An Israeli company that sells spyware to governments recently tried to hack into 1,400 users' devices by exploiting a vulnerability in WhatsApp, according to a new lawsuit from the messaging app.

WhatsApp is suing the company, NSO Group, for a barrage of high-profile attacks that occurred in May involving a scary (but now patched) vulnerability in the iOS and Android versions of the messaging app. The flaw was so bad it let an attacker send spyware to a WhatsApp-enabled phone simply by using a voice call to the victim. No pickup of the call was required.

At the time, the Facebook-owned messaging app refrained from calling out NSO Group. But on Tuesday, WhatsApp filed a lawsuit in US district court and published an op-ed in The Washington Post blaming the Israeli surveillance company for its spying attempts.

"We learned that the attackers used servers and internet-hosting services that were previously associated with NSO. In addition, as our complaint notes, we have tied certain WhatsApp accounts used during the attacks back to NSO," the messaging service's head Will Cathcart wrote in the op-ed.

The hacking attempts occurred between between April 29 and May 10. Of the 1,400 users targeted, at least 100 were human rights activists, journalists, and other members of civil society groups, according to Citizen Lab, a watchdog group from the University of Toronto that helped WhatsApp investigate the hacks.

These 100 users were based in 20 countries ranging from Africa, Asia, Europe, the Middle East, and North America, Citizen Lab said in its own report.

Although WhatsApp is well known for providing end-to-end encryption, the technology only scrambles your messages while they travel through app's pipeline. All the content will eventually be decrypted when it arrives on your WhatsApp client.

It's why NSO Group resorted to trying to install spyware on victims' mobile devices, according to the lawsuit. The Israeli company has been tied to a specific strain known as Pegasus, which is capable of recording phone calls, stealing files, tracking the device's location, and secretly snapping photos.

To exploit the vulnerability, NSO Group created fake WhatsApp accounts using phone numbers in countries such as Brazil, Cyprus, Israel, and the Netherlands, among others. At the same time, the company also leased servers from companies including Amazon Web Services to distribute the spyware packages on the mobile messaging app to victims' smartphones.

But according to WhatsApp, all the activities constitute computer hacking crimes that involved tampering with the company's servers. The lawsuit is now demanding the court order an injunction against NSO Group barring it from using WhatsApp and Facebook; it also requests damages.

The lawsuit probably won't stop NSO Group from secretly using WhatsApp services. But the Israeli company is denying the accusations, saying it plans to fight them. "The sole purpose of NSO is to provide technology to licensed government intelligence and law enforcement agencies to help them fight terrorism and serious crime. Our technology is not designed or licensed for use against human rights activists and journalists. It has helped to save thousands of lives over recent years," the company told PCMag in an email.

"We consider any other use of our products than to prevent serious crime and terrorism a misuse, which is contractually prohibited," the company added. "We take action if we detect any misuse."

Nevertheless, the Israeli company has for years faced accusations of helping oppressive regimes spy on human rights activists, journalists, and politicians. In response to the criticism, NSO Group recently adopted a human rights policy to prevent customers from abusing its technologies, but critics claim the company is doing little to rein in the abuses.

"The WhatsApp incident, and the more than 100 cases of abusive targeting that are associated with it, clearly verify the serious concerns Citizen Lab and others have raised. NSO Group spyware is being sold to government clients without appropriate controls over how it is employed by those clients," the watchdog group said on Tuesday.

WhatsApp has notified all 1,400 users targeted by the hack. The messaging app is also calling for all major tech firms to join a United Nations' expert call for a moratorium on the sale of surveillance software until safeguards can be put in place to prevent human rights violations.

"Governments and companies need to do more to protect vulnerable groups and individuals from these attacks," WhatsApp's Will Cathcart added in his op-ed post.