Actress Emma Watson arrives for the world premiere of Harry Potter and the Deathly Hallows - Part 2 in Trafalgar Square, central London.
When searching for the 22-year-old actress there is a one-in-eight chance of landing on a malicious site, the security firm said.
This is the sixth time the Intel-owned security technology company has conducted the study, which was last year topped by Heidi Klum.
Others among the riskiest celebrities to search online are Jessica Biel, Eva Mendes, Selena Gomez and Halle Berry, the report said.