Last week we wrote about some malicious spam containing Microsoft Publisher files, leading to the FlawedAmmyy RAT, where the actors behind the campaign were targeting banks.
The same actors have resumed business today, however this time, they embedded the Publisher files within a PDF document. Again, all the target domains were banks.
Below is a representation of how the Trustwave Secure Email Gateway unpacks the message, you can see the Publisher file within the PDF. The ultimate payload was still FlawedAmmyy, so same trick, different package.
21 August – PDF with embossed .pub file – Subject: PDF_2108 or Subject: Payment Notification
Embedding stuff within a PDF is a favorite trick with this group, which uses the Necurs botnet to distribute its malware. See here and here for previous examples.
We also got curious as to what other attachments this group may be spamming out, so we went back further and studied the historical data more carefully. What we saw was a range of attachments being spammed, again targeting banks, and reaching back as far as 10 August at least. We didn't pick up on these earlier as the gateway was blocking all these samples (as it should). Here are some samples:
10 August – Doc with Macro – Subject: Invoice 7616
14 & 15 August – IQY file embedded in a PDF – Subject: MIS 93807.15/08/2018 or Subject: dhfl
21 August – IQY File – Subject: Sua NF-e foi emitida n° ( 670806)
21 August – Pub file – Subject: confirmation
What all this shows is the actors trying their usual array of tricks and file formats, but in different combinations and different packages. And, of course, determined in their widespread targeting of banks.
Banks and, well everyone, should consider blocking IQY files at the email gateway, and closely inspect inbound document files for other suspect embedded files and macros.
For Trustwave Secure Email Gateway customers, the gateway is detecting all the samples seen to date.
Embedded PUB file
File from URL(hxxp://g78k[.]com/set)
Unpacked file (FlawedAmmyy RAT)
DOC file with Macro
PDF with embedded IQY file