Built to collect victims' banking credentials, Dridex is now one of the most dangerous pieces of financial malware in circulation, according a new Symantec whitepaper.

Since 2014, spam email campaigns facilitated Dridex's almost exclusive distribution method, the Mountain View, Calif.-based cybersecurity firm said in the whitepaper. "These email campaigns are notable for their massive scale, frequency, and professionalism," it read. "The attackers behind Dridex regularly send millions of spam emails in the course of one day."

The number of Dridex infections detected by Symantec rose during 2015. Between January and April, there were less than 2,000 infections per month. Infection numbers spiked considerably in the following months, hitting almost 16,000 in June before dropping and stabilizing at a rate of 3,000 to 5,000 per month in the final quarter. Symantec observed at least 145 Dridex spam campaigns during one sample 10-week period.