WhatsApp has said the software was used for the surveillance of 1,400 people, including journalists, human rights activists and dissidents.
The Supreme Court on Monday asked the Justice Department to file a brief offering its views on the legal issue.
Meta Platforms is the parent of WhatsApp and Facebook and was known as Facebook Inc when the suit was filed. WhatsApp in October 2019 sued NSO seeking an injunction and damages, accusing it of accessing WhatsApp servers without permission six months earlier to install the Pegasus software on the targeted people's mobile devices. NSO has argued that Pegasus helps law enforcement and intelligence agencies fight crime and protect national security.
NSO appealed a trial judge's July 2020 refusal to award it "conduct-based immunity," a common-law doctrine protecting foreign officials acting in their official capacity.
Upholding that ruling last November, the San Francisco-based 9th US Circuit Court of Appeals called it an "easy case" because NSO's mere licensing of Pegasus and offering technical support did not shield it from liability under a federal law called the Foreign Sovereign Immunities Act, which took precedence over common law.