Office (OOXML) / .XLSX malware analysis — malicious · 6e815471cf53862d

MALICIOUS

Office (OOXML) / .XLSX

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

MD5: 78752e96bec579d6449871ad074d9060 SHA-1: 4b1428c493bbdfb41db121fc92efd7176502b691 SHA-256: 6e815471cf53862dec85aab935c32ab03fe37b669672e44c7a61d47e6506f346

120 Risk Score

Malware Insights

MITRE ATT&CK

T1059.005 Visual Basic T1203 Exploitation for Client Execution

The file is an Excel spreadsheet containing Excel 4.0 macros. These macros are designed to reassemble a payload and execute it using 'mshta C:\ProgramData\excel.rtf'. This indicates the file's purpose is to download and execute a secondary malicious payload.

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` `c2ef92769e83bc07aecd4f98e20882c7f7925e5d0185bb22f8baa350adb506e7`	xlm-macrosheet	OOXML XLM macro sheet: xl/macrosheets/sheet1.bin	564426 bytes

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