Projects/OWASP Mobile Security Project - Top Ten Mobile Controls

OWASP/ENISA Collaboration

OWASP and the European Network and Information Security Agency (ENISA) collaborated to build a joint set of controls. ENISA has published the results of the collaborative effort as the "Smartphone Secure Development Guideline": http://www.enisa.europa.eu/activities/application-security/smartphone-security-1/smartphone-secure-development-guidelines

Contributors

This document has been jointly produced with ENISA as well as the following individuals:

• Vinay Bansal, Cisco Systems
• Nader Henein, Research in Motion
• Giles Hogben, ENISA
• Karsten Nohl, Srlabs
• Jack Mannino, nVisium Security
• Christian Papathanasiou, Royal Bank of Scotland
• Stefan Rueping, Infineon
• Beau Woods, Dell Secureworks

Top 10 mobile controls and design principles

1. Identify and protect sensitive data on the mobile device

Risks: Unsafe sensitive data storage, attacks on decommissioned phones unintentional disclosure: Mobile devices (being mobile) have a higher risk of loss or theft. Adequate protection should be built in to minimize the loss of sensitive data on the device.

• 1.1 In the design phase, classify data storage according to sensitivity and apply controls accordingly (e.g. passwords, personal data, location, error logs, etc.). Process, store and use data according to its classification. Validate the security of API calls applied to sensitive data.
• 1.2 Store sensitive data on the server instead of the client-end device. This is based on the assumption that secure network connectivity is sufficiently available and that protection mechanisms available to server side storage are superior. The relative security of client vs server-side security also needs to be assessed on a case-by-case basis (see ENISA cloud risk assessment (3) or the OWASP Cloud top 10 (4) for decision support).
• 1.3 When storing data on the device, use a file encryption API provided by the OS or other trusted source. Some platforms provide file encryption APIs which use a secret key protected by the device unlock code and deleteable on remote kill. If this is available, it should be used as it increases the security of the encryption without creating extra burden on the end-user. It also makes stored data safer in the case of loss or theft. However, it should be born in mind that even when protected by the device unlock key, if data is stored on the device, its security is dependent on the security of the device unlock code if remote deletion of the key is for any reason not possible.
• 1.4 Do not store/cache sensitive data (including keys) unless they are encrypted and if possible stored in a tamper-proof area (see control 2).
• 1.5 Consider restricting access to sensitive data based on contextual information such as location (e.g. wallet app not usable if GPS data shows phone is outside Europe, car key not usable unless within 100m of car etc...).
• 1.6 Do not store historical GPS/tracking or other sensitive information on the device beyond the period required by the application (see controls 1.7, 1.8).
• 1.7 Assume that shared storage is untrusted - information may easily leak in unexpected ways through any shared storage. In particular:
  • Be aware of caches and temporary storage as a possible leakage channel, when shared with other apps.
  • Be aware of public shared storage such as address book, media gallery and audio files as a possible leakage channel. For example storing images with location metadata in the media-gallery allows that information to be shared in unintended ways.
  • Do not store temp/cached data in a world readable directory.
• 1.8 For sensitive personal data, deletion should be scheduled according to a maximum retention period, (to prevent e.g. data remaining in caches indefinitely).
• 1.9 There is currently no standard secure deletion procedure for flash memory (unless wiping the entire medium/card). Therefore data encryption and secure key management are especially important.
• 1.10 Consider the security of the whole data lifecycle in writing your application (collection over the wire, temporary storage, caching, backup, deletion etc)
• 1.11 Apply the principle of minimal disclosure - only collect and disclose data which is required for business use of the application. Identify in the design phase what data is needed, its sensitivity and whether it is appropriate to collect, store and use each data type.
• 1.12 Use non-persistent identifiers which are not shared with other apps wherever possible - e.g. do not use the device ID number as an identifier unless there is a good reason to do so (use a randomly generated number – see 4.3). Apply the same data minimization principles to app sessions as to http sessions/cookies etc.
• 1.13 Applications on managed devices should make use of remote wipe and kill switch APIs to remove sensitive information from the device in the event of theft or loss. (A kill-switch is the term used for an OS-level or purpose-built means of remotely removing applications and/or data).
• 1.14 Application developers may want to incorporate an application-specific "data kill switch" into their products, to allow the per-app deletion of their application's sensitive data when needed (strong authentication is required to protect misuse of such a feature).

2. Handle password credentials securely on the device

Risks: Spyware, surveillance, financial malware. A user's credentials, if stolen, not only provide unauthorized access to the mobile backend service, they also potentially compromise many other services and accounts used by the user. The risk is increased by the widespread of reuse of passwords across different services.

• 2.1 Instead of passwords consider using longer term authorization tokens that can be securely stored on the device (as per the OAuth model). Encrypt the tokens in transit (using SSL/TLS). Tokens can be issued by the backend service after verifying

Smartphones secure development guidelines for app developers the user credentials initially. The tokens should be time bounded to the specific service as well as revocable (if possible server side), thereby minimizing the damage in loss scenarios. Use the latest versions of the authorization standards (such as OAuth 2.0). Make sure that these tokens expire as frequently as practicable.

• 2.2 In case passwords need to be stored on the device, leverage the encryption and key-store mechanisms provided by the mobile OS to securely store passwords, password equivalents and authorization tokens. Never store passwords in clear text. Do not store passwords or long term session IDs without appropriate hashing or encryption.
• 2.3 Some devices and add-ons allow developers to use a Secure Element e.g. (5) (6) – sometimes via an SD card module - the number of devices offering this functionality is likely to increase. Developers should make use of such capabilities to store keys, credentials and other sensitive data. The use of such secure elements gives a higher level of assurance with the standard encrypted SD card certified at FIPS 140-2 Level 3. Using the SD cards as a second factor of authentication though possible, isn't recommended, however, as it becomes a pseudo-inseparable part of the device once inserted and secured.
• 2.4 Provide the ability for the mobile user to change passwords on the device.
• 2.5 Passwords and credentials should only be included as part of regular backups in encrypted or hashed form.
• 2.6 Smartphones offer the possibility of using visual passwords which allow users to memorize passwords with higher entropy. These should only be used however, if sufficient entropy can be ensured. (7)
• 2.7 Swipe-based visual passwords are vulnerable to smudge-attacks (using grease deposits on the touch screen to guess the password). Measures such as allowing repeated patterns should be introduced to foil smudge-attacks. (8)
• 2.8 Check the entropy of all passwords, including visual ones (see 4.1 below).
• 2.9 Ensure passwords and keys are not visible in cache or logs.
• 2.10 Do not store any passwords or secrets in the application binary. Do not use a generic shared secret for integration with the backend (like password embedded in code). Mobile application binaries can be easily downloaded and reverse engineered.

3. Ensure sensitive data is protected in transit

Risks: Network spoofing attacks, surveillance. The majority of smartphones are capable of using multiple network mechanisms including Wi-Fi, provider network (3G, GSM, CDMA and others), Bluetooth etc. Sensitive data passing through insecure channels could be intercepted. (9) (10)

• 3.1 Assume that the provider network layer is not secure. Modern network layer attacks can decrypt provider network encryption, and there is no guarantee that the Wi-Fi network will be appropriately encrypted.
• 3.2 Applications should enforce the use of an end-to-end secure channel (such as SSL/TLS) when sending sensitive information over the wire/air (e.g. using Strict Transport Security - STS (11)).This includes passing user credentials, or other authentication equivalents. This provides confidentiality and integrity protection.
• 3.3 Use strong and well-known encryption algorithms (e.g. AES) and appropriate key lengths (check current recommendations for the algorithm you use e.g. (12) page 53).
• 3.4 Use certificates signed by trusted CA providers. Be very cautious in allowing self- signed certificates. Do not disable or ignore SSL chain validation.
• 3.5 For sensitive data, to reduce the risk of man-in-middle attacks (like SSL proxy, SSL strip), a secure connection should only be established after verifying the identity of the remote end-point (server). This can be achieved by ensuring that SSL is only established with end-points having the trusted certificates in the key chain.
• 3.6 The user interface should make it as easy as possible for the user to find out if a certificate is valid.
• 3.7 SMS, MMS or notifications should not be used to send sensitive data to or from mobile end-points.

Reference: Google vulnerability of Client Login account credentials on unprotected wifi - [1]

4. Implement user authentication/authorization and session management correctly

Risks: Unauthorized individuals may obtain access to sensitive data or systems. This can be done by circumventing authentication systems (logins) or by reusing valid tokens or cookies.

• 4.1 Require appropriate strength user authentication to the application. It may be useful to provide feedback on the strength of the password when it is being entered for the first time. The strength of the authentication mechanism used depends on the data being processed by the application and its access to valuable resources (e.g. costing money).
• 4.2 It is important to ensure that the session management is done correctly after the initial authentication. Require authentication credentials or tokens to be passed with any subsequent request (especially those granting privileged access or modification).
• 4.3 Use high entropy (unpredictable) session identifiers with .
• 4.4 Use context to add security to authentication - e.g. device ID, IP location, etc...
• 4.5 Consider using additional authentication factors for applications giving access to sensitive data or interfaces where possible - e.g. voice, fingerprint (if available), who-you-know, behavioural etc...
• 4.6 Use authentication that ties back to the end user identity (rather than the device identity).

Reference: Google's ClientLogin implementation [2]

5. Keep the backend APIs (services) and the platform (server) secure

Risks: Attacks on backend systems, loss of data via cloud storage. Majority of the mobile applications interact with the backend APIs using REST/Web Services or other proprietary protocols. Insecure implementation of backend APIs or services, and not keeping the back-end platform hardened/patched will allow bad guys to directly attack/compromise the back-ends.

• 5.1 Carry out a specific check of all data transferred betwen the mobile device and web-server backends and other external interfaces - (e.g. is location or other information transferred within file metadata)
• 5.2 All back-end services (WebServices/REST) for mobile apps should be tested for vulnerabilities periodically e.g. using static code analyzer tools and fuzzing tools for testing and finding security flaws.
• 5.3 Ensure that the back-end platform (server) is running with a hardened configuration with the latest security patches applied to the OS, Web Server and other application components.
• 5.4 Ensure adequate logs are retained on the back-end in order to detect and respond to incidents and perform forensics (within the limits of data protection law).
• 5.5 Employ rate limiting and throttling on a per-user/IP basis (if user identification is available) to reduce the risk from DDoS attack.
• 5.6 Test for DoS vulnerabilities where the server may become overwhelmed by certain resource intensive application calls.
• 5.7 Web Services, REST and APIs can have similar vulnerabilities to Web Applications. Perform testing of the backend Web Service, REST or API to determine vulnerabilities may exist. Perform abuse case testing, in addition to use case testing.

Reference: [3] [4]

6. Perform data integration with third party services/applications securely

Risks: Data Leakage

• 6.1 Vet the security/authenticity of any third party code/libraries used in your mobile application (reliable source, supported, no backend Trojans, licensing)
• 6.2 Track all third party frameworks/APIs used in the mobile application for security patches. A corresponding security update must be done for the mobile applications using these third party APIs/frameworks.
• 6.3 Pay particular attention to validating all data received from and sent to non-trusted third party apps (e.g. check ad network software) before processing in the application.

7. Pay specific attention to the collection and storage of consent for the collection and use of the user’s data

Risks: Unintentional disclosure of personal or private information. In the European Union, it is mandatory to obtain user consent for the collection of personally identifiable information (PII).

• 7.1 Create a privacy policy covering the usage of personal data and make it available to the user especially when making consent choices.
• 7.2 Consent may be collected in 3 main ways:
  • At install time
  • At run-time when data is sent
  • Via “opt-out” mechanisms where a default setting is implemented and the user has to turn it off.
• 7.3 Check whether your application is collecting PII - it may not always be obvious - for example do you use persistent identifiers linked to central data stores containing personal information?
• 7.4 Audit communication mechanisms to check for unintended leaks (e.g. image metadata).
• 7.5 Keep a record of consent to the transfer of PII available to the user (consider also the value of keeping server-side records attached to any user data stored).
• 7.6 Check whether your consent collection-mechanism overlaps or conflicts with any other consent-collection within the same stack (e.g. APP-native + webkit HTML)

Reference: EU Law [5]

8. Implement controls to prevent unauthorised access to paid-for resources (wallet, SMS, phone calls etc...) Risks: Smartphone apps give programmatic access to premium rate phone calls, SMS, roaming data, NFC payments etc. Apps with privileged access to such API’s should take particular care to prevent abuse given the financial impact of vulnerabilities giving attackers access to the user’s financial resources.

• 8.1 Maintain logs of access to paid resources in a non-repudiable format and make them available to the end-user for monitoring (e.g. signed receipt sent to server back-end). Logs should be protected from unauthorised access.
• 8.2 Check for anomalous usage patterns in paid resource usage and require reauthentication. E.g. when significant change in location occurs, user-language changes etc.
• 8.3 Consider using a white-list model by default for paid resource addressing - e.g. address book only unless specifically authorised for phone calls.
• 8.4 Authenticate all API calls to paid resources (e.g. using an app developer certificate).
• 8.5 Ensure that wallet API callbacks do not pass cleartext account/pricing/ billing/item information.
• 8.6 Warn user and obtain consent for any cost implications for app behaviour.
• 8.7 Minimise data transfers using techniques such as fast dormancy (3GPP), caching etc... to minimise signalling load on base stations.

Reference: Google Wallet Security [6]

9. Ensure secure distribution/provisioning of mobile applications

Risks:

• 9.1 Applications must be designed and provisioned to allow updates for security patches, taking into account the requirements for approval by app-stores and the extra delay this may imply.
• 9.2 Provide remote kill functionality
• 9.3 Feedback channels
• 9.4 Code signing for some mobile platforms implies inherent trust between applications (with same code signatures), installed on the same mobile device. Plan code signing mechanisms properly. //Needs elaboration.

10. Carefully check any runtime interpretation of code for errors

Risks: Runtime interpretation of code covers any opportunity an app gives for untrusted parties to provide unverified text or binary which is interpreted as code. For example, extra levels in a game, scripts, interpreted SMS headers. This gives an opportunity for malware to circumvent walled garden controls. Injection attacks leading to Data leakage, Surveillance, Spyware, Diallerware

• 10.1 Minimise runtime interpretation and capabilities offered to runtime interpreters.
• 10.2 Run interpreters at minimal privilege levels.
• 10.3 Fuzz test interpreters.
• 10.4 Note that it is not always obvious that your code is interpreting text. Look for any capabilities accessible via user-input data and use of third party API’s - e.g. javascript interpreters.
• 10.5 Define a comprehensive escape syntax as appropriate.

Candidates (for consideration)

A: Some general coding best practices are particularly relevant to mobile coding too

• Input Validation and Output Encoding
• Minimise lines of code.
• Use safe languages (e.g. from buffer-overflow).
• Implement a security report handling point (address) [email protected]
• Use static and binary code analyzers to find security flaws.
• Use safe string functions, avoid buffer and Integer overflow.
• Run with the minimum privilege required for the application on the operating system. Be aware of privileges granted by default by API's and disable them.
• Don't authorize code/app to execute with root/sa privilege
• Always perform testing as a standard as well as a privileged user
• Avoid opening application specific server sockets (listener ports) on the client device. Use the communication mechanisms provided by the OS.
• Context aware security: may be able to decrease/increase access based on the context (e.g. location, network) -
• Remove all test code before releasing the application
• Ensure logging is done appropriately but do not record excessive logs, especially including sensitive user information.
• //What sort of information should be recorded in the logs. (Keep audit data on the server, no user specific data - link to the Apple Issue - Signed Timestamps)

B. Enforce higher security posture on the device for sensitive apps used in an enterprise context: // Vinay to check

• If a sensitive application needs to be provisioned on a device, application can employ enforcement of the certain security posture on the device (such as PIN, remote management/wipe) // Vinay - Still needs to be clarified
• Enterprise applications can employ this principle of doing a security posture check before deployment of sensitive applications
• Banking Apps
• //(Remote Management, PIN enforcement, encryption, application monitoring)
  • Device cert can be used for stronger device identity. // How to make sure that this does not provide linkability between transactions (i.e. using the same cert across different service providers leaks data). I guess zero-knowledge certificates are too far-out for this guidance? Is this a common feature - device-cert - I have not come across it.

C. Protect your application from other malicious applications on the device

Risk: User's are prone to install applications that look cool (may be malicious) and can transmit data about user (or stored data) for malicious purpose.

• (?? What guidelines could be provided to developers)

U*ser education on using due diligence while installing third party applications on mobile devices

D. Provide or use an existing reporting channel for phishing from apps

(e.g. if you are a browser plugin developer). //APWG? Can we recommend one?