Weak or non-existent authentication can grant an attacker unauthorized access to an app.
A password should not be simplistic. It’s best to require, if not at least support, complex passwords, including length of at least six alphanumeric characters (more characters is always stronger). Requiring the selection of a secret word or icon (which the user does not create themselves) as part of the log-in process can help protect users' accounts in the event they re-use passwords and their password was exposed as part of another data compromise.
In some cases, a username and password does not provide sufficient security for a mobile app. When sensitive data or transactions are involved, implement two-factor authentication. This may not be feasible every time a user logs in but can be used at intervals or when accessing selected functions. Consider step-up authentication methods to provide normal access to non-transactional areas but require a second layer of authentication for sensitive functions.
Options for enhanced authentication include:
- Additional secret word/icon
- Additional code provided by SMS or email -- but beware that an attacker will likely have access to both on a stolen device
- Password plus additional user-known value, for example user-selected personal factor
- Security questions and answers, selected by the user in advance (e.g. during registration)
For the highest level of security, use one-time passwords that require the user to not only possess the correct credentials, but also a physical token including the one time password.