Trojan.Silentbanker intercepts account information, redirects traffic to phishing sites, and even alters transactions to send money to the attacker's bank account.
The long list of targets include banks in the US, UK, Ireland, Spain and France.
"The scale and sophistication of this emerging banking Trojan is worrying, even for someone who sees them on a daily basis," wrote Symantec researcher Liam O'Murchu in a
company blog.
The most troubling feature of the Trojan is its ability to perform man-in-the-middle attacks to intercept and alter data travelling between the browser and the bank.
This allows the Trojan to modify data and reroute money to another account without the user's knowledge.
The malware also has the ability to steal and upload account data, change DNS settings and reroute users to phishing sites.
O'Murchu noted that the Trojan can display more than 600 pornographic URLs, which he suspects is designed to raise the Trojan's author referral payments.
The current attacks may be just the beginning, however. O'Murchu said that the Trojan is constantly updating itself, checking for new configuration files several times a day.
Symantec urged users to protect against the Trojan by keeping antivirus software up to date. Administrators can also configure firewalls to block the URLs to which the Trojan connects.