WhatsApp has taken a step toward wining its widely publicized lawsuit against the NSO Group after the Israeli spyware merchant failed to show up in court, according to a notice of default entered Monday in California.

WhatsApp sued NSO in October after finding evidence that the hacking firm had abused a flaw in the Facebook Inc.-owned messaging app's popular chat program to remotely hijack hundreds of smartphones.

The lawsuit - along with alert messages that WhatsApp sent to hundreds users alleged affected by the hacking - sparked disclosures about NSO's surveillance work across the world.

The case was closely watched, both because of the high-tech surveillance angle but also because it was unusual for a major service provider to sue a hacking company on behalf of its users.

NSO promised to "vigorously fight" the allegations, but the firm was a no-show in the Northern District of California, where the case was filed.

Legal documents filed by WhatsApp detail repeated efforts to serve the company with legal documents, including emails to senior executives, FedEx-delivered copies to NSO board members, and even a hand-delivered copy of the suit left with NSO cofounder Omri Lavie's wife at their New Jersey home.

In a statement, WhatsApp noted that NSO had failed to show up before a judge and said it would "continue to pursue swift accountability from the courts in the U.S."

NSO said in response that WhatsApp had "prematurely moved for default before properly serving NSO with the lawsuit" and that "this default notice will not stand."

A notice of default paves the way for a default judgment against NSO - which could involve injunctions and damages - but a litigator who specializes in cybersecurity issues said that was some way off.

Scott Watnik of Wilk Auslander in New York noted that courts were uncomfortable with default judgments and were usually generous about overturning them when challenged.

"If NSO came forward in a timely way to vacate the default judgment, there's a very strong chance that the court would grant such a motion," Watnik said.

On the other hand, Watnik said he found it extraordinary that NSO was publicly commenting on a lawsuit that it said it had not been properly served in.

"I've never seen that before," he said. "It’s a high risk maneuver because it really cuts away at their ability to move to vacate the default judgment."