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

Static analysis result for SHA-256 3aaf6bead6dd9378…

MALICIOUS

Office (OOXML) / .XLSX

235.0 KB Created: 2021-02-03 15:28:44 UTC Authoring application: Microsoft Excel 16.0300
MD5: fc7972c7162cd5c9d5d603cb0e596df1 SHA-1: 9a40872efc1bfd1435fb3760c61d492d73411658 SHA-256: 3aaf6bead6dd93784618497665724c21869be54131009f852afa4a2f672183c5
180 Risk Score

Malware Insights

MITRE ATT&CK
T1059.005 Visual Basic for Applications

The sample is an Excel file containing Excel 4.0 macros, as indicated by the OOXML_XLM_MACROSHEET and OOXML_XLSB_INTL_MACROSHEET_IN_XLSX heuristics. ClamAV detection confirms it as a downloader. The extracted macro sheet content is heavily obfuscated, but the presence of XLM macros strongly suggests a malicious downloader attempting to execute a second-stage payload. The specific macro sheet name 'xlm_sheet_00.bin' and the ClamAV detection name 'Xls.Downloader.DarkTheme122101-9913615-0' are the primary indicators.

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
5b5828f1601a1e1e99d79c1c1a8e2deae809db7189b0f478c8635956a9f27946		 xlm-macrosheet OOXML XLM macro sheet: xl/macrosheets/intlsheet1.bin 141726 bytes

Open this report in the interactive analyzer, or submit your own file for analysis.