New Emotet spam campaign continues to bypass email security products

Posted by   Martijn Grooten on   Sep 18, 2019

Following the resumption of activity by Emotet's C&C servers in August, it was only a matter of time before the botnet started sending out spam again. This did indeed happen on Monday morning.

Many, though not all, of the emails Emotet sent out this week used a trick that the malware has used before: sending the malicious spam as apparent replies to previously harvested emails.

This has a number of advantages for the spammers. First, it makes the emails seem credible. Not only do they appear to come from known contacts, they also tend to contain messages such as "find the new document attached" that would seem an appropriate reply to many emails.

Secondly, because most of the email body contains the quoted email that is being replied to, content filters are less likely to block it.

And thirdly, by sending these emails as replies to harvested emails, they are likely to evade most spam traps. Spam campaigns hitting a large number of spam traps is probably the factor that contributes most to the emails in the campaign getting blocked; this is why most of today's malicious spam campaigns are sent in much smaller volumes.

Emotet's recent campaign didn't manage to evade spam traps entirely though, because some of the harvested emails were spam themselves; possibly with forged senders. Some of the emails we saw in our tests were such replies which, amusingly, included emails with an 'updated document' in response to dating spam.

Even so, we noted that many products in our test lab failed to recognise the emails as either spam or malicious. This is part of a worrying trend we have seen for a while, with malicious spam campaigns having much higher delivery rates than regular spam, sometimes as much as ten per cent of the emails piercing through the first defence layer.

Apart from 'hijacking' email threads, Emotet also avoids detection by sending most of its spam campaigns through compromised but legitimate mail servers. Such servers aren't likely to be listed on IP blocklists; IP-based defences are a very effective first defence layer in any email security product.

Spam is often seen as a problem of volume, but this has long since ceased to be the case: Emotet and other malware spreading via email campaigns is sent out in relatively low-volume campaigns, resulting in relatively high delivery rates and likely better returns on investment.

emotet_20190916.png
No fewer than nine email security products missed this Emotet campaign, despite it being a reply to a dating spam email.

Malwarebytes performed a technical analysis of the most recent campaigns, in which obfuscated macros in the attached Office files downloaded the Emotet binary from compromised WordPress sites.

At VB2019 in London next month, Sophos researcher Luca Nagy will present a paper on Emotet, while ZEROSPAM researchers Pierre-Luc Vaudry and Olivier Coutu will present a last minute paper on tackling the conversation thread hijacking Emotet is doing.

You can still register to see these and 50 other presentations on all aspects of threat intelligence. VB2019 takes place 2-4 October at the Novotel London West hotel in London, UK.



vb2019-register-now-2.jpg

twitter.png
fb.png
linkedin.png
hackernews.png
reddit.png

 

Latest posts:

VB2021 localhost call for last-minute papers

The call for last-minute papers for VB2021 localhost is now open. Submit before 20 August to have your paper considered for one of the slots reserved for 'hot' research!

New article: Run your malicious VBA macros anywhere!

Kurt Natvig explains how he recompiled malicious VBA macro code to valid harmless Python 3.x code.

New article: Dissecting the design and vulnerabilities in AZORult C&C panels

In a new article, Aditya K Sood looks at the command-and-control (C&C) design of the AZORult malware, discussing his team's findings related to the C&C design and some security issues they identified.

VB2021 localhost call for papers: a great opportunity

VB2021 localhost presents an exciting opportunity to share your research with an even wider cross section of the IT security community around the world than usual, without having to take time out of your work schedule (or budget) to travel.

New article: Excel Formula/Macro in .xlsb?

In a follow-up to an article published last week, Kurt Natvig takes us through the analysis of a new malicious sample using the .xlsb file format.

We have placed cookies on your device in order to improve the functionality of this site, as outlined in our cookies policy. However, you may delete and block all cookies from this site and your use of the site will be unaffected. By continuing to browse this site, you are agreeing to Virus Bulletin's use of data as outlined in our privacy policy.