Malicious Office (OOXML) / .XLSX — malware analysis report

Static analysis result for SHA-256 3ad6c74adeb3461e…

MALICIOUS

Office (OOXML) / .XLSX

235.1 KB Created: 2021-02-03 15:28:44 UTC Authoring application: Microsoft Excel 16.0300
MD5: 08ccc0a5851a5f2ff6032173fe9627b1 SHA-1: 1beead8a9b2477e32d665029bbf0e92d3482ae32 SHA-256: 3ad6c74adeb3461e006f9eea820804cd7006664bc3472db0981381aab525d10e
180 Risk Score

Malware Insights

MITRE ATT&CK
T1059.005 Visual Basic

The sample is an Excel file containing hidden Excel 4.0 macros, identified by multiple critical heuristics. The ClamAV signature 'Xls.Downloader.DarkTheme122101-9913615-0' indicates a downloader functionality. Although the macro content is truncated, the presence of XLM macros strongly suggests an intent to download and execute a second-stage payload.

Heuristics 3

  • Excel 4.0 macro sheet (1 sheet(s)) critical OOXML_XLM_MACROSHEET
    Spreadsheet contains an Excel 4.0 (XLM) macro sheet — XLM was a major Office malware vector during 2020-2022 and evaded many VBA-focused controls before Microsoft tightened XLM defaults. Even legitimate XLM use is rare in modern workbooks. The macro sheet is stored as XLSB/BIFF12 binary content, which many XML-only OOXML scanners miss.
  • XLSB international XLM macro sheet hidden in .xlsx critical OOXML_XLSB_INTL_MACROSHEET_IN_XLSX
    OOXML package is named .xlsx but contains XLSB workbook parts and an international Excel 4.0 macro sheet. This hides XLM macro execution from scanners that trust the extension or only inspect XML worksheet parts. The technique is macro execution, not a document-parser CVE.
  • ClamAV: Xls.Downloader.DarkTheme122101-9913615-0 critical CLAMAV_DETECTION
    ClamAV detected this file as malware: Xls.Downloader.DarkTheme122101-9913615-0

Extracted artifacts 1

Files carved from inside the sample during analysis.

FilenameKindSourceSize
xlm_sheet_00.bin
abc1c20f70d49d66e0ac887cab544b14c8e59404956c4baf9e15d5bd9464e7da		 xlm-macrosheet OOXML XLM macro sheet: xl/macrosheets/intlsheet1.bin 141726 bytes

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