Penetration Testing for iPhone Applications – Part 3
In the first part of this article, we have discussed the iPhone application traffic analysis. The second part of the article covered the privacy issues and property list data storage. In this part, we will make an in-depth analysis of the keychain data storage. Apple has designed the keychain with many security measures in place to protect the user’s data, however it is broken at every level. So complete understanding of the keychain and its security & weaknesses will help penetration testers to provide proper remediation suggestions during iOS application security assessments.
Local Data 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. In general, iOS applications store the user’s credentials in the keychain to provide transparent authentication and to not prompt the user every time for login. 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 (shown in Figure 1). 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.
Example: genp table columns shown in Figure 2 are mapped to Generic password keychain item class attributes as shown in Table 1.
|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|
|scrp||kSecAttrScriptCode||String script code (such as encoding type)|
|labl||kSecAttrLabel||Label to be displayed to the user (print name)|
|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|
|Actual data (such as password, crypto key…)|
|agrp||kSecAttrAccessGroup||Keychain access group|
|pdmn||kSecAttrAccessible||Access restrictions (Data protection classes)|
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)|
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.
sed -n ‘//,//p’ [AppDirectory]/[ApplicationBinary]
Example: Below command list out the entitlements of Facebook iOS application.
> sed -n ‘//,//p’ Facebook.app/Facebook application-identifier T84QZS65DQ.com.facebook.Facebook aps-environment production keychain-access-groups T84QZS65DQ.platformFamily
* 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 indicates 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.
*The above screen shot is captured by copying the keychain-2.db file from a JailBroken iPhone to a windows machine and opening it with a Sqlite browser.
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 –
- 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.
- 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.
- 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.
- Keychain item is accessible only after the device is unlocked, and the item cannot be migrated between devices.
- Keychain item is accessible after the first unlock of the device and the item cannot be migrated between devices.
- 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|
Sample keychain-2.db file is shown in the Figure 4 and pdmn column is highlighted.
*The above screen shot is captured by copying the keychain-2.db file from a JailBroken iPhone to an OS X machine and opening it with a SQLite browser.
Third party applications usually store the plain text credentials in the keychain to not prompt the user every time for login and to preserve the data across re-installation or up-gradation of the application. So while penetration testing, we have to look at the keychain items to see what kind of information is being stored by the applications in the keychain. But the keychain service does not allow viewing the keychain items of any application without proper entitlements. On a JailBroken device this restriction can be broken and it is possible to dump all the keychain items by signing an application with one of the below entitlements files.
- An entitlements file which has all the keychain access groups
- Fetch all the keychain access groups from the keychain by running a SQL query: select agrp from genp and place them in the entitlements file.
- An entitlements file with wild card ‘*’ keychain access group
- An entitlements file with com.apple.keystore.access-keychain-keys keychain access group
Link identity editor – ldid (Cydia Package) can be used to the sign the application with the entitlement file: ldid –Sentitlements.xml [app-name]
Patrick Toomey has developed a keychain_dumper tool that can be used to dump all the keychain items on a JailBroken iOS device. Recent version of the keychain_dumper uses the wildcard ‘*’ keychain access group entitlement to access all the keychain items. Steps listed below explain the usage of keychain dumper tool on the iPhone.Keychain Dumper
1. Jailbreak the iPhone.
2. Install openssh from Cydia.
3. On Mac OS X, download keychain_dumper and cyberduck tools.
4. Connect the iPhone and the computer to the same WI-FI network.
5. On Mac OS X, run Cyberduck and connect to the iPhone by typing the iPhone IP address, username as root and password as alpine (shown in Figure 5).
6. Copy keychain_dumper executable to the iPhone root directory (Shown in Figure 6).
7. Run the terminal and connect to the iPhone over SSH by typing the iPhone IP address, username as root and password as alpine.
8. On SSH terminal, type chmod 777 keychain_dumper and ./keychain_dumper commands. It will execute the keychain_dumper tool and displays all the keychain items from genp & inet tables (Shown in Figure 7).
9. ./keychain_dumper –c command dumps the keychain items from cert & keys tables (Shown in Figure 8).
Note: keychain-dump & keychain viewer tools can also be used to dump all the keychain items. The tools are developed by Jean Sigwald and they use the com.apple.keystore.access-keychain-keys keychain group entitlement to dump all the keychain items.
Third party applications also set data protection accessibility constants while adding the keychain items to make it available only after unlock. Data protection is designed to protect the data at rest by adding another layer of encryption. Data protection is tied to the user passcode and it secures the data only when a user sets a passcode as illustrated in Figure 9.
Upon obtaining physical access to the device which is not protected with a passcode, an attacker can read all the keychain items by JailBreaking the device and running tools like keychain dumper. So even if third party applications use data protection API, data at rest is not protected without a user’s passcode and leaves the security to the end user.
Even if a user sets a simple passcode for the device, an attacker can still read all the keychain items by brute forcing the user’s passcode. A simple 4 digit passcode can be brute forced in less than 20 minutes by booting the device with a custom ram disk. More details about the iPhone passcode brute forcing are documented at iPhone Forensics article. Data protection is useful only when a user sets a complex passcode (minimum of 6 alpha numeric characters in length).
Note: Boot Rom level vulnerability is required to boot the device with a custom ram disk. At the time of writing this article, Boot Rom level vulnerability was not identified in A5 chips which are used in the iPhone 4s & iPad 2. So currently it is not possible to perform a passcode brute force attack on the iPhone 4s and iPad 2.
Techniques explained above conclude that the keychain items are secure only if a user sets a complex passcode for the device and the keychain item is protected with an accessibility constant which makes it available only after unlock.
Though Apple has designed the iOS keychain with best security features in place, it is broken at every level. Techniques explained in the article demonstrate that all the keychain items can be dumped upon gaining physical access to the device. It is also quite possible to design a remote application (targeting JailBroken devices) which can read all the keychain items and transmit the data to a remote server.
In the recent versions of iOS (4 & 5), by default, the keychain items are stored using the kSecAttrAccessibleWhenUnlocked data protection accessibility constant. However the data protection is effective only with a device passcode, which implies that sensitive data stored in the keychain is secure only when a user sets a complex passcode for the device. But iOS applications cannot enforce the user to set a device passcode. So if iOS applications relyonly on the Apple provided security they can be broken if iOS security is broken.
iOS application security can be improved by understanding the shortcomings of the current implementation and writing one’s own implementation that works better. In the case of the keychain, iOS application security can be improved by using the custom encryption (using built-in crypto API) along with the data protection API while adding the keychain entries. If custom encryption is implemented it is recommended to not to store the encryption key on the device.
Penetration Testing For iPhone Applications is going to be covered in a series of articles. In the Part 4, we will take a look at the files stored in the application home directory, caching issues and error log analysis.
1. iPhone data protection tools
2. iOS Keychain weaknesses FAQs
3. Keychain services concepts
4. Ptoomey keychain dumper