The former general counsel of Monster Worldwide Inc., the company that runs the Monster job search Web site, admitted in court Thursday that he illegally backdated millions of dollars in employee stock option grants — a practice that cheats shareholders.

Myron Olesnyckyj, 45, of New Providence, N.J., pleaded guilty to securities fraud and conspiracy to commit securities fraud, charges that carry potential penalties of up to 25 years in prison and fines of more than $5.2 million. He promised to cooperate, which can earn him leniency.