The messaging platform has directly notified affected users and provided them with security guidance, including resources from digital rights research group Citizen Lab at the University of Toronto.

It has also sent a cease and desist letter to Israeli-based Paragon Solutions.

"This is the latest example of why spyware companies must be held accountable for their unlawful actions," a WhatsApp spokesperson said. "WhatsApp will continue to protect peoples' ability to communicate privately."

WhatsApp declined to give further details on the incident, including the country or countries targeted in the campaign.

The development highlights growing concerns over commercial spyware firms.

Paragon Solutions is among several firms that have emerged in recent years offering sophisticated surveillance tools to government clients.

Critics argue these companies enable human rights abuses under the guise of national security operations.

The Israeli-founded NSO Group, maker of the notorious Pegasus spyware, has faced multiple lawsuits and sanctions after its software was allegedly used to surveil journalists, activists, and government officials worldwide.

In 2021, WhatsApp won a US court case against NSO Group over a similar targeting campaign.

WhatsApp said it has successfully blocked the exploitation method used in this campaign and continues to pursue legal action against spyware vendors who breach its terms of service.

Paragon Solutions did not reply to a request for comment.

The company on its website says it provides "ethically based tools, teams, and insights to disrupt intractable threats."

"History shows that secret surveillance technology carries a temptation to be used against civil society and journalists that is impossible to resist. It was true 50 years ago and it's true today," said John Scott-Railton, senior researcher at the Citizen Lab.