Office (OOXML) / .XLSX malware analysis — malicious · 021eac36e976fa6a

MALICIOUS

Office (OOXML) / .XLSX

277.5 KB Created: 2021-10-27 10:31:49 UTC Authoring application: Microsoft Excel 12.0000

MD5: 31e9f1ca57cd3303577afbe0ab277034 SHA-1: 725df06354d1d99e637146ef760a62c29b6c565e SHA-256: 021eac36e976fa6a3d107280dddd3d8fc29c710a1dee8c02d925c0281142b535

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 used to reassemble and execute a payload. The reassembled payload is 'mshta C:\ProgramData\excel.rtf', indicating that the macro attempts to download and execute a second-stage payload using mshta.exe. This is a common technique for delivering malware.

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.

Filename	Kind	Source	Size
`xlm_sheet_00.bin` `46aacd695fd17182bd60923542c8cb326479a1baa06b65e282f4dcc40031a98f`	xlm-macrosheet	OOXML XLM macro sheet: xl/macrosheets/sheet1.bin	598606 bytes

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