A security issue regarding WhatsApp for Android was posted by Bas Bosschert which allows any other app on your phone to read your WhatsApp conversations.
This issue still exists even after WhatsApp received a major update for Android. One way you could be secure is by not installing any malicious apps on your phone.
How hackers can read your conversations on WhatsApp?
WhatsApp for Android, like many other apps stores it data(images, videos, conversations) on the phone's SD card. When other apps on the phone are granted permission to access all the data on the phone, they can also access WhatsApp database.
If you were to install a malicious app or a game, you WhatsApp conversations could be uploaded in the back while you busy checking out the features of the new app.
WhatsApp team recently worked on encrypting the WhatsApp conversations database so that it could not be accessible even by SQLite. But, According to Bas Bosschert, it is possible to decrypt the database using scripting.
This is a problem of the Android infrastructure. Any app can access data of any other app if it has full permissions. We would like to this issue being fixed in the future version of Android.