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

Static analysis result for SHA-256 056809e596895320…

MALICIOUS

Office (OOXML) / .XLSX

328.4 KB Created: 2021-03-16 13:14:06 UTC Authoring application: Microsoft Excel 16.0300
MD5: d17e780a23c19a5ce5c2a0d4abc19b55 SHA-1: d3d73e9f08cb6d880dc163149d352a197dac7300 SHA-256: 056809e596895320397378f7f3ff4958107e48f4890a960229dcfbc32b7379b7
120 Risk Score

Malware Insights

MITRE ATT&CK
T1059.005 Visual Basic T1105 Ingress Tool Transfer

The sample contains Excel 4.0 macros, identified by the OOXML_XLM_MACROSHEET heuristic. These macros are designed to reassemble a payload using CHAR() and split formulas, as indicated by OOXML_XLM_REASSEMBLED_PAYLOAD. The script explicitly calls 'certutil -f -decode .d0 .0 && rundll32 .0,DF', which is a common technique to decode and execute a downloaded payload, likely from a remote source.

Heuristics 2

  • 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.
  • XLM payload reassembled from CHAR()/split formulas critical OOXML_XLM_REASSEMBLED_PAYLOAD
    An Excel 4.0 macro sheet builds its payload inside the formula token stream by concatenating per-character CHAR() calls and string fragments, so no WinAPI name, shell command, or URL is ever contiguous in the .bin for a literal-bytes scan to find. Reassembling the formulas recovered download/execute API names, LOLBin commands (regsvr32/rundll32/mshta/wmic/powershell), or a payload URL — the de-obfuscated download-and-run kill chain.

Extracted artifacts 1

Files carved from inside the sample during analysis.

FilenameKindSourceSize
xlm_sheet_00.bin
03134a02221733c260eb40ad667353117338d28ccfb1b88e9da3e39ea8ff3f2f		 xlm-macrosheet OOXML XLM macro sheet: xl/macrosheets/sheet1.bin 1995 bytes

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