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

Static analysis result for SHA-256 697c77dfb6b077d0…

MALICIOUS

Office (OOXML) / .XLSX

162.5 KB Created: 2021-10-04 08:20:54 UTC Authoring application: Microsoft Excel 12.0000
MD5: 58514092e1e6d885d966c5096520826b SHA-1: b700184bef1ade35f960e29d8504be637eca4652 SHA-256: 697c77dfb6b077d0b44c7665607db612b7fa92006b78aca8ff422fc2dfcf6a0a
120 Risk Score

Malware Insights

MITRE ATT&CK
T1059.005 Visual Basic T1203 Exploitation for Client Execution

The file contains Excel 4.0 macros, which are known to be used for malicious purposes. The macros were reassembled from split formulas and executed a command to launch 'mshta' with a specific RTF file, indicating a likely attempt to download and execute a second-stage payload. The specific command observed is 'wmic process call create 'mshta C:\ProgramData\FXIusWBj.rtf''.

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
a859339f36a8a605b989ed6c7e45f5d7245ad616954ef3fe5d2741904571fd91		 xlm-macrosheet OOXML XLM macro sheet: xl/macrosheets/sheet1.bin 710958 bytes

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