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SpiderLabs Blog

Grandoreiro Banking Malware Resurfaces for Tax Season

Trustwave SpiderLabs in early April observed a Grandoreiro malware campaign targeting bank users from Brazil, Spain, and Mexico. The campaign exploits the tax season in target countries by sending out tax-themed phishing emails.

Grandoreiro was first detected in 2016 is one of the largest banking trojan families developed to strike targets Latin America. The malware can log keystrokes, capture clipboard data, steal cookies and other user information using malicious browser extensions, and monitor online banking activity. 

Spam Delivery

An attack begins with threat actor distributing email spam in Portuguese. The email pretends to be a memo coming from “Serviço de Administração Fiscal” or “Tax Administration Service” as translated. It has a link that downloads a malicious PDF document hosted on a compromised website.

18697_picture11

Figure 1. Email spam in Portuguese

The PDF purports to come from DocuSign and tricks the user into clicking the link leading to a ZIP archive containing an MSI installer. The same compromised website as the PDF document also hosts the ZIP archive.

18698_picture2

Figure 2. DocuSign-themed PDF file

The MSI Installer

Digging deeper into the MSI installer using the Orca MSI Editor, we observed a suspicious code embedded in the CustomAction table. The CustomAction enables the author of an installation package to extend the capabilities of standard actions by including executables, dynamic-link libraries, and scripts.

18699_picture3

Figure 3. CustomAction table view in Orca MSI Editor

The block of the custom code written in JScript serves as a downloader of the final payload. The code will perform host and IP address location checks before it downloads, extracts, and executes the final payload.

The JScript code avoids infecting a host with “dx” as the network username. It uses the geolocation service http[://]ip-api[.]com/json to gather the target’s IP address location data. Distribution of the final payload is only limited to IP addresses located in Brazil, Spain, and Mexico. After passing the checks, the final payload will be downloaded from hxxp[://]167[.]114[.]43[.]27:4433/mrrrpx2503[.]zip and extracted at %AppData%\Roaming\RIPIOSDS in the background.

18700_picture4

Figure 4. Code snippet of the JScript custom action

18701_picture5

Figure 5. JScript block that checks for IP address location

The downloader retrieves a ZIP package with an executable and multiple DLL components. The executable (kitbootnetsuuui.exe) appears to be legitimate software called the Advanced Installer Intune Tool used in the deployment of software packages and signed with a valid signature.

The executable is bundled with three DLLs. Two of these are uires.dll, the resource module of the Advanced Installer tool, and the Zlib data compression library – zlibai.dll, both of which are legitimate libraries. The third DLL masqueraded as a debug help library, dbghelp.dll, is the Grandoreiro banking trojan.

Table 1. Filenames and hashes of zip package

Filename

SHA1

kitbootnetsuuui.exe (Advanced Installer Intune Tool)

5dd0b062dda3991c09e439f0688ba94004573d6e

uires.dll (Resource module for Advanced Installer)

be3bebab8db0087d92316b5f54b5aaf5f51fbf46

zlibai.dll (Zlib data compression library)

aadc8a089d1288e91e6ba9e095d37d30de3bbb18

dbghelp.dll (Grandoreiro banking trojan)

ff908727cc1b5335e541fbcd80a327565f308bc7

 

18702_picture6

Figure 6. Extracted files of the final payload

Grandoreiro banking trojan

We have seen several common characteristics of Grandoreiro in this variant, as documented here. Written in Delphi, the threat actor inflated the DLL to 300MB by adding bitmap images to the resource section. This technique is known as binary padding and is used to evade detection. Other Latin American banking trojans like Javali have implemented a similar technique.

After running the signed executable, it will load the malicious DLL payload and perform the malicious routines. Grandoreiro leverages the DLL side-loading technique to conceal malicious actions under a legitimate software process. This method will likely evade detection by AV scanners since the benign executable used to side-load the DLL may not raise alerts during execution.

The trojan sets up persistence using the registry run key to enable automatic execution at every startup:

Key: HKCU\Software\Microsoft\Windows\CurrentVersion\Run

Value: HYNCDLXKKIO={SAMPLE_PATH}\kitbootnetsuuui.exe

 

Grandoreiro may gather host information such as operating system version, hostname, display monitor information, keyboard layout, and mouse type. Moreover, it is capable of enumerating installed security programs and web browsers. It can execute shell commands like pinging a domain or an IP address and kill running processes. Keylogging, monitoring users’ browsing activity, capturing clipboards, and stealing session cookies are part of its backdoor capabilities.

Target bank names, executable names, installation locations, and other internal strings are encrypted with the key "F5454DNBVXCCEFD3EFMNBVDCMNXCEVXD3CMBKJHGFM" using XOR-based encryption. Grandoreiro resolves these strings and API calls during execution to evade detection.

Table 2. List of several encrypted strings with equivalent plaintext

Encrypted String

Plaintext

34C116B01045F71235F1

Santander

7B89EE046F849392A4BF8747CA789F40964FF429

TRAVASITE |Bradesco

C34B8C35AF16C0689EE8739745F519C3187DF90509

Internet Banking BNB

3FC6025AF66E8B4EF53AF177AA49EB15BB5A8FF018D26E

AplicativoBradesco.exe

102A4B89CB0B3BEE16DE

~\Trusteer

4DD31260968283B86B9AD475

HSBC España

9BB814B217B052CE76B846

chrome.exe

A5A022B71EA6BD7A9F9B5992B4

AvastSvc.exe

 

We wrote a string decryptor and created a list of decrypted notable strings available at this link.

Domain Generation Algorithm (DGA)

Grandoreiro's DGA implementation relies on the current date as a seed variable. It queries an NTP server, e.g., time.nist.gov, to retrieve the current date and time. The day combined with the letter corresponding to the month in Table 3 and the rightmost digit of the current year as a suffix will act as the initial value. For instance, the date 13/04/2022 will yield "13W2".

Table 3. Monthly key

1

2

3

4

5

6

7

8

9

10

11

12

N

S

L

W

B

K

Z

O

D

E

P

V

 

The initial value is XORed with the current date's key; refer to Table 4 below. The hex equivalent of the product of the XOR operation, all characters should be upper case, will be concatenated to the static string, nuu, serving as a prefix. The output is base64-encoded using the custom alphabet set, "g-yA-Za-fz0-9+/=", with the padding removed, generating the subdomain name.

Table 4. Daily XOR keys for each month

Date

Jan (1)

Feb (2)

Mar (3)

Apr (4)

May (5)

Jun (6)

Jul (7)

Aug (8)

Sep (9)

Oct (10)

Nov (11)

Dec (12)

1

iota

fxcj

vdjq

dinu

wcmt

qbgo

pdha

xejr

ewdj

tyio

hzel

wbmt

2

bcmp

egxa

oruz

eglo

ikpu

wyej

gjmr

txak

behm

ehkp

gvzf

vzcm

3

jorh

ndgy

nthy

mrdw

orwi

uxci

insh

fwbh

takp

ehyd

lotz

ptyk

4

adgn

duze

clqd

lqtv

fknu

vyin

qthy

xafk

ejmt

knsg

loty

ckpd

5

nqta

orwb

loty

ckpd

zehn

adgn

orev

seva

zdgm

knsx

ails

ruzj

6

lqty

hmpu

mpty

cgkp

pswc

waej

fvyd

cfin

lqty

nqva

svyk

mpsz

7

hwbg

aejo

jmrw

quyi

qehz

nsfw

vybg

adin

hlqu

adgl

gjnt

hkou

8

ilqe

fuze

cglp

vyin

mpsx

nrdu

hwze

dhkp

gjmr

vybg

vybi

sfwa

9

orub

knsg

fuxc

qvyi

swak

nrfv

qtfw

vycg

rduz

knqe

bjot

uxcm

10

fuze

mpsx

ckod

vycn

uxam

aimr

yblq

ptgx

loth

orva

vybn

dhkq

11

rwzj

dhmq

zcmr

osxc

fimr

uxak

orwb

ailq

swbk

wzdi

gjot

afin

12

jmpw

euxc

knsx

loty

uxch

dhkp

cgjo

gjns

knrw

mqev

wzdj

qdgx

13

kpsx

knqg

aehn

ilot

xakp

xadi

mptg

vybg

gvze

ptyi

rduz

bfin

14

aein

qdgx

zehm

adgl

mpdu

gvaf

zcmr

ilot

zckp

gjot

pthy

nqth

15

uxch

ailq

xadi

wbej

ails

lpsg

ruzj

ilot

impu

hwbg

jmpd

ilot

16

mrdu

vyin

xadk

sehy

svyk

yblq

vzcm

bjnt

nqua

bjmr

psxc

nqev

17

akns

jnsx

seva

jnrf

adin

twak

impu

mquz

mrfx

swbk

swzk

ruyi

18

nqva

dglq

imqe

zjnr

nqth

rvyi

knqv

ehlp

ilqe

fjmr

chkp

ybel

19

mrdv

ydgl

txcl

ruyj

jnsg

qtyi

mpsx

hwze

psgx

mpsx

vaej

jorf

20

rwam

hwbg

bfio

hlqu

mrva

orvb

adhl

tfwb

cfip

ruxc

xaek

aeho

21

qthy

rehz

xadi

ybel

ejmr

bejo

ychl

jmpd

fils

mpuz

nrfv

fuxc

22

yblq

bjot

tgwb

dhwb

mqeu

reuz

xafk

korf

nqva

psxc

ckpd

ruxc

23

ilqe

uybm

sehy

jmpd

zjmr

wzcm

dgva

jmpf

qvyi

ruxc

svak

uxch

24

rehy

gjmt

knsg

xbjo

orwb

rtyc

ilpt

wzjo

xain

svzj

fkns

nqdv

25

qtfw

zehm

psxc

pswb

bejo

gjmr

ilqe

wzjo

losy

uxcm

ilqv

lpsy

26

lorf

xcgl

knsx

zehm

dgwc

behm

ilqe

mpsx

loty

zcmr

psxc

hxbg

27

jmqv

clqd

mpdu

dgxc

twbl

zcmr

loty

uxch

dglq

nrva

lpdu

ybfk

28

hlou

lorh

qduz

orfw

hkou

lorf

vybl

sfuz

xadi

ruzj

psgx

nqev

29

zcgl

zclr

xaip

psgx

uxch

uxbk

jnqv

rdgx

ehxb

svzk

nqev

orvb

30

tfuz

uxbl

qvyi

uxaf

rdgx

uxbm

jmpd

nrdu

loth

quxc

ilqe

fwze

31

jmpd

orwb

kosx

aehm

vzjn

pswb

pthx

aimr

seva

puxc

bkpt

blot

32

orub

kpsx

losy

ehmr

bfjo

zchm

orfw

uxak

jnsf

knqv

twal

oruz

 

The subdomain is prefixed to free Dynamic DNS providers such as freedynamicdns[.]org to serve as command-and-control infrastructure for data retrieval and exfiltration. We have precalculated the domains and reported them to the DDNS service provider for a takedown action, but it has not yet responded to our request. Our Python code implementation of Grandoreiro’s DGA is available here:

18703_picture7

Figure 7. Process of Grandoreiro's domain generation algorithm

Conclusion

In this recent campaign, Grandoreiro consistently exhibited a variety of tricks to evade detection, such as code obfuscation, binary padding, and the use of MSI's custom action to execute its payload, to name a few. The re-emergence of the Grandoreiro banking trojan indicates that it evolved and remains a threat in the Latin American and Portuguese/Spanish-speaking countries.

IoCs

URLs:

hxxp[://]belfaro[.]com[.]br/admin/PROCESSO-02028[.]82655[.]2019[.]550[.]pdf

hxxps[://]belfaro[.]com[.]br/admin/nota[.]php?file=docprocesso27032022[.]zip

hxxp[://]167[.]114[.]43[.]27:4433/mrrrpx2503[.]zip

167[.]114[.]88[.]99 (C2 IP Address)

iuc1[\da-z]{11}\.freedynamicdns\.org (C2 DGA)

Files:

SHA1: 1e81d73ff946560692a01c38649227897339dd5a

docprocesso27032022i512l3j0i271l2.3130j0j15&sourceid=chrome&ie=UTF-8.msi (downloader)

SHA1: ff908727cc1b5335e541fbcd80a327565f308bc7

dbghelp.dll (Grandoreiro banking trojan)

 

References

Grandoreiro Decryptor: https://github.com/SpiderLabs/Grandoreiro-decryptor/blob/main/grandoreiro_dga_gen.py

Grandoreiro DGA code: https://github.com/SpiderLabs/Grandoreiro-decryptor/blob/main/grandoreiro_string_decryptor.py

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