BEDFORD, Mass. — Credit unions accounted for nearly half of the targets chosen for phishing, pharming and Trojan attacks detected against American financial institutions in December, according to a leading cyber-security firm.

RSA Security says credit unions accounted for 45% of the attacks noted by its Anti-Fraud Command Center (AFCC), up from 34% in November. Nationwide banks, meanwhile, fell to 26% of the targeted attacks by brand in December, down from 44% the month before. Regional banks accounted for 29% of the attacks, up from 7% in November, RSA Security says.

RSA Security, whose client base includes financial institutions and other e-commerce entities serving millions of end users, attributes the sharp rise in December phishing attacks to the Rock Phish group, which has been active for months and by some accounts are responsible for half the phishing attacks worldwide. The report also notes the emergence of other groups using similar methods, including proxy servers that mask the primary host.

The AFCC detects, monitors, tracks and shuts down phishing, pharming and Trojan attacks for more than 250 organizations worldwide, and has recorded more than 60,000 site takedowns, the company says. It also participates in global anti-cyberfraud networks.