Almost a year after students ended pro-democracy street protests in Hong Kong, they face an online battle against what Western security experts say are China-sponsored hackers using techniques rarely seen elsewhere.

Hackers have expanded their attacks to parking malware on popular file-sharing services including Dropbox and Google Drive to trap victims into downloading infected files and compromising sensitive information. They also use more sophisticated tactics, honing in on specific targets through so-called white lists that only infect certain visitors to compromised websites.

Security experts say such techniques are only used by sophisticated hackers from China and Russia, usually for surveillance and information extraction.