WhatsApp keeps your conversations stored on a microSD card. If you have allowed other apps to access your microSD card, past conversations can be accessed. This is not just a problem with WhatsApp, but is something that afflicts other Android apps as well. And despite WhatsApp's encryption process, a savvy hacker can still crack the code.
While WhatsApp uses the microSD slot to capture conversations, it doesn't have to save them in this manner. If WhatsApp can change the way it stores conversations, they could be made more secure. Until then, it is just another security issue to worry about.
source: BasBosschert via RedmondPie