According to researcher Tavis Ormandy, who is part of the team that discovered the issue, the bug is a big deal as it could force the popular messaging app to crash and close.
The bug is actually only found on Android and Apple smartphones, but WhatsApp's owner Facebook has insisted that it's already been resolved.
The tech giant insisted it responded "promptly" as soon as the problem was flagged up.
The company explained: "We routinely engage with security researchers from around the world to ensure WhatsApp remains safe and reliable."
The issue has come to light shortly after it was revealed that Facebook could face action from Germany's anti-trust watchdog.
Andreas Mundt, the head of the organisation, accused the social networking website of abusing its market dominance to gather user's data without their consent.
Speaking at a conference on competition law in Berlin, the Federal Cartel Office President revealed the body was looking into what steps it could take.
He said: "We are currently evaluating Facebook's opinion on our preliminary assessment and I'm very optimistic that we are going to take further steps, even this year, whatever this would mean."