WhatsApp, a Facebook-owned messaging app used by more than 1.5 billion people around the world, recently found a major security flaw, the Financial Times reported.

The hack is reportedly as simple as receiving a WhatsApp phone call, even if you don't pick up the call. A record of the call can even be remotely erased, the report says.

The WhatsApp exploit enables the installation of software from the NSO Group, a secretive firm from Israel that reportedly bills itself as a leader in cyberwarfare and is behind a notoriously invasive software tool called Pegasus.

The NSO Group denied involvement in the WhatsApp exploit, though that doesn't preclude the possibility that someone else used its products to exploit the WhatsApp security hole.

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A security flaw in the massively popular WhatsApp messaging platform used by more than 1.5 billion people exposed users to software from NSO Group, the maker of one of the world's most malicious spyware programs, called Pegasus.

The spy software enables remote access to your phone's most private information, from text messages to call logs to location data.

Pegasus surfaced in 2016, when it was reportedly used to spy on a human-rights activist in the United Arab Emirates. In the years since, it's been linked to the death of the Washington Post reporter Jamal Khashoggi, as well as the Mexican government's capture of the drug trafficker Joaquín "El Chapo" Guzmán.

The NSO Group is notoriously secretive. The Israeli firm sells sophisticated hacking tools to governments, militaries, and intelligence agencies — and it tries to keep such a low profile that it even changes its name regularly.

Here's everything we know about the secretive firm behind one of the world's most effective spyware applications.