Necurs Recurs

The Necurs botnet, which was responsible for millions of malicious spam messages last year, has recently been extremely active again. For the past three weeks it has spammed emails with a malicious PDF attachment that drops a word document with a macro that, in turn, downloads rebranded ransomware. The chart below, which is data from our Spam Research Database, shows the daily spam received from the previous weeks to present. In typical Necurs fashion, you can see the short high volume bursts.

001_Graph

The campaign uses various subjects in the spammed emails.

Week 1 (May 11-12)
Scanned image
Receipt to print
File_ {random numbers}
Copy_{random numbers}
Document_{random numbers}
PDF_{random numbers}
Scan_{random numbers}
Week 2 (May 15 -17)
Your Invoice # {random numbers}
XX_Invoice_XXXX
Emailing: {random numbers}.pdf
Invoice {random numbers} {mm/dd/yyy}
Week 3 (May 22-25)
Invoice(XX-XXXX)
Copy of Invoice {random numbers}
IMG_XXXX.pdf
Payment Receipt XXX
Payment Receipt#XXX
Payment Receipt_XXX
Payment XXX
Payment#XXX
Payment_XXX
Payment-XXX
Receipt XXX
Receipt#XXX
Receipt_XXX
Receipt-XXX
***Note that X is any random number

Sample emails:

1st week

002_1stweekemail

2nd week

003_2ndweekemail

3rd week

004_3rdweekemail

The PDF campaigns have been evolving, almost daily. Recent documents have a larger number of embedded files inside the pdf. These additional files do nothing, and are probably just decoys. But the main.docm file, with its malicious macro, still acts as the malware downloader. Below you can see how the Trustwave Secure Email Gateway sees these messages. Note the docm file with its vbaProject macro component.

1st and 2nd Week3rd Week
005_Prevweekembed006_Thisweekembed

Saving and opening the PDF attachment, as shown below, has an exportDataObject Launch instruction to open the embedded .docm file

007_PDF

The PDF File will drop and launch the embedded .docm file

008_macro

If the macro is enabled, it will start to download a malicious file from URL which is the Jaff Ransomware. The table below shows differences from variants from week to week:

1st and 2nd Week3rd Week
Dropped ransom notes on every folder that it encrypted files in it, the image file was also used as a desktop wallpaper once ransomware done encrypting your files.

009_01_Jaff_bmp
Readme.bmp

012_02_Jaff_bmp
README_TO_DECRYPTl.bmp

010_01_Jaff_html
ReadMe.html

013_02_Jaff_html
ReadMe.html

011_01_Jaff_txt
ReadMe.txt

014_02_Jaff_txt
README_TO_DECRYPTl.txt

Encrypted files will have an appended file extension

{Original FileName}.jaff

{Original FileName}.wlu

Even though the appearance is different, both variants have the same URL where you are able to recover your encrypted files.

015_tor

Earlier versions asks for 2.01 BTC and later versions it only asks for 0.31 BTC

015_earlypay015_lastpay


To conclude Necurs is a large botnet and when active it distributes massive volumes of malicious spam. As observed, it tends to take breaks on weekends and it currently has an ongoing campaign using malicious PDFs to download Jaff ransomware. These malicious PDFs are continuously evolving by adding more layers of embedded files and obfuscation.

The Trustwave Secure Email Gateway can recognize and block this threat.

MD5 hashes of the malware:
PDF Droppers
d364eb043e01f61822c9d2906a36ad2f902c60d7
8e4f36e0710aee26f125acc69b14cac44467238f
2001971c7ddaa9b2550d1b870f5e377c56f15f70
DOC Downloaders
ee4fef6b870d0baa3a503aa8594dc16920f7b8a3
f66680aac290ad5febd6bc5b40efe16817bd6850
5045d532a951af205d0e0d91805b2bc38ee6aedd
Jaff Ransomware 1
03b17da93cf91f61c9dbb4d25182016cefec0659
Jaff Ransomware 2
551f953db4ba48452a4f7de9f5f7149c98ddf52f

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