A recently observed email campaign is abusing compromised FTP servers as download locations for malicious documents and infecting users with the Dridex banking Trojan, Forcepoint has discovered.
Dridex has been one of the most prolific banking Trojans over the past several years, with the actors behind it constantly adopting new techniques and improving their malware for increased efficiency. The malware is focused on stealing user’s online banking credentials to perform financial fraud.
Malicious emails distributed as part of the new campaign were observed on January 17, 2018, primarily sent to .com top level domains (TDLs). Analysis of the top affected TDLs revealed that major regional targets included France, the UK, and Australia.
The emails were sent from compromised accounts, where the sender names were rotated around a list of names, in an attempt to make the emails look more convincing to unsuspecting recipients, Forcepoint reveals.
The malicious actor(s) behind the attack used two types of malicious documents as delivery mechanisms, namely a Word document abusing Dynamic Data Exchange (DDE) for malware execution, and a XLS file with macro code to fetch the banking Trojan.
The compromised servers abused in this campaign don’t appear to be running the same FTP software, and the security researchers believe that the attackers obtained the login credentials as part of other attacks.
“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. This may suggest that the attackers have an abundant supply of compromised accounts and therefore view these assets as disposable,” Forcepoint notes.
The emails sent in this campaign appear to come from the Necurs botnet, currently considered the largest spam botnet out there. The domains used for distribution were associated with other Necurs campaigns and the document downloaders are similar to those used by the botnet before. Furthermore, Necurs is known to have distributed Dridex for a long time.
What Forcepoint noticed regarding this campaign, however, was that the spam volume was very low compared to typical Necurs campaigns. Only around 9,500 emails were observed in this attack, while normal Necurs campaigns involve millions of emails. The use of FTP servers for download is also new.
“Cybercriminals constantly update their attack methods to try and ensure maximum infection rates. 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. The presence of FTP credentials in the emails highlights the importance of regularly updating passwords,” Forcepoint notes.