Facebook Inc. (FB - Get Report) was on the defensive again Tuesday following a Financial Time report that said its WhatsApp messaging app could be hacked with spyware developed by Israeli cyber surveillance experts.

WhatsApp officials acknowledged the vulnerability, which the FT said allowed hackers to install the spyware through an unanswered phone call and then upload personal data such as location services and details of private conversations, and urged its 1.5 billion global users to upgrade their messaging app software.

"The attack has all the hallmarks of a private company reportedly that works with governments to deliver spyware that takes over the functions of mobile phone operating systems," WhatsApp said in a statement. "We have briefed a number of human rights organizations to share the information we can and to work with them to notify civil society."

"Whatsapp encourages people to upgrade to the latest version of our app, as well as keep their mobile operating system up to date, to protect against potential targeted exploits designed to compromise information stored on mobile devices," the company said. "We are constantly working alongside industry partners to provide the latest security enhancements to help protect our users." 

Facebook shares were marked 0.75% higher in pre-market trading Tuesday, indicating an opening bell price of $182.90, after falling more than 3.6% on Wall Street yesterday amid steep declines for the so-called FAANG complex of tech stocks.

The FT report said the spyware was developed by Israel's NSO Group, a cyber surveillance firm whose flagship Pegasus product can allegedly find it way into mobile phones through either a microphone or a camera and then access sensitive personal data.

NSO said its products are used for the "sole purpose" of "fighting crime and terror", in remarks published by Reuters, and vowed to investigate the allegations in the Financial Times piece.