The announcement follows reports from technology websites ZDnet and The Register that the vulnerability, which affected WhatsApp applications on Apple and Android smartphones, was discovered in late August and was fixed by Facebook in early October.

“We routinely engage with security researchers from around the world to ensure WhatsApp remains safe and reliable. We promptly issued a fix to the latest version of WhatsApp to resolve this issue,” Ann Yeh, spokeswoman for WhatsApp, told Reuters in an email.

WhatsApp is used by more than 1.2 billion people around the world and is a key tool for communications and commerce in many countries. The service was acquired by Facebook in 2014 for $19 billion.

“This is a big deal,” Travis Ormandy, a researcher at Google Project Zero which discovered the bug, said on Twitter. “Just ++answering a call from an attacker could completely compromise WhatsApp.”

Facebook has suffered a string of security-related problems in the last year. The social media company last week disclosed its worst-ever security breach affecting nearly 50 million accounts.

Facebook shares were down 1.8 percent at $155.02 on Wednesday.