Tag: XLS

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")