The Trojan was first spotted back in 2014 after targeting banks in the United Kingdom.
Since then, Dridex has become infamous for striking financial institutions across Europe.
The malware spreads through phishing campaigns, duping victims into downloading and executing malicious macros hidden in Microsoft documents, as well as attacks by way of web injections.
Once the Trojan has compromised a PC, it steals online banking credentials which can then used fraudulently by operators to plunder bank accounts.
Spam and phishing campaigns utilizing the Trojan usually rely on HTTP download locations for malware payloads. However, Forcepoint Security Labs said on Thursday that a "peculiar" email campaign distributing a Dridex variant has chosen a more unusual method.
In a blog post, the team said compromised FTP websites are now being used to distribute the malware, which also exposes the credentials of the vulnerable domains in the process.
In this particular campaign, malicious emails were distributed on January 17 this year and remained active throughout the day. The emails were sent primarily to top-level domains including .com, .fr, and .co.uk.
France, the UK, and Australia were the most targeted countries.
According to the team, the sender domains were from compromised accounts from the FTP websites, and sender names -- such as admin@, billing@, and no-reply@ were rotated to appear more authentic.
The Dridex campaign uses two types of documents; an XLS file with a malicious macro that downloads the Trojan, and the second is a DOC file which abuses Dynamic Data Exchange (DDE) to execute shell commands.
"The perpetrators of the campaign do not appear to be worried about exposing the credentials of the FTP sites they abuse, potentially exposing the already-compromised sites to further abuse by other groups," Forcepoint says. "This may suggest that the attackers have an abundant supply of compromised accounts and therefore view these assets as disposable."
It may be that the latest Dridex campaign originates from the Necurs botnet, as the domains used for distribution are the same as previous Necurs campaigns; the downloaders are similar, and the download locations for the XLS file have been connected to Necurs in the past.
"Necurs has recently been recorded using malicious links (as opposed to malicious attachments) to distribute Dridex, but the switch to FTP-based download URLs is an unexpected change," the team says. "In this case, FTP sites were used, perhaps in an attempt to prevent being detected by email gateways and network policies that may consider FTPs as trusted locations."