Video: 3-Minutes Self-Decrypting Excel Malware Analysis

The script below shows how to brute-force the decryption of the code. It is not necessary, as in the video we calculate the correct value of the W86 cell manually, but the snippet might be useful for future samples.

from Pro.SiliconSpreadsheet import *
from Pro.UI import proContext

v = proContext().findView("Analysis [selfdecrxls]")
if v.isValid():
    view = SiliconSpreadsheetWorkspaceView(v)
    emu = view.getExcelEmulator()
    formula = """CHAR(A1-W86)&CHAR(A2-W86)&CHAR(A3-W86)&CHAR(A5-W86)&CHAR(A6-W86)&CHAR(A7-W86)&CHAR(A8-W86)&CHAR(A9-W86)&CHAR(A11-W86)&CHAR(A12-W86)&CHAR(A13-W86)&CHAR(A14-W86)&CHAR(A16-W86)&CHAR(A17-W86)&CHAR(A18-W86)&CHAR(A19-W86)&CHAR(A20-W86)&CHAR(A22-W86)&CHAR(A23-W86)&CHAR(A24-W86)&CHAR(A25-W86)&CHAR(A27-W86)&CHAR(A28-W86)&CHAR(A29-W86)&CHAR(A30-W86)&CHAR(A32-W86)&CHAR(A33-W86)&CHAR(A34-W86)&CHAR(A35-W86)&CHAR(A36-W86)&CHAR(A38-W86)&CHAR(A39-W86)&CHAR(A40-W86)&CHAR(A41-W86)&CHAR(A42-W86)&CHAR(A44-W86)&CHAR(A45-W86)&CHAR(A46-W86)&CHAR(A47-W86)&CHAR(A48-W86)"""
    cell_index = SiliconSpreadsheetUtil.cellIndex("'rZVUfQRQoV'!W86")
    ws = emu.getWorkspace()
    sheet = ws.getSheet(ws.sheetIndexFromName(cell_index.sheet))
    for i in range(1, 270):       
        sheet.addCell(cell_index.column, cell_index.row, SiliconSpreadsheetValueType_Number, str(i))
        res = emu.evaluate(formula, cell_index)
        print(str(i) + ":", res.getValue())
else:
    print("error: couldn't find view")

Video: 1.5-Minutes QakBot Excel Malware Analysis (2nd sample)

The script extends the Silicon Excel Emulator by implementing th “FORMULA” function:

from Pro.SiliconSpreadsheet import *
from Pro.UI import proContext

class EmulatorHelper(SiliconExcelEmulatorHelper):

    def __init__(self):
        super(EmulatorHelper, self).__init__()
        
    def evaluateFunction(self, emu, ctx, opts, depth, e):
        function_name = e.toString()
        if function_name == "FORMULA":
            if emu.expectedArguments(e, 2, 2):
                ve = emu.argToValue(ctx, opts, depth, e, 0)
                v = emu.valueToSpreadsheetValue(ve)
                idxstr = emu.argToValue(ctx, 0, depth, e, 1).toString()
                idx = SiliconSpreadsheetUtil.cellIndex(idxstr)
                print("FORMULA:", idxstr, "=", emu.valueToString(ve))
                # add the cell to the sheet
                ws = emu.getWorkspace()
                sheet_idx = ws.sheetIndexFromName(idx.sheet if idx.sheet else ctx.idx.sheet)
                sheet = ws.getSheet(sheet_idx)
                sheet.addCell(idx.column, idx.row, v.type, v.value)
                return SiliconExcelEmulatorValue(SiliconSpreadsheetValueType_Null, 0)
        return SiliconExcelEmulatorValue()

v = proContext().findView("Analysis [qakbot_xls_2]")
if v.isValid():
    view = SiliconSpreadsheetWorkspaceView(v)
    helper = EmulatorHelper()
    emu = view.getExcelEmulator()
    emu.setHelper(helper)
else:
    print("error: couldn't find view")

Video: 2-Minutes QakBot Excel Malware Analysis

The script extends the Silicon Excel Emulator by implementing the “NOW” and “FORMULA.FILL” functions:

from Pro.SiliconSpreadsheet import *
from Pro.UI import proContext

class EmulatorHelper(SiliconExcelEmulatorHelper):

    def __init__(self):
        super(EmulatorHelper, self).__init__()
        
    def evaluateFunction(self, emu, ctx, opts, depth, e):
        function_name = e.toString()
        if function_name == "FORMULA.FILL":
            if emu.expectedArguments(e, 2, 2):
                ve = emu.argToValue(ctx, opts, depth, e, 0)
                v = emu.valueToSpreadsheetValue(ve)
                idxstr = emu.argToValue(ctx, 0, depth, e, 1).toString()
                idx = SiliconSpreadsheetUtil.cellIndex(idxstr)
                print("FORMULA.FILL:", idxstr, "=", emu.valueToString(ve))
                # add the cell to the sheet
                ws = emu.getWorkspace()
                sheet_idx = ws.sheetIndexFromName(idx.sheet if idx.sheet else ctx.idx.sheet)
                sheet = ws.getSheet(sheet_idx)
                sheet.addCell(idx.column, idx.row, v.type, v.value)
                return SiliconExcelEmulatorValue(SiliconSpreadsheetValueType_Null, 0)
        elif function_name == "NOW":
            return SiliconExcelEmulatorValue(SiliconSpreadsheetValueType_Number, "44249.708602")
        return SiliconExcelEmulatorValue()

v = proContext().findView("Analysis [qakbot_xls_0]")
if v.isValid():
    view = SiliconSpreadsheetWorkspaceView(v)
    helper = EmulatorHelper()
    emu = view.getExcelEmulator()
    emu.setHelper(helper)
else:
    print("error: couldn't find view")

Cerbero Suite 4.5 is out!

Because of some bureaucratic slowdowns this release took longer than usual. On the upside, it comes packed with news!

We have added support for the XLSB format (via pyxlsb2), so that now Cerbero Suite decompiles both XLS and XLSB formulas. Not only that, Cerbero Suite now previews spreadsheets even better than Microsoft Excel does!

But even more exciting, we have introduced the Silicon Excel Emulator to emulate Microsoft Excel formulas.

In order to emulate a formula, it’s enough to select it in the spreadsheet and to press “Ctrl+E” (or using the context menu).

While the emulator doesn’t support all the functions available in Excel, we made it extremely simple to extend it from Python. When a function is not supported, the emulator simply prints out its arguments:

warning: unimplemented function 'CALL'
    arg_0: "Shell32"
    arg_1: "ShellExecuteA"
    arg_2: "JJCCCCJ"
    arg_3: 0
    arg_4: "Open"
    arg_5: "C:\ProgramData\nCjBmqQ.exe"
    arg_6: 
    arg_7: 0
    arg_8: 0

In most cases, this is enough to understand what’s happening. When it isn’t and we need something more, we can easily hook into the emulator engine via Python and extend it:

from Pro.SiliconSpreadsheet import *
from Pro.UI import proContext

class EmulatorHelper(SiliconExcelEmulatorHelper):

    def __init__(self):
        super(EmulatorHelper, self).__init__()
        
    def evaluateFunction(self, emu, ctx, opts, depth, e):
        function_name = e.toString()
        print(function_name)
        return SiliconExcelEmulatorValue()

v = proContext().findView("Analysis [file name]")
if v.isValid():
    view = SiliconSpreadsheetWorkspaceView(v)
    helper = EmulatorHelper()
    emu = view.getExcelEmulator()
    emu.setHelper(helper)
else:
    print("error: couldn't find view")

This little snippet of Python code does nothing else than to hook the Silicon Excel Emulator being used in a spreadsheet preview and to log every function being called. We can, however, just as easily modify the behavior of a function or implement its functionality by returning a valid value. We’ll show you how to do this in future articles and videos.

The new version of Cerbero Suite comes also with other improvements. For instance, it is now possible to edit the prototype of functions in the decompiler by pressing “Y”.

And you can now edit the prototype of a function in the same way also in the native UI for Ghidra.

This is the complete list of news for version 4.5:

– added spreadsheet preview for XLS and XLSB files
– added XLSB format support
+ added Microsoft Excel macro emulator
+ added editing of function prototypes in the decompiler
+ added editing of function prototypes in the native UI for Ghidra
+ improved Ghidra native UI
– improved XLS macro decompiler
– improved XLS format support
– exposed XLS classes to Python
– exposed new cell table view to Python
– minor improvements

We’ll soon publish material to demonstrate how to reverse engineer malicious Excel documents.

Happy hacking!

Cerbero Suite 4.4 is out!

This time it took a bit longer for the release as we’re undergoing some organizational changes, but we’ll make up for it in the upcoming months!

This is the list of news for version 4.4:

– added Excel macro decompiler
– added word highlighting to text editor
– added password brute-forcers
+ improved C++ support in the decompiler
+ improved disassembly view
– improved detection of Excel malware

Excel Macro Decompiler

The major news of this release is the addition of a decompiler for Excel macros. We’ll continue to build and improve upon this feature in the upcoming releases!

C++ Decompiler Improvements

We have improved C++ support in the decompiler. A small improvement makes the code much easier to read!

Text Editor: Word Highlighting

Cerbero Suite now provides the word highlighting feature available in the disassembly/decompiler also in the text editor. This makes the analysis of VBA macros, JavaScript and other managed languages much easier.

Password Brute-Forcers

“infected” is such an ubiquitous password for sharing Zip archives containing malware that starting with this release you don’t even have to enter it when opening a file.

We’ve also added optional common passwords dictionary-based brute-forcers as a bonus. You can activate them from the extensions page.

More is coming soon…

Happy hacking!

Video: In-Depth Obfuscated VBA Analysis

This script concatenates strings such as “a” + “b”:

from Pro.UI import *
import re

ctx = proContext()
v = ctx.getCurrentView()
if v.isValid() and v.hasSelection():
    s = v.getSelectedText().replace('" &', '" +')
    s = eval(s)
    v.setSelectedText('"' + s + '"')

This second script decrypts strings the same way as the “NobosMeik” function:

from Pro.UI import *
import base64

ctx = proContext()
v = ctx.getCurrentView()
if v.isValid() and v.hasSelection():
    s = v.getSelectedText()
    s = base64.b64decode(s)
    key = b"versache"
    s2 = bytearray(s)
    y = 0
    tire = lambda r, g: (r & ~g) | (~r & g)
    for x in range(len(s)):
        s2[x] = tire(s2[x], key[y])
        if y < len(key) - 1:
            y += 1
        else:
            y = 0
    print(s2)

Malicious Windows Link with Embedded Microsoft Cabinet

You can find the original analysis for this malware at malwarebytes. As a bonus, in the video we show how to improve the static analysis of the final payload by resolving API calls.

This script converts the decrypted blob hashes into a call index → api name dictionary.

blob = bytes([
    0x6B, 0x65, 0x72, 0x6E, 0x65, 0x6C, 0x33, 0x32, 0x2E, 0x64, 0x6C, 0x6C, 0x00, 0x31, 0x33, 0x38,
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    0x32, 0x36, 0x38, 0x39, 0x38, 0x32, 0x38, 0x34, 0x36, 0x00, 0x31, 0x39, 0x35, 0x34, 0x30, 0x32,
    0x32, 0x34, 0x38, 0x35, 0x00, 0x32, 0x33, 0x35, 0x36, 0x39, 0x38, 0x35, 0x39, 0x39, 0x00, 0x35,
    0x34, 0x31, 0x37, 0x35, 0x37, 0x33, 0x35, 0x39, 0x00, 0x31, 0x31, 0x34, 0x32, 0x32, 0x31, 0x30,
    0x39, 0x38, 0x30, 0x00, 0x2D, 0x33, 0x31, 0x34, 0x36, 0x35, 0x30, 0x32, 0x38, 0x00, 0x2D, 0x36,
    0x30, 0x39, 0x31, 0x37, 0x39, 0x32, 0x31, 0x00, 0x2D, 0x36, 0x34, 0x35, 0x33, 0x37, 0x30, 0x34,
    0x38, 0x39, 0x00, 0x35, 0x38, 0x36, 0x36, 0x30, 0x38, 0x39, 0x31, 0x36, 0x00, 0x38, 0x31, 0x33,
    0x35, 0x33, 0x35, 0x38, 0x36, 0x37, 0x00, 0x2D, 0x32, 0x31, 0x33, 0x31, 0x33, 0x31, 0x32, 0x31,
    0x35, 0x38, 0x00, 0x2D, 0x32, 0x31, 0x33, 0x30, 0x36, 0x30, 0x33, 0x32, 0x38, 0x34, 0x00, 0x31,
    0x35, 0x32, 0x37, 0x33, 0x39, 0x30, 0x30, 0x32, 0x00, 0x49, 0x70, 0x68, 0x6C, 0x70, 0x61, 0x70,
    0x69, 0x2E, 0x64, 0x6C, 0x6C, 0x00, 0x38, 0x30, 0x36, 0x34, 0x37, 0x33, 0x33, 0x35, 0x36, 0x00,
    0x43, 0x72, 0x79, 0x70, 0x74, 0x33, 0x32, 0x2E, 0x64, 0x6C, 0x6C, 0x00, 0x31, 0x34, 0x33, 0x34,
    0x31, 0x38, 0x33, 0x31, 0x38, 0x39, 0x00, 0x2D, 0x31, 0x32, 0x30, 0x38, 0x36, 0x30, 0x33, 0x32,
    0x00, 0x53, 0x68, 0x6C, 0x77, 0x61, 0x70, 0x69, 0x2E, 0x64, 0x6C, 0x6C, 0x00, 0x2D, 0x31, 0x31,
    0x36, 0x30, 0x32, 0x39, 0x30, 0x30, 0x37, 0x30, 0x00, 0x55, 0x72, 0x6C, 0x6D, 0x6F, 0x6E, 0x2E,
    0x64, 0x6C, 0x6C, 0x00, 0x2D, 0x37, 0x37, 0x36, 0x37, 0x36, 0x35, 0x30, 0x36, 0x34, 0x00, 0x6D,
    0x73, 0x76, 0x63, 0x72, 0x74, 0x2E, 0x64, 0x6C, 0x6C, 0x00, 0x31, 0x38, 0x30, 0x31, 0x31, 0x32,
    0x30, 0x31, 0x31, 0x35, 0x00, 0x31, 0x36, 0x35, 0x36, 0x39, 0x34, 0x35, 0x34, 0x39, 0x35, 0x00,
    0x31, 0x32, 0x36, 0x36, 0x38, 0x37, 0x31, 0x33, 0x34, 0x37, 0x00, 0x31, 0x32, 0x36, 0x32, 0x31,
    0x35, 0x32, 0x35, 0x36, 0x33, 0x00, 0x2D, 0x31, 0x34, 0x34, 0x31, 0x36, 0x31, 0x33, 0x32, 0x32,
    0x32, 0x00, 0x2D, 0x37, 0x31, 0x38, 0x33, 0x31, 0x36, 0x37, 0x33, 0x36, 0x00, 0x2D, 0x33, 0x34,
    0x35, 0x38, 0x31, 0x37, 0x37, 0x39, 0x38, 0x00, 0x2D, 0x33, 0x34, 0x37, 0x31, 0x39, 0x38, 0x35,
    0x32, 0x38, 0x00, 0x31, 0x38, 0x30, 0x37, 0x36, 0x35, 0x37, 0x39, 0x36, 0x38, 0x00, 0x31, 0x31,
    0x36, 0x30, 0x31, 0x39, 0x39, 0x30, 0x39, 0x38, 0x00, 0x2D, 0x39, 0x38, 0x37, 0x32, 0x38, 0x34,
    0x36, 0x30, 0x38, 0x00, 0x31, 0x31, 0x31, 0x32, 0x39, 0x31, 0x31, 0x38, 0x33, 0x32, 0x00, 0x2D,
    0x35, 0x32, 0x30, 0x37, 0x30, 0x31, 0x30, 0x33, 0x31, 0x00, 0x2D, 0x35, 0x32, 0x30, 0x37, 0x30,
    0x34, 0x37, 0x31, 0x31, 0x00, 0x00
    ])

from Pro.Core import *
from Pro.PE import *
from Pro.ccast import *

ror = lambda val, r_bits, max_bits: \
    ((val & (2**max_bits-1)) >> r_bits%max_bits) | \
    (val << (max_bits-(r_bits%max_bits)) & (2**max_bits-1))

def getAPIs(dllpath):
    apis = {}
    c = createContainerFromFile(dllpath)
    dll = PEObject()
    if not dll.Load(c):
        print("error: couldn't load dll")
        return apis
    ordbase = dll.ExportDirectory().Num("Base")
    functions = dll.ExportDirectoryFunctions()
    names = dll.ExportDirectoryNames()
    nameords = dll.ExportDirectoryNameOrdinals()
    n = functions.Count()
    it = functions.iterator()
    for x in range(n):
        func = it.next()
        ep = func.Num(0)
        if ep == 0:
            continue
        apiord = str(ordbase + x)
        n2 = nameords.Count()
        it2 = nameords.iterator()
        name_found = False
        for y in range(n2):
            no = it2.next()
            if no.Num(0) == x:
                name = names.At(y)
                offs = dll.RvaToOffset(name.Num(0))
                name, ret = dll.ReadUInt8String(offs, 500)
                apiname = name.decode("ascii")
                apis[apiname] = apiord
                apis[apiord] = apiname
                name_found = True
                break
        if not name_found:
            apis[apiord] = apiord
    return apis
    
def hash(name):
    x = 0
    for c in name:
        x = ror(x, 0xD, 32)
        x += ord(c)
    x = ror(x, 0xD, 32)
    return x
    
def hashAPIs(apis):
    hapis = {}
    for i, name in apis.items():
        hapis[hash(name)] = name
    return hapis
    
def walkBlob():
    i = 0
    idxs = {}
    pos = 0
    while i < len(blob):
        e = blob.find(b"\x00", i)
        if i == e:
            break
        s = blob[i:e].decode("ascii")
        i = e + 1
        if "." in s:
            dllname = s
            apis = getAPIs("C:\\Windows\\System32\\" + dllname)
            apis = hashAPIs(apis)
        else:
            x = dword(int(s))
            apiname = apis[x]
            idxs[pos] = apiname
            pos += 0x10
            print(apiname)
    return idxs
            
idxs = walkBlob()
print(idxs)

This script uses the index dictionary from the previous script to comment register-based call instructions in the disassembly with the resolved API name.

idxs = {0: 'VirtualAlloc', 16: 'Sleep', 32: 'CreateThread', 48: 'CloseHandle', 64: 'ReadFile', 80: 'CreateFileA', 96: 'WriteFile', 112: 'GetFileSize', 128: 'lstrlenA', 144: 'lstrlenW', 160: 'lstrcpyA', 176: 'lstrcpyW', 192: 'lstrcatA', 208: 'lstrcatW', 224: 'lstrcmpA', 240: 'lstrcmpW', 256: 'VirtualFree', 272: 'WaitForSingleObject', 288: 'TerminateThread', 304: 'GetTickCount', 320: 'FormatMessageA', 336: 'GetLastError', 352: 'EnterCriticalSection', 368: 'LeaveCriticalSection', 384: 'InitializeCriticalSection', 400: 'DeleteCriticalSection', 416: 'LocalFree', 432: 'MultiByteToWideChar', 448: 'WideCharToMultiByte', 464: 'GetComputerNameW', 480: 'GetModuleFileNameW', 496: 'GetCurrentProcessId', 512: 'GetLocalTime', 528: 'QueryPerformanceFrequency', 544: 'QueryPerformanceCounter', 560: 'IsWow64Process', 576: 'GetCurrentProcess', 592: 'GetVersionExA', 608: 'GlobalFree', 624: 'VirtualFreeEx', 640: 'DuplicateHandle', 656: 'DebugBreak', 672: 'CreateEventW', 688: 'DeviceIoControl', 704: 'DeleteFileA', 720: 'GetTempPathA', 736: 'GetTempFileNameA', 752: 'SetErrorMode', 768: 'FreeLibrary', 784: 'RtlGetNtVersionNumbers', 800: 'RtlNtStatusToDosError', 816: 'RtlDecompressBuffer', 832: 'RtlCompressBuffer', 848: 'RtlGetCompressionWorkSpaceSize', 864: 'NtQuerySystemInformation', 880: 'NtQueryObject', 896: 'PeekMessageW', 912: 'GetMessageW', 928: 'PostThreadMessageW', 944: 'send', 960: 'recv', 976: 'closesocket', 992: 'WSAStartup', 1008: 'socket', 1024: 'bind', 1040: 'listen', 1056: 'accept', 1072: 'connect', 1088: 'WSACleanup', 1104: 'inet_addr', 1120: 'inet_ntoa', 1136: 'htons', 1152: 'getaddrinfo', 1168: 'freeaddrinfo', 1184: 'WSAAddressToStringA', 1200: 'setsockopt', 1216: 'getsockopt', 1232: 'recvfrom', 1248: 'sendto', 1264: 'shutdown', 1280: 'WSAGetLastError', 1296: 'select', 1312: 'getpeername', 1328: 'CryptAcquireContextW', 1344: 'CryptDestroyHash', 1360: 'CryptCreateHash', 1376: 'CryptHashData', 1392: 'CryptGetHashParam', 1408: 'CryptDeriveKey', 1424: 'CryptEncrypt', 1440: 'CryptDecrypt', 1456: 'GetUserNameW', 1472: 'UuidCreate', 1488: 'WinHttpGetIEProxyConfigForCurrentUser', 1504: 'WinHttpOpen', 1520: 'WinHttpGetProxyForUrl', 1536: 'WinHttpCloseHandle', 1552: 'WinHttpConnect', 1568: 'WinHttpOpenRequest', 1584: 'WinHttpAddRequestHeaders', 1600: 'WinHttpSendRequest', 1616: 'WinHttpWriteData', 1632: 'WinHttpQueryDataAvailable', 1648: 'WinHttpQueryOption', 1664: 'WinHttpReceiveResponse', 1680: 'WinHttpReadData', 1696: 'WinHttpSetOption', 1712: 'WinHttpSetCredentials', 1728: 'WinHttpQueryAuthSchemes', 1744: 'GetAdaptersInfo', 1760: 'CryptBinaryToStringA', 1776: 'CryptStringToBinaryA', 1792: 'StrStrIA', 1808: 'URLDownloadToFileA', 1824: 'memset', 1840: 'memmove', 1856: 'memcpy', 1872: 'memcmp', 1888: '_wcsicmp', 1904: 'time', 1920: 'strstr', 1936: 'atoi', 1952: '_itow', 1968: 'srand', 1984: 'rand', 2000: '_wcsnicmp', 2016: 'sprintf', 2032: 'printf'}

from Pro.UI import proContext
from Pro.Carbon import *
from Pro.capstone import *
import re

def commentAPIs():
    md = Cs(CS_ARCH_X86, CS_MODE_64)
    v = proContext().getCurrentView()
    ca = v.getCarbon()
    db = ca.getDB()
    e = caASEntry()
    e.end = 0
    while db.getNextASEntry(e.end, e):
        if e.type_id != CarbonType_I_x64:
            continue
        buf = ca.read(e.start, e.end - e.start)
        insns = md.disasm(buf, 0)
        i = next(insns, None)
        if i.mnemonic != "call":
            continue
        print(i.mnemonic, i.op_str)
        if i.op_str.find("rip") != -1:
            continue
        j = i.op_str.find("+ 0x")
        if j == -1:
            continue
        idx = int(i.op_str[j+2:-1], 16)
        if idx > 0x1000:
            continue
        apiname = idxs.get(idx, None)
        if not apiname:
            continue
        c = caComment()
        if db.getComment(e.start, c) and c.text:
            continue
        c.address = e.start
        c.text = apiname
        db.setComment(c)
        print("   ", apiname)
    # update the view
    v.update()
    
commentAPIs()

Cerbero Suite 4.2 is out!

The new version of Cerbero Suite is out with the following news:

– added CAB format support
– added convert/from_array and convert/to_array filters
– added detection of external resources in Office documents
+ improved renaming of variables in the decompiler
– improved dev/array filter
– included OpenSSL executable
– fixed bugs

We’ll be soon publishing videos detailing some of these additions!

Cerbero Suite 4.1 is out!

The new version of Cerbero Suite is out with the following news:

+ added Carbon option to import PDB types into the project header
+ improved x86/x64 decompiler
– fixed bugs

The option in Carbon to import types into the header of a project when loading a PDB comes handy when a crash dump is associated to a structure.

In this case the BugCheck information tells us that at a specific address there’s an EPROCESS structure. By importing the types along with the symbols of ntoskrnl.exe, we import the correct EPROCESS structure to map onto memory.

Of course, Cerbero Suite contains kernel symbols for all version of Windows x86/x64, but by relying on the structures in the PDB we don’t need to load the correct Windows header from the headers folder: just import the types and then open the header of the project!