Dridex activity continues

Dridex, a banking malware which attempts to steal the victim's banking credentials and system information, continues to remain active in the wild after the recent takedown attempt. Dridex activity went down significantly in September after the takedown operation but we started seeing an uptick in the number of samples in our sandboxes starting early October 2015.

Dridex is distributed via e-mail with a Microsoft Office attachment that leads to the download and installation of the Dridex Trojan executable. The e-mail attachments in these campaigns varied from regular Microsoft Office document files to MHTML files as seen in Figure 1.

Figure 1: Malicious document sent in MHTML format

MHTML, also known as MIME HTML, is a web page archive format used to combine in a single document the HTML code and its companion resources that are otherwise represented by external links (such as images, Flash animations, Java applets, and audio files). The malware authors are known to send the documents containing malicious macro in MHTML format to evade antivirus detection.

The content of the embedded macro was both obfuscated and protected using a basic password that is intended to prevent modification of the macros.  We were able to bypass the password protection and extract the various document components by standard means.

Figure 2 shows an example embedded malicious macro that downloads the Dridex executable from a predetermined server:

Figure 2: Embedded malicious macro to download Dridex

The samples we are seeing in these new campaigns are a mix of unsigned as well as some digitally signed using valid certificates. The format of the URLs hosting the signed Dridex samples changed to 195.37.231[.]2:8080/uniq/load[.]php instead of previously seen URL format 94.250.252[.]13/bt/bt/stata[.]php

The Dridex executable downloaded by above macro (Figure 2) appears to be packaged using a custom packer. This instance of the Dridex Trojan is signed using a certificate with the common name of “Favorite-III” which appears specific to the Dridex malware and which is currently also present in Comodo’s Certificate Revocation List.

Figure 3: Dridex Certificate issued with CN Favorite-III

We also saw following additional certificates used to sign the newer Dridex executables:

• INTEX
• KASHTAN OOO
• Promtorg
• KONSALTING PLUS OOO
• Brand IT 
• AVTOZVIT Scientific Production Private Company
• Afet
• 3 AM CHP
• PJSC "BIZNES AVTOMATYKA"
• Favorite-III
• Private Person Parobii Yuri Romanovich
• SWIFT Weather
• PJSC "BIZNES AVTOMATYKA
• AVTOZVIT Scientific Production Private Company
• Private Person Parobii Yuri Romanovich
• Favorite-III [NEW - seen in October 2015]
• promtorg  [NEW - seen in October 2015]

The following are some recent URLs from which Dridex executables were downloaded:

• 93.170.104.168:443/uniq/load[.]php
• 178.62.7.183:443/uniq/load[.]php
• 195.37.231.2:443/uniq/load[.]php
• 5.2.199.30:8080/uniq/load[.]php
• 113.30.152.165:8080/uniq/load[.]php
• 168.243.33.195:8080/uniq/load[.]php
• 199.175.55.116:8080/uniq/load[.]php

The following are some MD5 sums of signed and unsigned Dridex executables that we have encountered in past month:

Signed

82BB00DCAC6411669CE6AE5A60CBB3B3
9D4225ECDCDA7FE9A5EAE48601919114
2485C741AF50DE986079B6AD9B6C948A
290CD720AECF28773960C8E41172513C
1C21AEB3DC0E30E05630A3F61AAE83F9
0DA24BD7B49A955D8E4624371CCB8E9F
AFADE4E50D147A1FE18ACA8942E3E679

Unsigned

1DE3889FDE95E695ADF6EADCB4829C6D
D7A31449E5F808FDBA0F6D3CF0D6E91C
7519F0D9D5C3B8D072FFCA7DDF213DDA
F2D6DEC39DAEF7ED90AAABB725590B02

Following are the statistics of Dridex executable downloads that we have seen in past six weeks:

Figure 4: Dridex executable downloads in past six weeks

Figure 5: Dridex executable hosting servers

Conclusion

Dridex infections went down considerably after the global takedown operation but we are starting to see a steady increase in the infections this month, which indicates the malware gang's attempt at resurrecting this highly lucrative Botnet. The authors continue to use the tactic of digitally signed malware executable to evade detection with legitimate certificates created specifically for this purpose.

Zscaler ThreatLabZ is actively monitoring this Botnet and ensuring that Zscaler customers are protected.

Research by Tarun Dewan and Nirmal Singh

Signed Dridex Campaign

Introduction

Malware authors use various means to make their malware look similar to legitimate software. One such approach involves signing a malware sample with a digital certificate. Recently we saw Dridex malware authors using this technique while reviewing the samples in our Cloud Sandbox. Dridex is a banking Trojan which typically arrives to a system via malicious spam email with a Microsoft Office file as an attachment. These files will have embedded macros that lead to the download and installation of the Dridex Trojan. Dridex then attempts to steal the victim's banking credentials and system information.

Signed Dridex campaign

Here we came across one malicious attachment with an encrypted macro that downloads signed Dridex samples from 81.17.28.101/bt/bt/sti[.]php. This Dridex sample is packed using a custom packer, which is is compiled with .NET. The current Dridex is signed with a certificate that is issued to Private Person Parobii Yuri Romanovich. This certificate as been specially created for spreading the Dridex malware.

Certificate

We also saw the following signer information in the certificates used for signing the Dridex executables:

• PJSC "BIZNES AVTOMATYKA
• AVTOZVIT Scientific Production Private Company
• Private Person Parobii Yuri Romanovich

The certificates were all issued by COMODO and we observed the following URLs serving the signed Dridex malware:

• 185.14.29.214/bt/bt/sdp [.] php
• 81.17.28.101/bt/bt/sti [.] php
• 5.196.241.204/bt/bt/ched [.] php
•  217.12.203.171/bt/bt/freda [.] php
• 94.250.252.13/bt/bt/stata [.] php
• 149.202.146.176/bt/bt/chdid [.] php
• 93.170.105.60/bt/bt/grtes [.] php

Below are the Dridex samples served from the aformentioned URLs. All tof he samples are packed with the same .NET packer mentioned above:

• 5CA1DBA1C72AC999E221DE98BBC584C4
• 9E73E0C4B92253C5F8B6648F29B28B5B
• CD243B30B9BBD682C082CFEFDBF79ACD
• E578618F2D38FC251D52D1366144404F
• 5F907702CE229937955B4DCE92EC4575
• DC443FBB5FB6125EBEEEBEC2E4BAA372
• 0BBC8CD08E9958ACDE0519A2B2840CD7
• 9D1D0632329F04D8B1EC21AFF4CE6493
• 32230D747829DCF77841F594AA54915A
• 8F1A9A9830FF02C5C2BA4C17DFE8B09D
• 00DCA835BB93708797A053A3B540DB16
• 393E2145F4C3E9B5697A2AAEB25AA8D3
• 1992170FDC642D4A99A7BC82BA82FA31
• 9261B8EAF1DA3D9CFF522875A7198667
• FB67C85F3F42D3E48B9E7B7637D30858
• E578618F2D38FC251D52D1366144404F

Dridex Packer:
The Dridex sample is embedded in the resource section of the packer. After unpacking, it drops a Borland Delphi executable file. The following is the snapshot of encrypted resource section:

Encrypted Resource Section

Dridex Activity:
The current Dridex sample tries to connect to different IPs included in the config file. The config file for the sample is embedded in the sample itself. In the config file we observed a botnet ID and list of  C&C servers. Below is a snapshot of config file:

Configuration file

Dridex collects and sends the following system information to one of the C&C mentioned in the config file:

• Computer Name
• User Name
• Windows Version
• Botnet ID

Information sent to the server

Below is the complete list of C&Cs it tries to connect.

• 80.247.233.18
• 91.121.82.113
• 69.164.213.85
• 79.143.191.147
• 199.241.30.233
• 162.243.12.14
• 188.93.73.90
•  195.154.184.240

Conclusion

The use of a legitimate certificate in signing malware executables to evade security detection is not new but is still very effective. The malware author aims to exploit the Code-Signing Certificate based whitelisting approach by signing their samples. Zscaler ThreatlabZ is actively monitoring these signed malware campaigns and ensuring coverage for our customers.

Analysis by Tarun Dewan and Nirmal Singh