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Thousands of spam emails are going out, designed to deliver a piece of malware to organizations via Dropbox, researchers said.

The attack starts with a fake email that appears to come from the Automated Clearing House (ACH), said researchers at Trend Micro. The messages tell recipients their ACH transfer ended up declined by the bank and instructs them to click on a link for more details.

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The links point to a malicious document hosted on Dropbox. Trend Micro reported seeing over one thousand Dropbox links used in this attack. It is not new for bad guys to use Dropbox to host malware, but researchers said this is the first time the file hosting service hosted macro-based malware.

The malicious documents observed in the recent attacks hide a variant of the Bartalex malware (W2KM_BARTALEX.SMA). The document instructs users to enable macros in order to view its content. Once macros end up enabled, the malware downloads a variant of the Dyre banking Trojan to infected systems.

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Over the last three months, most of the Bartalex infections spotted by Trend Micro have been in the United States (35 percent), followed by Canada (11 percent) and Australia (11 percent).

“We noticed that this attack used an old Microsoft Office 2010 logo. Given that many enterprises do not immediately upgrade to the latest Office versions, it is possible that users within enterprise organizations may fall victim to this technique,” Trend Micro researchers said in a blog post.

Dyre is a popular tool among bad guys.

In January, Trend Micro reported attackers had been distributing the threat with the Upatre downloader and a worm designed to send out malicious emails via Microsoft Outlook. Earlier this month, IBM said a cybercrime ring stole over $1 million from the corporate accounts of U.S. businesses using Dyre.

Using macros to infect computers is an old technique, but it can still be highly efficient for targeting consumers and enterprises. In the past period, researchers noticed macros seeing action in operations leveraging pieces of malware such as Dridex, Rovnix and Vawtrak.

“Addressing macro malware in an enterprise (and small and medium-sized business) setting involves reevaluating and revisiting existing security policies. It’s also advisable to decrease the attack surface area by making sure systems within the corporation have the necessary security measures in place: for instance, it may be wise to disable Windows Scripting Host on users’ systems if it serves no substantial purpose,” Trend Micro said. “Lastly, user education will go a long way in defending against these types of threats, in particular, those that exploit human error, e.g., enabling malicious macros in Word documents.”

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