Penetration Testing for iPhone Applications- Part 2

April 18, 2012 by Satish B.

In the first part of this article, we discussed the iPhone application traffic analysis. In this part, we will take a look at the privacy issues and the application local data storage.

Privacy issues

Every iPhone has an associated unique device Identifier derived from a set of hardware attributes called UDID. UDID is burned into the device and one cannot remove or change it. However, it can be spoofed with the help of tools like UDID Faker.

UDID of the latest iPhone is computed with the formula given below –
UDID = SHA1(Serial Number + ECID + LOWERCASE (WiFi Address) + LOWERCASE(Bluetooth Address))

UDID is exposed to application developers through an API which would allow them to access the UDID of an iPhone without requiring the device owner’s permission. The code snippet shown below is used to collect the UDID of a device, later which can used to track the user’s behavior.

NSString *uniqueIdentifier = [device uniqueIdentifier]

Current research shows that, with the help of UDID, it is possible to observe the user’s browsing patterns and trace out the user’s geo location. As it is possible to locate the user’s exact location with the help of a device UDID, it became a big privacy concern. More possible attacks are documented in Eric Smith-iPhone application privacy issues whitepaper. Eric’s research shows that 68% of applications silently send UDIDs to the servers on the internet. A perfect example of a serious privacy security breach is social gaming network Openfient.
Openfient collected device UDID’s and misused them by linking it to real world user identities (like email address, geo locations latitude & longitude, Facebook profile picture) and making them available for public access, resulting in a serious privacy breach. More details about this security breach are documented at this link.

While penetration testing, observe the network traffic for UDID transmission. UDID in the network traffic indicates that the application is collecting the device identifier or might be sending it to a third party analytic company to track the user’s behaviour. In iOS 5, Apple has deprecated the API that gives access to the UDID, and it will probably remove the API completely in future iOS releases. Development best practice is not to use the API that collects the device UDIDs, as it breaches the privacy of the user. If the developers want to keep track of the user’s behaviour, create a unique identifier specific to the application instead of using UDID. The disadvantage with the application specific identifier is that it only identifies an installation instance of the application, and it does not identify the device.

Apart from UDID, applications may transmit personal identifiable information like age, name, address and location details to third party analytic companies. Transmitting personal identifiable information to third party companies without the user’s knowledge also violates the user’s privacy. So, during penetration testing carefully observe the network traffic for the transmission of any important data.
Example: Pandora application was used to transmit user’s age and zip code to a third party analytic company (doubleclick.net) in clear text. For the applications which require the user’s geo location (ex: check-in services) to serve the content, it is always recommended to use the least degree of accuracy necessary. This can be achieved with the help of accuracy constants defined in core location framework (ex: CLLocationAccuracy kCLLocationAccuracyNearestTenMeters).

Local data storage

Mobile applications store the data locally on the device to maintain essential information across the application execution or for a better performance or offline access. Also, developers use the local device storage to store information such as user preferences and application configurations. As device theft is becoming an increasing concern, especially in the enterprise, insecure local storage is considered to be the top risk in mobile application threats. A recent survey conducted by Viaforensics revealed that 76 percent of mobile applications are storing user’s information on the device. 10 percent of them are even storing the plain text passwords on the phone.

Sensitive information stored on the iPhone can be obtained by attackers in several ways. A few of the ways are listed below –

  • From Backups
    When an iPhone is connected to iTunes, iTunes automatically takes a backup of everything on the device. Upon backup, sensitive files will also end up on the workstation. So an attacker who gets access to the workstation can read the sensitive information from the stored backup files.
  • Physical access to the device
    People lose their phones and phones get stolen very easily. In both cases, an attacker will get physical access to the device and read the sensitive information stored on the phone. The passcode set to the device will not protect the information as it is possible to brute force the iPhone simple passcode within 20 minutes. To know more details about iPhone passcode bypass go through the iPhone Forensics article available at – https://resources.infosecinstitute.com/iphone-forensics/.
  • Malware
    Leveraging a security weakness in iOS may allow an attacker to design a malware which can steal the files on the iPhone remotely. Practical attacks are demonstrated by Eric Monti in his presentation on iPhone Rootkit? There’s an App for That! .

iPhone application directory structure:

In iOS, applications are treated as a bundle represented within a directory. The bundle groups all the application resources, binaries and other related files into a directory. In iPhone, applications are executed within a jailed environment (sandbox or seatbelt) with mobile user privileges. Unlike Android UID based segregation, iOS applications runs as one user. Apple says The sandbox is a set of fine-grained controls limiting an application’s access to files, preferences, network resources, hardware, and so on. Each application has access to the contents of its own sandbox but cannot access other applications’ sandboxes. When an application is first installed on a device, the system creates the application’s home directory, sets up some key subdirectories, and sets up the security privileges for the sandbox. A sandbox is a restricted environment that prevents applications from accessing unauthorized resources; however, upon iPhone JailBreak, sandbox protection gets disabled.

When an application is installed on the iPhone, it creates a directory with a unique identifier under /var/mobile/Applications directory. Everything that is required for an application to execute will be contained in the created home directory. Typical iPhone application home directory structure is listed below.

In iPhone, applications might store the information in any of the locations listed below.

  • Plist files
  • Keychain
  • Application’s home directory
  • Cache
  • Logs

Plist files:

Property List is a structured binary formatted file which contains the essential configuration of a bundle executable in nested key value pairs. Plist files are used to store the user preferences and the configuration information of an application. For example, Gaming applications usually store game levels and game scores in the Plist files. In general, applications store the Plist files under [Application’s Home Directory]/documents/preferences folder. Plist can either be in XML format or in binary format. As XML files are not the most efficient means of storage, most of the applications use binary formatted Plist files. Binary formatted data stored in the Plist files can be easily viewed or modified using Plist editors (ex: plutil). Plist editors convert the binary formatted data into an XML formatted data, later it can be edited easily. Plist files are primarily designed to store the user preferences & application configuration; however, the applications may use Plist files to store clear text usernames, passwords and session related information. So, while penetration testing, view all the Plist files available under application’s home directory and look for sensitive information, like usernames, passwords, user’s personal information and session cookies, etc… Developers can assign any extension to the Plist files. A Plist file can be easily identified by looking at the file contents using cat command. The content of a Plist file starts with bplist’.

Along with the sensitive information storage, application may also take authentication & authorization decisions based on the values stored in Plist files. For example, if you notice a Plist entry like admin=0 during penetration testing, change the admin key value to 1 and open the application. If the application does not validate the user input properly and takes the authorization decision based on the Plist entry, you may log into the application as an administrator. Development best practice is to not store any sensitive information in Plist files. Also, do not take authentication & authorization decisions based on the information stored in Plist files. Plist files contain user controlled input, and it should be validated properly like any other user input.

WordPress iPhone application used to store clear text username and password in a Plist file. The video below here demonstrates the WordPress vulnerability. This vulnerability was reported by SANS and WordPress fixed it immediately.

Plist files can be viewed and modified easily on both the JailBroken and non JailBroken iPhones. The examples listed below illustrate the various ways of editing Plist files on the both JailBroken and non JailBroken devices.

Tampering Plist files on a non JailBroken iPhone:

On a non JailBroken iPhone, Plist files can be viewed & modified using tools like iExplorer and iBackupBot.

Modifying Plist entries with iExplorer

iExplorer (formerly iPhone Explorer) gives access to the iPhone in disk mode and allows browsing all the folders on the iPhone directly.

Stick Cricket iPhone game is used for the demo.

Stick Cricket iPhone game stores the game score in a Plist file under application’s home directory. As the application is storing the game score locally in a Plist file, it can be altered by editing the Plist file.

Screenshot shown below displays the actual score before the Plist modification.

Steps shown below will demonstrate the usage of iExplorer tool to modify the game scores stored in the Plist file –

1. On your workstation download and install iExplorer.
2. Connect the iPhone to the workstation over USB.
3. In iExplorer, browse to Apps->com.sticksports.stickcricket folder.

4. Navigate to stick cricket Library->Preferences folder.

5. Copy com.sticksports.stickcricket.Plist file to the workstation by dragging it to the desktop.
6. On the workstation, open the Plist file using a Plist editor and modify the yourBest5Overs key value.

For this demo, I have modified the value to 180 from 30 and saved the Plist file.

7. From iExplorer, delete the com.sticksports.stickcricket.Plist on the iPhone and drag the newly saved file onto the iPhone.
8. In iPhone, terminate the Stick Cricket application and reopen it. The Stick Cricket welcome screen now displays the modified score as shown in the screenshot below.

Modifying Plist entries with iBackupBot
When the iPhone is connected to a computer, iTunes takes a backup of everything on the phone including configuration files (Plist files). iBackupBot tool can be used to view and modify the Plist file entries on the iPhone backup and restore the modified backup onto the iPhone..

Steps shown below will demonstrate the use of iBackupBot tool to modify the game scores stored in the Plist file –

      1. Connect the iPhone to the workstation over USB cable.
      2. On Workstation, open iTunes and take a backup of the iPhone.
      3. Close iTunes.
      4. Open iBackupBot. It automatically identifies the existing backups and displays the files inside the backup to the user.
      5. Click on Stick Cricket and open /Library/Preferences/com.sticksports.stickcricket.Plist file.

      1. Modify the score stored in the Plist file.
      2. Click on Export icon to save the modified Plist file.
      3. Click the restore icon in iBackupBot toolbar. It will restore the iPhone with the modified backup. Now on iPhone, reopening the Stick Cricket game will display the modified score.

Tampering Plist files on a JailBroken iPhone:

On a JailBroken iPhone, Plist files can be viewed & modified using tools like plutil and iFile. Both these tools can be downloaded from Cydia (packages – com.eric.tool & iFile). iFile would allow to modify the Plist files directly on the iPhone.

The iPhone camera application is used for the demo. In iOS camera application, Apple has hidden the panorama mode feature and planned to include this feature in future iOS versions. Panorama mode basically allows the users to take continuous photos while panning the camera from left to right. Apple stored the panorama mode switch in a Plist file. iOS hackers Conard & Chpwn exposed the panorama mode in iOS 5 by modifying an entry in com.apple.mobileslideshow.Plist file.

Screenshot shown below displays the list of options available in the iPhone camera application.

Steps shown below will demonstrate the usage of plutil tool to change the panorama switch stored in the Plist file –

      1. SSH to the iPhone and login as a root user (password: alpine).
      2. Navigate to /private/var/mobile/Library/Preferences/ directory.
      3. View com.apple.mobileslideshow.Plist file content with the help of plutil tool.
> Plutil com.apple.mobileslideshow.plist

      1. Add ‘EnableFirebreak’ key to the com.apple.mobileslideshow.plist file with the below command.
> Plutil –key EnableFirebreak –value yes com.apple.mobileslideshow.plist

      1. It turns on the panorama feature in the iPhone camera application.Screenshot below shows different options available in the iPhone camera after the modification-

Penetration Testing for iPhone Applications is going to be covered in a series of articles. Part 3 will cover the keychain data storage and error log analysis.

Posted: April 18, 2012
Satish B.
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Satish B (@satishb3) is an Information Security Professional with 6 years of experience in penetration testing of web applications and mobile applications. He is currently a security researcher at Infosec Institute. Satish's blog is located at - http://www.securitylearn.net Email: satishb3@securitylearn.net

5 responses to “Penetration Testing for iPhone Applications- Part 2”

  1. I like your post very much about iPhone. The information in this post is very interesting and more useful for the developers. Thanks for share this valuable info.

  2. Arvind says:

    Thanks Satish. Its a really good series of articles for someone who is new to IPhone testing. Its easy to read, there are screenshots provided when needed and no complex jargon used anywhere. Keep writing.

  3. Ishan Girdhar says:

    The only blog post I could found on iPhone Application Security. Good Read Very Practical, though I would prefer to read more about testing on Simulators as well. I know, It’s restricted and we can not perform much things like we can perform on real device but still it would be good to know what all we could do on simulators as well.

  4. satish b says:

    Ishan, currently I am focusing on live devices.
    May be I will write on simulator in future.

  5. shyamkumar says:

    very clear explanation.
    looking forward for more tutorials

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