

Commercial Disassemblers

IDA Pro can deal with iOS binaries. It has a built-in iOS debugger. IDA is widely seen as the gold standard for GUI-based interactive static analysis, but it isn't cheap. For the more budget-minded reverse engineer, Hopper offers similar static analysis features.

Reverse Engineering iOS Apps

iOS reverse engineering is a mixed bag. On one hand, apps programmed in Objective-C and Swift can be disassembled nicely. In Objective-C, object methods are called via dynamic function pointers called "selectors," which are resolved by name during run time. The advantage of run-time name resolution is that these names need to stay intact in the final binary, making the disassembly more readable. Unfortunately, this also means that no direct cross-references between methods are available in the disassembler and constructing a flow graph is challenging.

In this guide, we'll introduce static and dynamic analysis and instrumentation. Throughout this chapter, we refer to the OWASP UnCrackable Apps for iOS, so download them from the MSTG repository if you're planning to follow the examples.

Static Analysis

Recovering an IPA File From an Installed App

From Jailbroken Devices

You can use Saurik's IPA Installer Console to recover IPAs from apps installed on the device. To do this, install IPA Installer Console via Cydia. Then, SSH into the device and look up the bundle ID of the target app. For example through listing of the available apps:

iPhone:~ root# ipainstaller -l
com.apple.Pages
com.example.targetapp
com.google.ios.youtube
com.spotify.client

Generate the IPA file via the following command:

iPhone:~ root# ipainstaller -b com.example.targetapp -o /tmp/example.ipa

From Non-Jailbroken Devices

If the app is available on iTunes, you can recover the IPA on macOS:

• Download the app through iTunes.

• Go to your iTunes Apps Library.

• Right-click on the app and select "Show in Finder".

Dumping Decrypted Executables

Besides being code-signed, apps distributed via the App Store are also protected by Apple's FairPlay DRM system. This system uses asymmetric cryptography to ensure that any app (including free apps) obtained from the App Store executes only on the device it is approved to run on. The decryption key is unique to the device and burned into the processor. As of now, the only way to obtain the decrypted code from a FairPlay-decrypted app is to dump it from memory while the app is running. On a jailbroken device, this can be done with the Clutch tool that's included in standard Cydia repositories [2]. Use clutch in interactive mode to get a list of installed apps, decrypt them, and pack them into an IPA file:

$ Clutch -i

NOTE: Only applications distributed via the AppStore are protected by FairPlay DRM. If your application was compiled in and exported directly from Xcode, you don't need to decrypt it. The easiest way to disassemble is to load the application into Hopper, which can be used to make sure that it's being correctly disassembled. You can also check it with otool:

$  otool -l yourbinary | grep -A 4 LC_ENCRYPTION_INFO

If the output contains cryptoff, cryptsize, and cryptid fields, the binary is encrypted. If the output of this command is empty, the binary is not encrypted. Remember to use otool on the binary, not on the IPA file.

Getting Basic Information with Class-dump and Hopper Disassembler

You can use class-dump to get information about methods in the application's source code. The example below uses the Damn Vulnerable iOS App to demonstrate this. Our binary is a so-called fat binary, which means that it can be executed on 32- and 64-bit platforms:

$ unzip DamnVulnerableiOSApp.ipa
​
$ cd Payload/DamnVulnerableIOSApp.app
​
$ otool -hv DamnVulnerableIOSApp
​
DamnVulnerableIOSApp (architecture armv7):
Mach header
     magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
  MH_MAGIC     ARM         V7  0x00     EXECUTE    38       4292   NOUNDEFS DYLDLINK TWOLEVEL WEAK_DEFINES BINDS_TO_WEAK PIE
​
DamnVulnerableIOSApp (architecture arm64):
Mach header
     magic cputype cpusubtype  caps    filetype ncmds sizeofcmds      flags
MH_MAGIC_64   ARM64        ALL  0x00     EXECUTE    38       4856   NOUNDEFS DYLDLINK TWOLEVEL WEAK_DEFINES BINDS_TO_WEAK PIE

Note the architectures: armv7 (which is 32-bit) and arm64. This design of a fat binary allows an application to be deployed on all devices. To analyze the application with class-dump, we must create a so-called thin binary, which contains one architecture only:

iOS8-jailbreak:~ root# lipo -thin armv7 DamnVulnerableIOSApp -output DVIA32

And then we can proceed to performing class-dump:

iOS8-jailbreak:~ root# class-dump DVIA32
​
@interface FlurryUtil : ./DVIA/DVIA/DamnVulnerableIOSApp/DamnVulnerableIOSApp/YapDatabase/Extensions/Views/Internal/
{
}
+ (BOOL)appIsCracked;
+ (BOOL)deviceIsJailbroken;

Note the plus sign, which means that this is a class method that returns a BOOL type. A minus sign would mean that this is an instance method. Refer to later sections to understand the practical difference between these.

Alternatively, you can easily decompile the application with Hopper Disassembler. All these steps would be executed automatically, and you'd be able to see the disassembled binary and class information.

The following command is listing shared libraries:

$ otool -L <binary>

Debugging

Debugging on iOS is generally implemented via Mach IPC. To "attach" to a target process, the debugger process calls the task_for_pid function with the process ID of the target process and receives a Mach port. The debugger then registers as a receiver of exception messages and starts handling exceptions that occur in the debugger. Mach IPC calls are used to perform actions such as suspending the target process and reading/writing register states and virtual memory.

The XNU kernel implements the ptrace system call, but some of the call's functionality (including reading and writing register states and memory contents) has been eliminated. Nevertheless, ptrace is used in limited ways by standard debuggers, such as lldb and gdb. Some debuggers, including Radare2's iOS debugger, don't invoke ptrace at all.

Using lldb

iOS ships with the console app debugserver, which allows remote debugging via gdb or lldb. By default, however, debugserver can't be used to attach to arbitrary processes (it is usually used only for debugging self-developed apps deployed with Xcode). To enable debugging of third-party apps, the task_for_pid entitlement must be added to the debugserver executable. An easy way to do this is to add the entitlement to the debugserver binary shipped with Xcode.

To obtain the executable, mount the following DMG image:

/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/<target-iOS-version/DeveloperDiskImage.dmg

You'll find the debugserver executable in the /usr/bin/ directory on the mounted volume. Copy it to a temporary directory, then create a file called entitlements.plist with the following content:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/ PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
 <key>com.apple.springboard.debugapplications</key>
 <true/>
 <key>run-unsigned-code</key>
 <true/>
 <key>get-task-allow</key>
 <true/>
 <key>task_for_pid-allow</key>
 <true/>
</dict>
</plist>

Apply the entitlement with codesign:

$ codesign -s - --entitlements entitlements.plist -f debugserver

Copy the modified binary to any directory on the test device. The following examples use usbmuxd to forward a local port through USB.

$ ./tcprelay.py -t 22:2222
$ scp -P2222 debugserver root@localhost:/tmp/

You can now attach debugserver to any process running on the device.

VP-iPhone-18:/tmp root# ./debugserver *:1234 -a 2670
debugserver-@(#)PROGRAM:debugserver  PROJECT:debugserver-320.2.89
for armv7.
Attaching to process 2670...

Cycript and Cynject

Cydia Substrate (formerly called MobileSubstrate) is the standard framework for developing run-time patches ("Cydia Substrate extensions") on iOS. It comes with Cynject, a tool that provides code injection support for C. Cycript is a scripting language developed by Jay Freeman (aka saurik). It injects a JavaScriptCore VM into the running process. Via the Cycript interactive console, users can then manipulate the process with a hybrid Objective-C++ and JavaScript syntax. Accessing and instantiating Objective-C classes inside a running process is also possible. Examples of Cycript usage are included in the iOS chapter.

First download, unpack, and install the SDK.

#on iphone
$ wget https://cydia.saurik.com/api/latest/3 -O cycript.zip && unzip cycript.zip
$ sudo cp -a Cycript.lib/*.dylib /usr/lib
$ sudo cp -a Cycript.lib/cycript-apl /usr/bin/cycript

To spawn the interactive Cycript shell, run "./cyript" or "cycript" if Cycript is on your path.

$ cycyript
cy#

To inject into a running process, we first need to find the process ID (PID). Running "cycript -p" with the PID injects Cycript into the process. To illustrate, we will inject into SpringBoard.

$ ps -ef | grep SpringBoard
501 78 1 0 0:00.00 ?? 0:10.57 /System/Library/CoreServices/SpringBoard.app/SpringBoard
$ ./cycript -p 78
cy#

We have injected Cycript into SpringBoard. Let's try to trigger an alert message on SpringBoard with Cycript.

cy# alertView = [[UIAlertView alloc] initWithTitle:@"OWASP MSTG" message:@"Mobile Security Testing Guide"  delegate:nil cancelButtonitle:@"OK" otherButtonTitles:nil]
#"<UIAlertView: 0x1645c550; frame = (0 0; 0 0); layer = <CALayer: 0x164df160>>"
cy# [alertView show]
cy# [alertView release]

Find the document directory with Cycript:

cy# [[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask][0]
#"file:///var/mobile/Containers/Data/Application/A8AE15EE-DC8B-4F1C-91A5-1FED35212DF/Documents/"

Use the following command to get the application's delegate class:

cy# [UIApplication sharedApplication].delegate

The command [[UIApp keyWindow] recursiveDescription].toString() returns the view hierarchy of keyWindow. The description of every subview and sub-subview of keyWindow is shown. The indentation space reflects the relationships between views. For example, UILabel, UITextField, and UIButton are subviews of UIView.

cy# [[UIApp keyWindow] recursiveDescription].toString()
`<UIWindow: 0x16e82190; frame = (0 0; 320 568); gestureRecognizers = <NSArray: 0x16e80ac0>; layer = <UIWindowLayer: 0x16e63ce0>>
  | <UIView: 0x16e935f0; frame = (0 0; 320 568); autoresize = W+H; layer = <CALayer: 0x16e93680>>
  |    | <UILabel: 0x16e8f840; frame = (0 40; 82 20.5); text = 'i am groot!'; hidden = YES; opaque = NO; autoresize = RM+BM; userInteractionEnabled = NO; layer = <_UILabelLayer: 0x16e8f920>>
  |    | <UILabel: 0x16e8e030; frame = (0 110.5; 320 20.5); text = 'A Secret Is Found In The ...'; opaque = NO; autoresize = RM+BM; userInteractionEnabled = NO; layer = <_UILabelLayer: 0x16e8e290>>
  |    | <UITextField: 0x16e8fbd0; frame = (8 141; 304 30); text = ''; clipsToBounds = YES; opaque = NO; autoresize = RM+BM; gestureRecognizers = <NSArray: 0x16e94550>; layer = <CALayer: 0x16e8fea0>>
  |    |    | <_UITextFieldRoundedRectBackgroundViewNeue: 0x16e92770; frame = (0 0; 304 30); opaque = NO; autoresize = W+H; userInteractionEnabled = NO; layer = <CALayer: 0x16e92990>>
  |    | <UIButton: 0x16d901e0; frame = (8 191; 304 30); opaque = NO; autoresize = RM+BM; layer = <CALayer: 0x16d90490>>
  |    |    | <UIButtonLabel: 0x16e72b70; frame = (133 6; 38 18); text = 'Verify'; opaque = NO; userInteractionEnabled = NO; layer = <_UILabelLayer: 0x16e974b0>>
  |    | <_UILayoutGuide: 0x16d92a00; frame = (0 0; 0 20); hidden = YES; layer = <CALayer: 0x16e936b0>>
  |    | <_UILayoutGuide: 0x16d92c10; frame = (0 568; 0 0); hidden = YES; layer = <CALayer: 0x16d92cb0>>`

Hooking Native Functions and Objective-C Methods

• Install the application that will be hooked.

• Run the application and make sure the app is in the foreground (it shouldn't be paused).

• Find the PID of the app with the command ps ax | grep App.

• Hook into the running process with the command cycript -p PID.

• The Cycript interpreter will be provided after successful hooking. You can get the application instance by using the Objective-C syntax: [UIApplication sharedApplication].

  cy# [UIApplication sharedApplication]
  cy# var a = [UIApplication sharedApplication]

• To find this application's delegate class:

  cy# a.delegate

• Let's print out the AppDelegate class' methods :

  cy# printMethods ("AppDelegate")

Installing Frida

​Frida is a runtime instrumentation framework that lets you inject JavaScript snippets or portions of your own library into native Android and iOS apps. If you've already read the Android section of this guide, you should be quite familiar with this tool.

If you haven't already done so, install the Frida Python package on your host machine:

$ pip install frida

To connect Frida to an iOS app, you need to inject the Frida runtime into the app. This is easy to do on a jailbroken device: just install frida-server through Cydia. Once it is installed, frida-server will automatically run with root privileges, allowing you to easily inject code into any process.

Start Cydia and add Frida's repository by navigating to Manage -> Sources -> Edit -> Add and entering https://build.frida.re. You should then be able to find and install the Frida package.

Connect your device via USB and make sure that Frida works by running the frida-ps command. This should return a list of processes running on the device:

$ frida-ps -U
PID  Name
---  ----------------
963  Mail
952  Safari
416  BTServer
422  BlueTool
791  CalendarWidget
451  CloudKeychainPro
239  CommCenter
764  ContactsCoreSpot
(...)

We'll demonstrate a few more uses for Frida below, but let's first look at what you should do if you're forced to work on a non-jailbroken device.

Dynamic Analysis on Non-Jailbroken Devices

Automated Repackaging with Objection

​Objection is a mobile runtime exploration toolkit based on Frida. One of the biggest advantages about Objection is that it enables testing with non-jailbroken devices. It does this by automating the process of app repackaging with the FridaGadget.dylib library. A detailed explanation of the repackaging and resigning process can be found in the next chapter "Manual Repackaging". We won't cover Objection in detail in this guide, as you can find exhaustive documentation on the official wiki pages.

Manual Repackaging

If you don't have access to a jailbroken device, you can patch and repackage the target app to load a dynamic library at startup. This way, you can instrument the app and do pretty much everything you need to do for a dynamic analysis (of course, you can't break out of the sandbox this way, but you won't often need to). However, this technique works only if the app binary isn't FairPlay-encrypted (i.e., obtained from the App Store).

Thanks to Apple's confusing provisioning and code-signing system, re-signing an app is more challenging than you would expect. iOS won't run an app unless you get the provisioning profile and code signature header exactly right. This requires learning many concepts-certificate types, BundleIDs, application IDs, team identifiers, and how Apple's build tools connect them. Getting the OS to run a binary that hasn't been built via the default method (Xcode) can be a daunting process.

We'll use optool, Apple's build tools, and some shell commands. Our method is inspired by Vincent Tan's Swizzler project. The NCC group has described an alternative repackaging method.

To reproduce the steps listed below, download UnCrackable iOS App Level 1 from the OWASP Mobile Testing Guide repository. Our goal is to make the UnCrackable app load FridaGadget.dylib during startup so we can instrument the app with Frida.

Please note that the following steps apply to macOS only, as Xcode is only available for macOS.

Getting a Developer Provisioning Profile and Certificate

The provisioning profile is a plist file signed by Apple. It whitelists your code-signing certificate on one or more devices. In other words, this represents Apple explicitly allowing your app to run for certain reasons, such as debugging on selected devices (development profile). The provisioning profile also includes the entitlements granted to your app. The certificate contains the private key you'll use to sign.

Depending on whether you're registered as an iOS developer, you can obtain a certificate and provisioning profile in one of the following ways:

With an iOS developer account:

If you've developed and deployed iOS apps with Xcode before, you already have your own code-signing certificate installed. Use the security tool to list your signing identities:

$ security find-identity -v
 1) 61FA3547E0AF42A11E233F6A2B255E6B6AF262CE "iPhone Distribution: Vantage Point Security Pte. Ltd."
 2) 8004380F331DCA22CC1B47FB1A805890AE41C938 "iPhone Developer: Bernhard Müller (RV852WND79)"

Log into the Apple Developer portal to issue a new App ID, then issue and download the profile. An App ID is a two-part string: a Team ID supplied by Apple and a bundle ID search string that you can set to an arbitrary value, such as com.example.myapp. Note that you can use a single App ID to re-sign multiple apps. Make sure you create a development profile and not a distribution profile so that you can debug the app.

In the examples below, I use my signing identity, which is associated with my company's development team. I created the App ID "sg.vp.repackaged" and the provisioning profile "AwesomeRepackaging" for these examples. I ended up with the file AwesomeRepackaging.mobileprovision-replace this with your own filename in the shell commands below.

With a Regular iTunes Account:

Apple will issue a free development provisioning profile even if you're not a paying developer. You can obtain the profile via Xcode and your regular Apple account: simply create an empty iOS project and extract embedded.mobileprovision from the app container, which is in the Xcode subdirectory of your home directory: ~/Library/Developer/Xcode/DerivedData/<ProjectName>/Build/Products/Debug-iphoneos/<ProjectName>.app/. The NCC blog post "iOS instrumentation without jailbreak" explains this process in great detail.

Once you've obtained the provisioning profile, you can check its contents with the security tool. You'll find the entitlements granted to the app in the profile, along with the allowed certificates and devices. You'll need these for code-signing, so extract them to a separate plist file as shown below. Have a look at the file contents to make sure everything is as expected.

$ security cms -D -i AwesomeRepackaging.mobileprovision > profile.plist
$ /usr/libexec/PlistBuddy -x -c 'Print :Entitlements' profile.plist > entitlements.plist
$ cat entitlements.plist
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
 <key>application-identifier</key>
 <string>LRUD9L355Y.sg.vantagepoint.repackage</string>
 <key>com.apple.developer.team-identifier</key>
 <string>LRUD9L355Y</string>
 <key>get-task-allow</key>
 <true/>
 <key>keychain-access-groups</key>
 <array>
   <string>LRUD9L355Y.*</string>
 </array>
</dict>
</plist>

Note the application identifier, which is a combination of the Team ID (LRUD9L355Y) and Bundle ID (sg.vantagepoint.repackage). This provisioning profile is only valid for the app that has this App ID. The "get-task-allow" key is also important: when set to "true," other processes, such as the debugging server, are allowed to attach to the app (consequently, this would be set to "false" in a distribution profile).

Other Preparations

To make our app load an additional library at startup, we need some way of inserting an additional load command into the main executable's Mach-O header. Optool can be used to automate this process:

$ git clone https://github.com/alexzielenski/optool.git
$ cd optool/
$ git submodule update --init --recursive
$ xcodebuild
$ ln -s <your-path-to-optool>/build/Release/optool /usr/local/bin/optool

We'll also use ios-deploy, a tool that allows iOS apps to be deployed and debugged without Xcode:

$ git clone https://github.com/phonegap/ios-deploy.git
$ cd ios-deploy/
$ xcodebuild
$ cd build/Release
$ ./ios-deploy
$ ln -s <your-path-to-ios-deploy>/build/Release/ios-deploy /usr/local/bin/ios-deploy

The last line in both the optool and ios-deploy code snippets creates a symbolic link and makes the executable available system-wide.

Reload your shell to make the new commands available:

zsh: # . ~/.zshrc
bash: # . ~/.bashrc

To execute the examples below, you need FridaGadget.dylib:

$ curl -O https://build.frida.re/frida/ios/lib/FridaGadget.dylib

We'll be using standard tools that come with macOS and Xcode in addition to the tools mentioned above. Make sure you have the Xcode command line developer tools installed.

Patching, Repackaging, and Re-Signing

Time to get serious! As you already know, IPA files are actually ZIP archives, so you can use any zip tool to unpack the archive. Copy FridaGadget.dylib into the app directory and use optool to add a load command to the "UnCrackable Level 1" binary.

$ unzip UnCrackable_Level1.ipa
$ cp FridaGadget.dylib Payload/UnCrackable\ Level\ 1.app/
$ optool install -c load -p "@executable_path/FridaGadget.dylib"  -t Payload/UnCrackable\ Level\ 1.app/UnCrackable\ Level\ 1
Found FAT Header
Found thin header...
Found thin header...
Inserting a LC_LOAD_DYLIB command for architecture: arm
Successfully inserted a LC_LOAD_DYLIB command for arm
Inserting a LC_LOAD_DYLIB command for architecture: arm64
Successfully inserted a LC_LOAD_DYLIB command for arm64
Writing executable to Payload/UnCrackable Level 1.app/UnCrackable Level 1...

Of course, such blatant tampering invalidates the main executable's code signature, so this won't run on a non-jailbroken device. You'll need to replace the provisioning profile and sign both the main executable and FridaGadget.dylib with the certificate listed in the profile.

First, let's add our own provisioning profile to the package:

$ cp AwesomeRepackaging.mobileprovision Payload/UnCrackable\ Level\ 1.app/embedded.mobileprovision

Next, we need to make sure that the BundleID in Info.plist matches the one specified in the profile because the codesign tool will read the Bundle ID from Info.plist during signing; the wrong value will lead to an invalid signature.

$ /usr/libexec/PlistBuddy -c "Set :CFBundleIdentifier sg.vantagepoint.repackage" Payload/UnCrackable\ Level\ 1.app/Info.plist

Finally, we use the codesign tool to re-sign both binaries. You need to use your signing identity (in this example 8004380F331DCA22CC1B47FB1A805890AE41C938), which you can output by executing the command security find-identity -v.

$ rm -rf Payload/UnCrackable\ Level\ 1.app/_CodeSignature
$ /usr/bin/codesign --force --sign 8004380F331DCA22CC1B47FB1A805890AE41C938  Payload/UnCrackable\ Level\ 1.app/FridaGadget.dylib
Payload/UnCrackable Level 1.app/FridaGadget.dylib: replacing existing signature

entitlements.plist is the file you created for your empty iOS project.

$ /usr/bin/codesign --force --sign 8004380F331DCA22CC1B47FB1A805890AE41C938 --entitlements entitlements.plist Payload/UnCrackable\ Level\ 1.app/UnCrackable\ Level\ 1
Payload/UnCrackable Level 1.app/UnCrackable Level 1: replacing existing signature

Installing and Running an App

Now you should be ready to run the modified app. Deploy and run the app on the device:

$ ios-deploy --debug --bundle Payload/UnCrackable\ Level\ 1.app/

If everything went well, the app should start in debugging mode with lldb attached. Frida should then be able to attach to the app as well. You can verify this via the frida-ps command:

$ frida-ps -U
PID  Name
---  ------
499  Gadget

Troubleshooting

When something goes wrong (and it usually does), mismatches between the provisioning profile and code-signing header are the most likely causes. Reading the official documentation helps you understand the code-signing process. Apple's entitlement troubleshooting page is also a useful resource.

Method Tracing with Frida

Intercepting Objective-C methods is a useful iOS security testing technique (for data storage operations and network requests, for example). In the following example, we'll write a simple tracer for logging HTTP(S) requests made via standard iOS HTTP APIs. We'll also show you how to inject the tracer into the Safari web browser.

In the following examples, we'll assume that you're working on a jailbroken device. If that's not the case, you need to first follow the steps outlined in the previous section to repackage the Safari app.

Frida comes with frida-trace, a function tracing tool. frida-trace accepts Objective-C methods via the -m flag. You can pass it wildcards as well: given -[NSURL *], for example, frida-trace will automatically install hooks on all NSURL class selectors. We'll use this to get a rough idea of which library functions Safari calls when the user opens a URL.

Run Safari on the device and make sure the device is connected via USB. Then start frida-trace:

$ frida-trace -U -m "-[NSURL *]" Safari
Instrumenting functions...
-[NSURL isMusicStoreURL]: Loaded handler at "/Users/berndt/Desktop/__handlers__/__NSURL_isMusicStoreURL_.js"
-[NSURL isAppStoreURL]: Loaded handler at "/Users/berndt/Desktop/__handlers__/__NSURL_isAppStoreURL_.js"
(...)
Started tracing 248 functions. Press Ctrl+C to stop.

Next, navigate to a new website in Safari. You should see traced function calls on the frida-trace console. Note that the initWithURL: method is called to initialize a new URL request object.

          /* TID 0xc07 */
 20313 ms  -[NSURLRequest _initWithCFURLRequest:0x1043bca30 ]
20313 ms  -[NSURLRequest URL]
(...)
21324 ms  -[NSURLRequest initWithURL:0x106388b00 ]
21324 ms     | -[NSURLRequest initWithURL:0x106388b00 cachePolicy:0x0 timeoutInterval:0x106388b80

We can look up the declaration of this method on the Apple Developer Website:

- (instancetype)initWithURL:(NSURL *)url;

The method is called with a single argument of type NSURL. According to the Apple Developer documentation, the NSURL class has a property called absoluteString, whose value should be the absolute URL represented by the NSURL object.

We now have all the information we need to write a Frida script that intercepts the initWithURL: method and prints the URL passed to the method. The full script is below. Make sure you read the code and inline comments to understand what's going on.

import sys
import frida
​
​
// JavaScript to be injected
frida_code = """
​
    // Obtain a reference to the initWithURL: method of the NSURLRequest class
    var URL = ObjC.classes.NSURLRequest["- initWithURL:"];
​
    // Intercept the method
    Interceptor.attach(URL.implementation, {
        onEnter: function(args) {
            // Get a handle on NSString
            var NSString = ObjC.classes.NSString;
​
            // Obtain a reference to the NSLog function, and use it to print the URL value
            // args[2] refers to the first method argument (NSURL *url)
            var NSLog = new NativeFunction(Module.findExportByName('Foundation', 'NSLog'), 'void', ['pointer', '...']);
​
            // We should always initialize an autorelease pool before interacting with Objective-C APIs
            var pool = ObjC.classes.NSAutoreleasePool.alloc().init();
​
            try {
                // Creates a JS binding given a NativePointer.
                var myNSURL = new ObjC.Object(args[2]);
​
                // Create an immutable ObjC string object from a JS string object.
                var str_url = NSString.stringWithString_(myNSURL.toString());
                NSLog(str_url);
            } finally {
                pool.release();
            }
        }
    });
"""
​
process = frida.get_usb_device().attach("Safari")
script = process.create_script(frida_code)
script.on('message', message_callback)
script.load()
​
sys.stdin.read()

Start Safari on the iOS device. Run the above Python script on your connected host and open the device log (we'll explain how to open device logs in the following section). Try opening a new URL in Safari; you should see Frida's output in the logs.

Of course, this example illustrates only one of the things you can do with Frida. To unlock the tool's full potential, you should learn to use its JavaScript API. The documentation section of the Frida website has a tutorial and examples of Frida usage on iOS.

Please also take a look at the Frida JavaScript API reference.

Patching React Native Applications

If the React Native framework has been used for development, the main application code is in the file Payload/[APP].app/main.jsbundle. This file contains the JavaScript code. Most of the time, the JavaScript code in this file is minified. With the tool JStillery, a human-readable version of the file can be retried, which will allow code analysis. The CLI version of JStillery and the local server are preferable to the online version because the latter discloses the source code to a third party.

At installation time, the application archive is unpacked into the folder /private/var/containers/Bundle/Application/[GUID]/[APP].app from iOS 10 onward, so the main JavaScript application file can be modified at this location.

To identify the exact location of the application folder, you can use the tool ipainstaller:

1. Use the command ipainstaller -l to list the applications installed on the device. Get the name of the target application from the output list.

2. Use the command ipainstaller -i [APP_NAME] to display information about the target application, including the installation and data folder locations.

3. Take the path referenced at the line that starts with Application:.

Use the following approach to patch the JavaScript file:

1. Navigate to the application folder.

2. Copy the contents of the file Payload/[APP].app/main.jsbundle to a temporary file.

3. Use JStillery to beautify and de-obfuscate the contents of the temporary file.

4. Identify the code in the temporary file that should be patched and patch it.

5. Put the patched code on a single line and copy it into the original Payload/[APP].app/main.jsbundle file.

6. Close and restart the application.

References

• Apple's Entitlements Troubleshooting - https://developer.apple.com/library/content/technotes/tn2415/_index.html​

• Apple's Code Signing - https://developer.apple.com/support/code-signing/​

• iOS Instrumentation without Jailbreak - https://www.nccgroup.trust/au/about-us/newsroom-and-events/blogs/2016/october/ios-instrumentation-without-jailbreak/​

• Frida iOS Tutorial - https://www.frida.re/docs/ios/​

• Frida iOS Examples - https://www.frida.re/docs/examples/ios/​

Tools

• Class-dump - http://stevenygard.com/projects/class-dump/​

• Class-dump-dyld - https://github.com/limneos/classdump-dyld/​

• Class-dump-z - https://code.google.com/archive/p/networkpx/wikis/class_dump_z.wiki​

• Cycript - http://www.cycript.org/​

• Damn Vulnerable iOS App - http://damnvulnerableiosapp.com/​

• Frida - https://www.frida.re​

• Hopper - https://www.hopperapp.com/​

• Hopper Disassembler - https://www.hopperapp.com/​

• ios-deploy - https://github.com/phonegap/ios-deploy​

• IPA Installer Console - https://cydia.saurik.com/package/com.autopear.installipa/​

• ipainstaller - https://cydia.saurik.com/package/com.slugrail.ipainstaller/​

• MachoOView - https://sourceforge.net/projects/machoview/​

• Objection - https://github.com/sensepost/objection​

• Optool - https://github.com/alexzielenski/optool​

• OWASP UnCrackable Apps for iOS - https://github.com/OWASP/owasp-mstg/tree/master/Crackmes#ios​

• Radare2 - https://rada.re/r/​

• Reverse Engineering tools for iOS Apps - http://iphonedevwiki.net/index.php/Reverse_Engineering_Tools​

• Swizzler project - https://github.com/vtky/Swizzler2/​

• Xcode command line developer tools - https://railsapps.github.io/xcode-command-line-tools.html​