Russia-linked hackers targeted the French presidential campaign of front-runner Emmanuel Macron, according to an anti-virus firm.
Researchers at Japan's Trend Micro said attempts to penetrate the Macron campaign by hackers linked to the Kremlin date back to December.
All attempts at intrusion were thwarted, however.
"It's serious, but nothing was compromised," Macron's digital campaign chief Mounir Mahjoubi said on Monday.
A Trend Micro report claimed that hackers set up a bogus website to harvest the passwords of Macron campaign staffers.
The firm attributed the online spying campaign to a group it calls Pawn Storm, but stopped short of saying who was pulling the strings.
Russian government officials have repeatedly denied claims of state-sanctioned hacking.