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

Static analysis result for SHA-256 7481bbb875d21794…

MALICIOUS

Office (OOXML) / .XLSX

235.0 KB Created: 2021-02-03 15:28:44 UTC Authoring application: Microsoft Excel 16.0300
MD5: 41f51884195209158472179655e3d56c SHA-1: a781a2688f1ba8ece55c7bbe36f95c0d75628f72 SHA-256: 7481bbb875d21794846f7bc3b3ebbecec8c4bc058d6ae2aea60bff793cecb2cb
180 Risk Score

Malware Insights

MITRE ATT&CK
T1059.005 Visual Basic

The file is an Excel XLSX containing an embedded Excel 4.0 macro sheet, 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 and the downloader signature strongly suggest the file's purpose is to fetch and execute additional malicious content.

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
ca6bdc3f1db291407ed33a635f3774886fd9ea2fb3d4212a620b9d817812bb96		 xlm-macrosheet OOXML XLM macro sheet: xl/macrosheets/intlsheet1.bin 141726 bytes

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