iOS Code Quality and Build Settings
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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
You can retrieve the signing certificate information from the application's .app file with . 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 to display the signing information:
Debugging iOS applications can be done using Xcode, which embeds a powerful debugger called lldb. Lldb is the default debugger since Xcode5 where it replaced GNU tools like gdb and is fully integrated in the development environment. While debugging is a useful feature when developing an app, it has to be turned off before releasing apps to the App Store or within an enterprise program.
Generating an app in Build or Release mode depends on build settings in Xcode; when an app is generated in Debug mode, a DEBUG flag is inserted in the generated files.
At first you need to determine the mode in which your app is to be generated to check the flags in the environment:
Select the build settings of the project
Under 'Apple LVM - Preprocessing' and 'Preprocessor Macros', make sure 'DEBUG' or 'DEBUG_MODE' is not selected (Objective-C)
Make sure that the "Debug executable" option is not selected.
Or in the 'Swift Compiler - Custom Flags' section / 'Other Swift Flags', make sure the '-D DEBUG' entry does not exist.
Check whether you can attach a debugger directly, using Xcode. Next, check if you can debug the app on a jailbroken device after Clutching it. This is done using the debug-server which comes from the BigBoss repository at Cydia.
Note: if the application is equipped with anti-reverse engineering controls, then the debugger can be detected and stopped.
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 bytecode 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).
Use gobjdump to inspect the main binary and any included dylibs for Stabs and DWARF symbols.
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.
Dynamic analysis is not applicable for finding debugging symbols.
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.
You can take the following static analysis approach for the logging statements:
Import the application's code into Xcode.
Search the code for the following printing functions: NSLog, println, print, dump, debugPrint.
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.
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:
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.
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.
As a developer, incorporating debug statements into your application's debug version should not be a problem as long as you make sure that the debug statements are never present in the application's release version.
In Objective-C, developers can use preprocessor macros to filter out debug code:
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:
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.
Run the application on a simulator and check for output in the console during the app's execution.
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.
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.
iOS applications often make use of third party libraries which accelerate development as the developer has to write less code in order to solve a problem. However, third party libraries may contain vulnerabilities, incompatible licensing, or malicious content. Additionally, it is difficult for organizations and developers to manage application dependencies, including monitoring library releases and applying available security patches.
Carthage is open source and can be used for Swift and Objective-C packages. It is written in Swift, decentralized and uses the Cartfile file to document and manage project dependencies.
CocoaPods is open source and can be used for Swift and Objective-C packages. It is written in Ruby, utilizes a centralized package registry for public and private packages and uses the Podfile file to document and manage project dependencies.
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 Alamofire.
These libraries can lead to 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 no longer be maintained or hardly be used, which is why no vulnerabilities are reported and/or fixed. This can lead to having bad and/or vulnerable code in your application through the library.
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.
Please note that this issue can hold on multiple levels: When you use webviews with JavaScript running in the webview, the JavaScript libraries can have these issues as well. The same holds for plugins/libraries for Cordova, React-native and Xamarin apps.
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.
Swift Package Manager
First, at the root of the project, where the Package.swift file is located, type
Next, check the file Package.resolved for the actual versions used and inspect the given libraries for known vulnerabilities.
CocoaPods
First, at the root of the project, where the Podfile is located, execute the following commands:
Next, now that the dependency tree has been built, you can create an overview of the dependencies and their versions by running the following commands:
The result of the steps above can now be used as input for searching different vulnerability feeds for known vulnerabilities.
Note:
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.
If the project uses CocoaPods in combination with Objective-C, SourceClear can be used.
Using CocoaPods with HTTP-based links instead of HTTPS might allow for man-in-the-middle attacks during the download of the dependency, allowing an attacker to replace (parts of) the library with other content. Therefore, always use HTTPS.
Carthage
First, at the root of the project, where the Cartfile is located, type
Next, check the Cartfile.resolved for actual versions used and inspect the given libraries for known vulnerabilities.
Discovered library vulnerabilities
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 process. 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.
In case frameworks are added manually as linked libraries:
Open the xcodeproj file and check the project properties.
In the case of copy-pasted sources: search the header files (in case of using Objective-C) and otherwise the Swift files for known method names for known libraries.
Last, if the application is a high-risk application, you will end up vetting the library manually. In that case there are specific requirements for native code, which are similar to the requirements established by the MASVS for the application as a whole. Next to that, it is good to vet whether all best practices for software engineering are applied.
In order to ensure that the copyright laws are not infringed, one can best check the dependencies installed by Swift Packager Manager, CocoaPods, or Carthage.
Swift Package Manager
When the application sources are available and Swift Package Manager is used, execute the following code in the root directory of the project, where the Package.swift file is located:
The sources of each of the dependencies have now been downloaded to /.build/checkouts/ folder in the project. Here you can find the license for each of the libraries in their respective folder.
CocoaPods
When the application sources are available and CocoaPods is used, then execute the following steps to get the different licenses: First, at the root of the project, where the Podfile is located, type
This will create a Pods folder where all libraries are installed, each in their own folder. You can now check the licenses for each of the libraries by inspecting the license files in each of the folders.
Carthage
When the application sources are available and Carthage is used, execute the following code in the root directory of the project, where the Cartfile is located:
The sources of each of the dependencies have now been downloaded to Carthage/Checkouts folder in the project. Here you can find the license for each of the libraries in their respective folder.
Issues with library licenses
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.
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.
The list_bundles command lists all of the application’s bundles that are not related to Frameworks. The output contains executable name, bundle id, version of the library and path to the library.
The list_frameworks command lists all of the application’s bundles that represent Frameworks.
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.
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:
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:
Handle the error with a do-catch statement. You can use the following pattern:
Handle the error as an optional value:
Use the try! expression to assert that the error won't occur.
Handle the generic error as a Result return:
Handle network and JSON decoding errors with a Result type:
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.
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.
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).
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.
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.
iOS applications have various ways to run into memory corruption bugs: first there are the native code issues which have been mentioned in the general Memory Corruption Bugs section. Next, there are various unsafe operations with both Objective-C and Swift to actually wrap around native code which can create issues. Last, both Swift and Objective-C implementations can result in memory leaks due to retaining objects which are no longer in use.
Are there native code parts? If so: check for the given issues in the general memory corruption section. Native code is a little harder to spot when compiled. If you have the sources then you can see that C files use .c source files and .h header files and C++ uses .cpp files and .h files. This is a little different from the .swift and the .m source files for Swift and Objective-C. These files can be part of the sources, or part of third party libraries, registered as frameworks and imported through various tools, such as Carthage, the Swift Package Manager or Cocoapods.
For any managed code (Objective-C / Swift) in the project, check the following items:
The doubleFree issue: when free is called twice for a given region instead of once.
Retaining cycles: look for cyclic dependencies by means of strong references of components to one another which keep materials in memory.
Using instances of UnsafePointer can be managed wrongly, which will allow for various memory corruption issues.
Trying to manage the reference count to an object by Unmanaged manually, leading to wrong counter numbers and a too late/too soon release.
Please note that with Swift 5 you can only deallocate full blocks, which means the playground has changed a bit.
There are various tools provided which help to identify memory bugs within Xcode, such as the Debug Memory graph introduced in Xcode 8 and the Allocations and Leaks instrument in Xcode.
Next, you can check whether memory is freed too fast or too slow by enabling NSAutoreleaseFreedObjectCheckEnabled, NSZombieEnabled, NSDebugEnabled in Xcode while testing the application.
There are various well written explanations which can help with taking care of memory management. These can be found in the reference list of this chapter.
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 - A memory management feature that adds retain and release messages when required
Stack Canary - Helps prevent buffer overflow attacks by means of having a small integer right before the return pointer. A buffer overflow attack often overwrites a region of memory in order to overwrite the return pointer and take over the process-control. In that case, the canary gets overwritten as well. Therefore, the value of the canary is always checked to make sure it has not changed before a routine uses the return pointer on the stack.
PIE - Position Independent Executable - enables full ASLR for binary
Stack-smashing protection
Steps for enabling Stack-smashing protection in an iOS application:
In Xcode, select your target in the "Targets" section, then click the "Build Settings" tab to view the target's settings.
Make sure that the "-fstack-protector-all" option is selected in the "Other C Flags" section.
Make sure that Position Independent Executables (PIE) support is enabled.
Steps for building an iOS application as PIE:
In Xcode, select your target in the "Targets" section, then click the "Build Settings" tab to view the target's settings.
Set the iOS Deployment Target to iOS 4.3 or later.
Make sure that "Generate Position-Dependent Code" is set to its default value ("NO").
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:
In Xcode, select your target in the "Targets" section, then click the "Build Settings" tab to view the target's settings.
Make sure that "Objective-C Automatic Reference Counting" is set to its default value ("YES").
Below are procedures for checking the binary security features described above. All the features are enabled in these examples.
PIE:
stack canary:
Automatic Reference Counting:
Dynamic analysis is not applicable for finding security features offered by the toolchain.
MSTG-CODE-1: "The app is signed and provisioned with a valid certificate, of which the private key is properly protected."
MSTG-CODE-2: "The app has been built in release mode, with settings appropriate for a release build (e.g. non-debuggable)."
MSTG-CODE-3: "Debugging symbols have been removed from native binaries."
MSTG-CODE-4: "Debugging code and developer assistance code (e.g. test code, backdoors, hidden settings) have been removed. The app does not log verbose errors or debugging messages."
MSTG-CODE-5: "All third party components used by the mobile app, such as libraries and frameworks, are identified, and checked for known vulnerabilities."
MSTG-CODE-6: "The app catches and handles possible exceptions."
MSTG-CODE-8: "In unmanaged code, memory is allocated, freed and used securely."
MSTG-CODE-9: "Free security features offered by the toolchain, such as byte-code minification, stack protection, PIE support and automatic reference counting, are activated."
There are various ways to distribute your app as described at , which include using the App Store or via Apple Business Manager for custom or in-house distribution. In case of an in-house distribution scheme, make sure that no ad hoc certificates are used when the app is signed for distribution.
Gobjdump is part of and can be installed on macOS via Homebrew.
A proper is possible because the system doesn't require any symbols in the application binary.
Depending on the application's setup, there may be more logging functions. For example, when is used, static analysis is a bit different.
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 based on the defined conditions:
There are three widely used package management tools , , and :
The Swift Package Manager is open source, included with the Swift language, integrated into Xcode (since Xcode 11) and supports packages. It is written in Swift, decentralized and uses the Package.swift file to document and manage project dependencies.
In case is used for managing third party dependencies, the following steps can be taken to analyze the third party libraries for vulnerabilities:
You can utilize the 's experimental to identify the naming scheme of all dependencies and any corresponding entries. Scan the application's Package.swift file and generate a report of known vulnerable libraries with the following command:
In case is used for managing third party dependencies, the following steps can be taken to analyze the third party libraries for vulnerabilities.
You can utilize the 's experimental to identify the naming scheme of all dependencies and any corresponding entries. Scan the application's *.podspec and/or Podfile.lock files and generate a report of known vulnerable libraries with the following command:
In case is used for third party dependencies, then the following steps can be taken to analyze the third party libraries for vulnerabilities.
Note, at the time of writing this chapter, there is no automated support for Carthage based dependency analysis known to the authors. At least, this feature was already requested for the OWASP DependencyCheck tool but not yet implemented (see the ).
Go to the tab Build Phases and check the entries in Link Binary With Libraries for any of the libraries. See earlier sections on how to obtain similar information using .
Next, note that for hybrid applications, you will have to check the JavaScript dependencies with . Similarly for Xamarin, you will have to check the C# dependencies.
When performing app analysis, it is important to also analyze the app dependencies (usually in form of libraries or so-called iOS Frameworks) and ensure that they don't contain any vulnerabilities. Even when you don't have the source code, you can still identify some of the app dependencies using tools like , or otool. Objection is the recommended tool, since it provides the most accurate results and it is easy to use. It contains a module to work with iOS Bundles, which offers two commands: list_bundles and list_frameworks.
Bear in mind that using NSException comes with memory management pitfalls: you need to from the try block that are in the . 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 . 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 handing in Swift (2 - 5) 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 . Methods that can throw errors use the throws keyword. The Result type represents a success or failure, see , and . There are four ways to :
and is provided by Ray Wenderlich on this subject.
See the .
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.
Codesign -