US national security officials told a private equity firm partly backed by a Russian billionaire named in the Steele dossier to sell its stake in Cofense, a cybersecurity firm used by major corporations, according to people familiar with the matter.

Pamplona Capital Management, a multinational firm that has raised money from investors including Mikhail Fridman, is selling its stake in Cofense after the Committee on Foreign Investment in the US raised national security concerns associated with foreign investors and the nature...