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Code Quality and Build Settings for iOS Apps

Making Sure that the App Is Properly Signed

Overview

Code signing your app assures users that the app has a known source and hasn't been modified since it was last signed. Before your app can integrate app services, be installed on a device, or be submitted to the App Store, it must be signed with a certificate issued by Apple. For more information on how to request certificates and code sign your apps, review the App Distribution Guide.

You can retrieve the signing certificate information from the application's .app file with codesign. Codesign is used to create, check, and display code signatures, as well as inquire into the dynamic status of signed code in the system.

After you get the application's .ipa file, re-save it as a ZIP file and decompress the ZIP file. Navigate to the Payload directory, where the application's .app file will be.

Execute the following codesign command:

$ codesign -dvvv <yourapp.app>
Executable=/Users/Documents/<yourname>/Payload/<yourname.app>/<yourname>
Identifier=com.example.example
Format=app bundle with Mach-O universal (armv7 arm64)
CodeDirectory v=20200 size=154808 flags=0x0(none) hashes=4830+5 location=embedded
Hash type=sha256 size=32
CandidateCDHash sha1=455758418a5f6a878bb8fdb709ccfca52c0b5b9e
CandidateCDHash sha256=fd44efd7d03fb03563b90037f92b6ffff3270c46
Hash choices=sha1,sha256
CDHash=fd44efd7d03fb03563b90037f92b6ffff3270c46
Signature size=4678
Authority=iPhone Distribution: Example Ltd
Authority=Apple Worldwide Developer Relations Certification Authority
Authority=Apple Root CA
Signed Time=4 Aug 2017, 12:42:52
Info.plist entries=66
TeamIdentifier=8LAMR92KJ8
Sealed Resources version=2 rules=12 files=1410
Internal requirements count=1 size=176

Finding Debugging Symbols

Overview

Generally, as little explanatory information as possible should be provided with the compiled code. Some metadata (such as debugging information, line numbers, and descriptive function or method names) makes the binary or byte-code easier for the reverse engineer to understand but isn't necessary in a release build. This metadata can therefore be discarded without impacting the app's functionality.

These symbols can be saved in "Stabs" format or the DWARF format. In the Stabs format, debugging symbols, like other symbols, are stored in the regular symbol table. In the DWARF format, debugging symbols are stored in a special "__DWARF" segment within the binary. DWARF debugging symbols can also be saved as a separate debug-information file. In this test case, you make sure that no debug symbols are contained in the release binary itself (in neither the symbol table nor the __DWARF segment).

Static Analysis

Use gobjdump to inspect the main binary and any included dylibs for Stabs and DWARF symbols.

$ gobjdump --stabs --dwarf TargetApp
In archive MyTargetApp:

armv5te:     file format mach-o-arm

aarch64:     file format mach-o-arm64

Gobjdump is part of binutils and can be installed on macOS via Homebrew.

Dynamic Analysis

Dynamic analysis is not applicable for finding debugging symbols.

Remediation

Make sure that debugging symbols are stripped when the application is being built for production. Stripping debugging symbols will reduce the size of the binary and increase the difficulty of reverse engineering. To strip debugging symbols, set Strip Debug Symbols During Copy to "YES" via the project's build settings.

A proper Crash Reporter System is possible because the system doesn't require any symbols in the application binary.

Finding Debugging Code and Verbose Error Logging

Overview

To speed up verification and get a better understanding of errors, developers often include debugging code, such as verbose logging statements (using NSLog, println, print, dump, and debugPrint) about responses from their APIs and about their application's progress and/or state. Furthermore, there may be debugging code for "management-functionality," which is used by developers to set the application's state or mock responses from an API. Reverse engineers can easily use this information to track what's happening with the application. Therefore, debugging code should be removed from the application's release version.

Static Analysis

You can take the following static analysis approach for the logging statements:

  1. Import the application's code into Xcode.
  2. Search the code for the following printing functions: NSLog, println, print, dump, debugPrint.
  3. When you find one of them, determine whether the developers used a wrapping function around the logging function for better mark up of the statements to be logged; if so, add that function to your search.
  4. For every result of steps 2 and 3, determine whether macros or debug-state related guards have been set to turn the logging off in the release build. Please note the change in how Objective-C can use preprocessor macros:
#ifdef DEBUG
    // Debug-only code
#endif

The procedure for enabling this behavior in Swift has changed: you need to either set environment variables in your scheme or set them as custom flags in the target's build settings. Please note that the following functions (which allow you to determine whether the app was built in the Swift 2.1. release-configuration) aren't recommended, as Xcode 8 and Swift 3 don't support these functions:

  • _isDebugAssertConfiguration
  • _isReleaseAssertConfiguration
  • _isFastAssertConfiguration.

Depending on the application's setup, there may be more logging functions. For example, when CocoaLumberjack is used, static analysis is a bit different.

For the "debug-management" code (which is built-in): inspect the storyboards to see whether there are any flows and/or view-controllers that provide functionality different from the functionality the application should support. This functionality can be anything from debug views to printed error messages, from custom stub-response configurations to logs written to files on the application's file system or a remote server.

Dynamic Analysis

Dynamic analysis should be executed on both a simulator and a device because developers sometimes use target-based functions (instead of functions based on a release/debug-mode) to execute the debugging code.

  1. Run the application on a simulator and check for output in the console during the app's execution.
  2. Attach a device to your Mac, run the application on the device via Xcode, and check for output in the console during the app's execution in the console.

For the other "manager-based" debug code: click through the application on both a simulator and a device to see if you can find any functionality that allows an app's profiles to be pre-set, allows the actual server to be selected or allows responses from the API to be selected.

Remediation

As a developer, incorporating debug statements into your application's debug version should not be a problem if you realize that the debugging statements should never

  1. be present in the application's release version or
  2. end up in the application's release configuration.

In Objective-C, developers can use preprocessor macros to filter out debug code:

#ifdef DEBUG
    // Debug-only code
#endif

In Swift 2 (with Xcode 7), you have to set custom compiler flags for every target, and compiler flags have to start with "-D." So you can use the following annotations when the debug flag DMSTG-DEBUG is set:

#if MSTG-DEBUG
    // Debug-only code
#endif

In Swift 3 (with Xcode 8), you can set Active Compilation Conditions in Build settings/Swift compiler - Custom flags. Instead of a preprocessor, Swift 3 uses conditional compilation blocks based on the defined conditions:

#if DEBUG_LOGGING
    // Debug-only code
#endif

Testing Exception Handling

Overview

Exceptions often occur after an application enters an abnormal or erroneous state. Testing exception handling is about making sure that the application will handle the exception and get into a safe state without exposing any sensitive information via its logging mechanisms or the UI.

Bear in mind that exception handling in Objective-C is quite different from exception handling in Swift. Bridging the two approaches in an application that is written in both legacy Objective-C code and Swift code can be problematic.

Exception handling in Objective-C

Objective-C has two types of errors:

NSException NSException is used to handle programming and low-level errors (e.g., division by 0 and out-of-bounds array access). An NSException can either be raised by raise or thrown with @throw. Unless caught, this exception will invoke the unhandled exception handler, with which you can log the statement (logging will halt the program). @catch allows you to recover from the exception if you're using a @try-@catch-block:

 @try {
 	//do work here
 }

@catch (NSException *e) {
	//recover from exception
}

@finally {
 	//cleanup

Bear in mind that using NSException comes with memory management pitfalls: you need to clean up allocations from the try block that are in the finally block. Note that you can promote NSException objects to NSError by instantiating an NSError in the @catch block.

NSError NSError is used for all other types of errors. Some Cocoa framework APIs provide errors as objects in their failure callback in case something goes wrong; those that don't provide them pass a pointer to an NSError object by reference. It is a good practice to provide a BOOL return type to the method that takes a pointer to an NSError object to indicate success or failure. If there's a return type, make sure to return "nil" for errors. If "NO" or "nil" is returned, it allows you to inspect the error/reason for failure.

Exception Handling in Swift

Exception handing in Swift (2 - 4) is quite different. The try-catch block is not there to handle NSException. The block is used to handle errors that conform to the Error (Swift 3) or ErrorType (Swift 2) protocol. This can be challenging when Objective-C and Swift code are combined in an application. Therefore, NSError is preferable to NSException for programs written in both languages. Furthermore, error-handling is opt-in in Objective-C, but throws must be explicitly handled in Swift. To convert error-throwing, look at the Apple documentation. Methods that can throw errors use the throws keyword. There are four ways to handle errors in Swift:

  • Propagate the error from a function to the code that calls that function. In this situation, there's no do-catch; there's only a throw throwing the actual error or a try to execute the method that throws. The method containing the try also requires the throws keyword:
func dosomething(argumentx:TypeX) throws {
	try functionThatThrows(argumentx: argumentx)
}
  • Handle the error with a do-catch statement. You can use the following pattern:
do {
    try functionThatThrows()
    defer {
    	//use this as your finally block as with Objective-c
    }
    statements
} catch pattern 1 {
    statements
} catch pattern 2 where condition {
    statements
}
  • Handle the error as an optional value:
	let x = try? functionThatThrows()
	//In this case the value of x is nil in case of an error.
  • Use the try! expression to assert that the error won't occur.

Static Analysis

Review the source code to understand how the application handles various types of errors (IPC communications, remote services invocation, etc.). The following sections list examples of what you should check for each language at this stage.

Static Analysis in Objective-C

Make sure that

  • the application uses a well-designed and unified scheme to handle exceptions and errors,
  • the Cocoa framework exceptions are handled correctly,
  • the allocated memory in the @try blocks is released in the @finally blocks,
  • for every @throw, the calling method has a proper @catch at the level of either the calling method or the NSApplication/UIApplication objects to clean up sensitive information and possibly recover,
  • the application doesn't expose sensitive information while handling errors in its UI or in its log statements, and the statements are verbose enough to explain the issue to the user,
  • high-risk applications' confidential information, such as keying material and authentication information, is always wiped during the execution of @finally blocks,
  • raise is rarely used (it's used when the program must be terminated without further warning),
  • NSError objects don't contain data that might leak sensitive information.
Static Analysis in Swift

Make sure that

  • the application uses a well-designed and unified scheme to handle errors,
  • the application doesn't expose sensitive information while handling errors in its UI or in its log statements, and the statements are verbose enough to explain the issue to the user,
  • high-risk applications' confidential information, such as keying material and authentication information, is always wiped during the execution of defer blocks,
  • try! is used only with proper guarding up front (to programmatically verify that the method that's called with try! can't throw an error).

Dynamic Testing

There are several dynamic analysis methods:

  • Enter unexpected values in the iOS application's UI fields.
  • Test the custom URL schemes, pasteboard, and other inter-app communication controls by providing unexpected or exception-raising values.
  • Tamper with the network communication and/or the files stored by the application.
  • For Objective-C, you can use Cycript to hook into methods and provide them arguments that may cause the callee to throw an exception.

In most cases, the application should not crash. Instead, it should

  • recover from the error or enter a state from which it can inform the user that it can't continue,
  • provide a message (which shouldn't leak sensitive information) to get the user to take appropriate action,
  • withhold information from the application's logging mechanisms.

Remediation

Developers can implement proper error handling in several ways:

  • Make sure that the application uses a well-designed and unified scheme to handle errors.
  • Make sure that all logging is removed or guarded as described in the test case "Testing for Debugging Code and Verbose Error Logging."
  • For a high-risk application written in Objective-C: create an exception handler that removes secrets that shouldn't be easily retrievable. The handler can be set via NSSetUncaughtExceptionHandler.
  • Refrain from using try! in Swift unless you're certain that there's no error in the throwing method that's being called.
  • Make sure that the Swift error doesn't propagate into too many intermediate methods.

Make Sure That Free Security Features Are Activated

Overview

Although Xcode enables all binary security features by default, it may be relevant to verify this for an old application or to check for the misconfiguration of compilation options. The following features are applicable:

  • ARC - Automatic Reference Counting - memory management feature
    • adds retain and release messages when required
  • Stack Canary - helps prevent buffer overflow attacks
  • PIE - Position Independent Executable - enables full ASLR for binary

Static Analysis

Xcode Project Settings
  • Stack-smashing protection

Steps for enabling Stack-smashing protection in an iOS application:

  1. In Xcode, select your target in the "Targets" section, then click the "Build Settings" tab to view the target's settings.

  2. Make sure that the "-fstack-protector-all" option is selected in the "Other C Flags" section.

  3. Make sure that Position Independent Executables (PIE) support is enabled.

Steps for building an iOS application as PIE:

  1. In Xcode, select your target in the "Targets" section, then click the "Build Settings" tab to view the target's settings.
  2. Set the iOS Deployment Target to iOS 4.3 or later.
  3. Make sure that "Generate Position-Dependent Code" is set to its default value ("NO").
  4. Make sure that "Don't Create Position Independent Executables" is set to its default value ("NO").
  • ARC protection

Steps for enabling ACR protection for an iOS application:

  1. In Xcode, select your target in the "Targets" section, then click the "Build Settings" tab to view the target's settings.
  2. Make sure that "Objective-C Automatic Reference Counting" is set to its default value ("YES").

See the Technical Q&A QA1788 Building a Position Independent Executable.

With otool

Below are procedures for checking the binary security features described above. All the features are enabled in these examples.

  • PIE:
$ 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
  • stack canary:
$ otool -Iv DamnVulnerableIOSApp | grep stack
0x0046040c 83177 ___stack_chk_fail
0x0046100c 83521 _sigaltstack
0x004fc010 83178 ___stack_chk_guard
0x004fe5c8 83177 ___stack_chk_fail
0x004fe8c8 83521 _sigaltstack
0x00000001004b3fd8 83077 ___stack_chk_fail
0x00000001004b4890 83414 _sigaltstack
0x0000000100590cf0 83078 ___stack_chk_guard
0x00000001005937f8 83077 ___stack_chk_fail
0x0000000100593dc8 83414 _sigaltstack
  • Automatic Reference Counting:
$ otool -Iv DamnVulnerableIOSApp | grep release
0x0045b7dc 83156 ___cxa_guard_release
0x0045fd5c 83414 _objc_autorelease
0x0045fd6c 83415 _objc_autoreleasePoolPop
0x0045fd7c 83416 _objc_autoreleasePoolPush
0x0045fd8c 83417 _objc_autoreleaseReturnValue
0x0045ff0c 83441 _objc_release
[SNIP]
With idb

IDB automates the processes of checking for stack canary and PIE support. Select the target binary in the IDB GUI and click the "Analyze Binary…" button.

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References

OWASP Mobile Top 10 2016

OWASP MASVS

  • V7.1: "The app is signed and provisioned with a valid certificate."
  • V7.4: "Debugging code has been removed, and the app does not log verbose errors or debugging messages."
  • V7.6: "The app catches and handles possible exceptions."
  • V7.7: "Error handling logic in security controls denies access by default."
  • V7.9: "Free security features offered by the toolchain, such as byte-code minification, stack protection, PIE support and automatic reference counting, are activated."

Tools

Checking for weaknesses in third party libraries

Overview

iOS applications often make use of third party libraries. These third party libraries accelerate development as the developer has to write less code in order to solve a problem. There are two categories of libraries:

  • Libraries that are not (or should not) be packed within the actual production application, such as OHHTTPStubs used for testing.
  • Libraries that are packed within the actual production application, such as Alomofire.

These libraries can have the following two classes of unwanted side-effects:

  • A library can contain a vulnerability, which will make the application vulnerable. A good example is AFNetworking version 2.5.1, which contained a bug that disabled certificate validation. This vulnerability would allow attackers to execute man-in-the-middle attacks against apps that are using the library to connect to their APIs.
  • A library can use a license, such as LGPL2.1, which requires the application author to provide access to the source code for those who use the application and request insight in its sources. In fact the application should then be allowed to be redistributed with modifications to its source code. This can endanger the intellectual property (IP) of the application.

Note: there are two widely used package management tools: Carthage and CocoaPods. Please note that this issue can hold on multiple levels: When you use webviews with JavaScript running in the webview. Then the JavaScript libraries can have these issues as well. The same holds for plugins/libraries for Cordova, React-native and Xamarin apps.

Static Analysis

Detecting vulnerabilities of third party libraries

In order to ensure that the libraries used by the apps are not carrying vulnerabilities, one can best check the dependencies installed by CocoaPods or Carthage.

In case CocoaPods is used for managing third party dependencies, the following steps can be taken to analyse the third party libraries for vulnerabilities:

  1. At the root of the project, where the Podfile is located, execute the following commands:
sudo gem install CocoaPods
pod install
  1. Now that the dependency tree has bene built, you can create an overview of the dependencies and their versions by running the following commands:
sudo gem install CocoaPods-dependencies
pod dependencies
  1. The result of the steps above can now be used as input for searching different vulnerability feeds for known vulnerabilities.

Note:

  1. If the developer packs all dependencies in terms of its own support library using a .podspec file, then this .podspec file can be checked with the experimental CocoaPods podspec checker.
  2. If the project uses CocaoPods in combination with Objective-C, SourceClear can be used.
  3. Using CocoaPods with http based links instead of https might allow for man-in-the-middle attacks during the download of the dependency, which might allow the attacker to replace (parts of) the library you download with other content. Therefore: always use https.

In case Carthage is used for third party dependencies, then the following steps can be taken to analyse the third party libraries for vulnerabilities:

  1. At the root of the project, where the Cartfile is located, type
brew install carthage
carthage update --platform iOS
  1. Check the Cartfile.resolved for actual versions used and inspect the given libraries for known vulnerabilities.

Note, at the time of writing of this chapter, there is no automated support for Carthage based dependency analysis known to the authors.

When a library is found to contain vulnerabilities, then the following reasoning applies:

  • Is the library packaged with the application? Then check whether the library has a version in which the vulnerability is patched. If not, check whether the vulnerability actually affects the application. If that is the case or might be the case in the future, then look for an alternative which provides similar functionality, but without the vulnerabilities.
  • Is the library not packaged with the application? See if there is a patched version in which the vulnerability is fixed. If this is not the case, check if the implications of the vulnerability for the build-proces. Could the vulnerability impede a build or weaken the security of the build-pipeline? Then try looking for an alternative in which the vulnerability is fixed.

Lastly, please note that for hybrid applications, one will have to check the JavaScript dependencies with RetireJS. Similarly for Xamarin, one will have to check the C# dependencies.

Detecting the licenses used by the libraries of the application

In order to ensure that the copyright laws are not infringed, one can best check the dependencies installed by CocoaPods or Carthage.

When the application sources are available and CocoaPods is used, then execute the following steps to get the different licenses:

  1. At the root of the project, where the Podfile is located, type
sudo gem install CocoaPods
pod install
  1. At the Pods folder you will find the libraries installed. Each in their own folder. Now you can check the licenses for each of the libraries by inspecting the license files in each of the folders.

When the application sources are available and Carthage is used, then execute the following steps to get the different licenses:

  1. At the root of the project, where the Cartfile is located, type
brew install carthage
carthage update --platform iOS
  1. The sources of each of the dependencies have been downloaded to Carthage/Checkouts folder in the project. Here you can find the license for each of the libraries in their respective folder.

When a library contains a license in which the app's IP needs to be open-sourced, check if there is an alternative for the library which can be used to provide similar functionalities.

Note: In case of a hybrid app, please check the build-tools used: most of them do have a license enumeration plugin to find the licenses being used.

Dynamic Analysis

The dynamic analysis of this section comprises of two parts: the actual license verification and checking which libraries are involved in case of missing sources.

It need to be validated whether the copyrights of the licenses have been adhered to. This often means that the application should have an about or EULA section in which the copy-right statements are noted as required by the license of the third party library.

When no source-code is available for library analysis, you can find some of the frameworks being used with otool and MobSF. After you obtain the library and Clutched it (e.g. removed the DRM), you can run oTool with at the root of the <Application.app> directory:

otool -L <Executable>

However, these do not include all the libraries being used. Next, with Class-dump (for Objective-C) you can generate a subset of the headerfiles used and derive which libraries are involved. But not detect the version of the library.

./class-dump <Executable> -r

References

OWASP Mobile Top 10 2016
OWASP MASVS
  • V7.5: "All third party components used by the mobile app, such as libraries and frameworks, are identified, and checked for known vulnerabilities."
CWE
  • CWE-937 - OWASP Top Ten 2013 Category A9 - Using Components with Known Vulnerabilities
Tools