Penetration Testing with iPhone Applications - Part 3

In third part we will make an in-depth analysis on the keychain data storage, understanding of the keychain and its security  and also its weakness will help penetration testers for further attacks.

Keychain Storage

Keychain is an encrypted container (128 bit AES algorithm) and a centralized SQLite database that holds identities & passwords for multiple applications and network services, with restricted access rights. On the iPhone, keychain SQLite database is used to store the small amounts of sensitive data like usernames, passwords, encryption keys, certificates and private keys.

iOS applications use the keychain service library/API (secItemAdd, secItemDelete, secItemCopyMatching & secItemUpdate methods) to read and write data to and from the keychain. Developers leverage the keychain services API to dictate the operating system to store sensitive data securely on their behalf, instead of storing them in a property list file or a plaintext configuration file. On the iPhone, the keychain SQLite database file is located at – /private/var/Keychains/keychain-2.db.

Keychain contains a number of keychain items and each keychain item will have encrypted data and a set of unencrypted attributes that describes it. Attributes associated with a keychain item depend on the keychain item class (kSecClass). In iOS, keychain items are classified into 5 classes – generic passwords (kSecClassGenericPassword), internet passwords (kSecClassInternetPassword), certificates (kSecClassCertificate), keys (kSecClassKey) and digital identities (kSecClassIdentity, identity=certificate + key). In the iOS keychain, all the keychain items are stored in 4 tables – genp, inet, cert and keys. Genp table contains generic password keychain items, inet table contains Internet password keychain items, and cert & keys tables contain certificates, keys and digital identity keychain items.

Columns in the keychain tables are mapped to the corresponding keychain item class attributes.
genp table columns shown in below image are mapped to Generic password keychain item class attributes as shown in Table 1.



(Figure 1)

column	Attribute	description
cdat	kSecAttrCreationDate	Item creation date in Unix epoch time format
mdat	kSecAttrModificationDate	Item modification date in Unix epoch time format
desc	kSecAttrDescription	User visible string that describes the item
icmt	kSecAttrComment	User editable comment for the item
crtr	kSecAttrCreator	Application created (4 char) code
type	kSecAttrType	Item type
scrp	kSecAttrScriptCode	String script code (such as encoding type)
labl	kSecAttrLabel	Label to be displayed to the user (print name)
alis	kSecAttrAlias	Item alias
invi	kSecAttrIsInvisible	Invisible
nega	kSecAttrIsNegative	Invalid item
cusi	kSecAttrHasCustomIcon	Existence of application specific icon (Boolean)
prot	kSecProtectedDataItemAttr ?	Item’s data is protected (Boolean)
acct	kSecAttrAccount	Account key (such as user id)
svce	kSecAttrService	Service name (such as Application identifier)
gena	kSecAttrGeneric	User defined attribute
data	kSecValueData	Actual data (such as password, crypto key…)
agrp	kSecAttrAccessGroup	Keychain access group
pdmn	kSecAttrAccessible	Access restrictions (Data protection classes)

Table 1



Attributes for all the keychain item classes are documented in the Keychain Item class keys and Values section in Apple’s documentation.The keychain database is encrypted with a hardware-specific key which is unique per the device. The hardware key cannot be extracted from the device, so the data stored in the keychain can only be accessible on the device itself and cannot be moved to another device. The keychain database is tied to the device, so even if an attacker obtains access to the keychain through physical access to the file system or in a remote attack, he cannot decrypt and view the file contents. The keychain file format is shown in Table 2.

Version

Protection class

len_wrapped key

AESWRAP (class key, item key, len_wrapped key)

AES256_GCM(item key, data)

Integrity tag(16 bytes)

(Table 2)



 Keychain data is logically zoned and data stored by one application is not accessible to another application. Keychain data of an iOS application is stored outside the application’s sandbox. So the operating system process securityd enforces the access control and regulates access to the keychain data in such a way that the applications with correct permissions can read their data. Keychain access permissions of an iOS application are defined in the code sign entitlements. Keychain Services uses these entitlements to grant permissions for the applications to access its own keychain items.

Entitlements of an application define the properties that provide access to the iOS features such as push notifications, keychain access and iCloud communication, etc… Entitlements grant specific capabilities or security permissions to iOS applications. An entitlement file for a keychain data sharing application contains an application-identifier and may contain a set of keychain-access-groups constants. In iOS, each application ships with a unique application-identifier. The keychain service restricts the keychain data access based on this application identifier. By default, applications can only access data associated with their own application-identifier.

Later, to share the keychain items with multiple applications, keychain-access-groups were introduced. Applications with the same keychain access group entitlement can access/share the keychain items. Entitlements of an application are embedded in the application binary and stored unencrypted. So, on a JailBroken iPhone, entitlements of an application can be extracted from the application binary using grep or sed commands (stream editor – sed can be downloaded from Cydia packages).

To list out the entitlements of an iOS application, connect to the iPhone over SSH, navigate to the application’s home directory (/var/mobile/Applications/[unique-id]/) and run the below command.

[CODE]

sed -n ‘//,/<\/dict>/p’  [AppDirectory]/[ApplicationBinary]
[/CODE]


For example below command will  list out the entitlements of Facebook iOS application.


[CODE]

> sed -n ‘//,/<\/dict>/p’  Facebook.app/Facebook         application-identifier         T84QZS65DQ.com.facebook.Facebook         aps-environment         production         keychain-access-groups                 T84QZS65DQ.platformFamily
[/CODE]

Facebook iOS application is a fat binary (built for ARM6 & ARM7 architectures), so the above command will print the entitlement details twice.

The above result shows that the Facebook iOS application uses ‘T84QZS65DQ.platformFamily’ keychain-access-group while storing the entries in the keychain.

When an application adds an entry to the keychain, an application identifier or keychain access group of the application also gets added to the keychain item agrp (access group) column. Later, when an application tries to access the keychain item, the keychain service verifies the application identifier or keychain access group against the agrp value of corresponding keychain item to permit the access. A sample keychain-2.db file is shown in Figure 3 and Facebook entitlements are highlighted.



(Figure2)


Note--> Applications that are built for the simulator use the same default keychain access group. So on the simulator, all the applications can access all the keychain items.

With the introduction of data protection mechanisms in iOS, sensitive data stored in the keychain item is protected with another layer of encryption which is tied to the user’s passcode. Data protection encryption keys (protection class keys) are derived from a device’s hardware key and a key generated from the user’s passcode. So encryption offered by data protection API is as good as the strength of a user’s passcode. Data protection is designed to protect the user’s data in case a device is lost or stolen. Data protection for the keychain items can be enabled by supplying an accessibility constant value to the kSecAttrAccessible attribute of SecItemAdd or SecItemUpdate methods.

Data protection accessibility constants determine when a keychain item should be readable by an application. They also determine whether a keychain item is allowed to migrate to other devices or not. During an iTunes backup, all the data stored in the iOS device is backed up to the computer including the keychain database. On the iTunes backups, the keychain SQLite database is stored as a Plist file (Keychain-backup.plist). Keychain items which are backed-up with the iTunes encrypted backup option can be moved/loaded to another device. However the keychain items which are protected with ThisDeviceOnly constants cannot be moved to other iOS devices.

Below is the list of keychain item accessibility constants –

• kSecAttrAccessibleWhenUnlocked
  • Keychain item is accessible only after the device is unlocked
  • Data protection class keys required to decrypt the keychain items are loaded into memory only when the device is unlocked and the encryption keys are automatically purged in 10 seconds once the device is locked.
• kSecAttrAccessibleAfterFirstUnlock
  • Keychain item is accessible only after the first unlock of the device until reboot
  • Data protection class keys required to decrypt the keychain items are loaded into memory only when the user unlocks the device after a reboot, and the keys remain in the memory til next reboot of the device.
• kSecAttrAccessibleAlways
  • Keychain item is accessible even when the device is locked
  • Data protection class keys required to decrypt the keychain items are always loaded into memory.
• kSecAttrAccessibleWhenUnlockedThisDeviceOnly
  • Keychain item is accessible only after the device is unlocked, and the item cannot be migrated between devices.
• kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly
  • Keychain item is accessible after the first unlock of the device and the item cannot be migrated between devices.
• kSecAttrAccessibleAlwaysThisDeviceOnly
  
  
  • Keychain item is accessible even when the device is locked and the item cannot be migrated between devices.

Data protection accessibility constants of a keychain item are mapped to pdmn (protection domain) column in the keychain tables – genp, inet… Mapping of the keychain data protection accessibility constants and pdmn values is shown in Table 3.

pdmn	Keychain accessibility constants
ak	kSecAttrAccessibleWhenUnlocked
ck	kSecAttrAccessibleAfterFirstUnlock
dk	kSecAttrAccessibleAlways
aku	kSecAttrAccessibleWhenUnlockedThisDeviceOnly
cku	kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly
dku	kSecAttrAccessibleAlwaysThisDeviceOnly

(Table 3)

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Penetration Testing with iPhone Applications - Part 2

Privacy Issues

Every iPhone has an associated unique device Identifier derived from a set of hardware attributes called UDID. It 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.

There's a  code snippet which is used to collect the UDID of a device, later which can used to track the user’s behavior.

Code: NSString *uniqueIdentifier = [device uniqueIdentifier]

It is known 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.

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.

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.

A few ways from which data stored in iPhone can be obtained by attacker's are:

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 20-30 minutes.

Malware

Leveraging a security weakness in iOS may allow an attacker to design a malware which can steal the files on the iPhone remotely.

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. Each and almost every application has access to the contents of its own sandbox but cannot access other applications’. 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, information could be stored  in any of the locations listed below :

   1. Plist files 
   2. Keychain

   3. Application’s home directory

   4. Cache

   5. Logs.

Property list files

Itis a structured binary formatted file which contains the essential configuration of a bundle executable in nested key value pairs. These 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 can be viewed and modified in both JailBroken and non JailBroken iPhones.Below are examples of both of them.

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 (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 example.

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 tellthe 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.

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 tell 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.

6.   Modify the score stored in the Plist file.

7.   Click on Export icon to save the modified Plist file.

8.   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 tell 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

4.   Add ‘EnableFirebreak’ key to the com.apple.mobileslideshow.plist file with the below command.

Plutil –key EnableFirebreak –value yes com.apple.mobileslideshow.plist

5.   It turns on the panorama feature in the iPhone camera application.

Screenshot below shows different options available in the iPhone camera after the modification

Hope you all enjoyed this tutorial and if you have any  problem or question then you may ask in comments.

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