Office (OOXML) / .XLSX malware analysis — malicious · c766c4e5bf9694ac

MALICIOUS

Office (OOXML) / .XLSX

225.1 KB Created: 2015-06-05 18:19:34 UTC Authoring application: Microsoft Excel 16.0300

MD5: 7ecd9bde17a647d33ed06ae4ddc6cc34 SHA-1: 68d1a9133f758a70ab188b55ee1078e0e0d0d718 SHA-256: c766c4e5bf9694aca28705d0563c4333de8137cac564dcb21aef57ab72b0c96d

120 Risk Score

Malware Insights

MITRE ATT&CK

T1059.005 Visual Basic

The file is an XLSX document containing an embedded Excel 4.0 macro sheet, indicated by the 'OOXML_XLM_MACROSHEET' and 'OOXML_XLSB_INTL_MACROSHEET_IN_XLSX' heuristics. While the macro content is heavily obfuscated and unreadable, the presence of these critical heuristics strongly suggests that the macro is designed to execute arbitrary commands. This is a common technique for downloading and executing a second-stage 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.
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.

Extracted artifacts 1

Files carved from inside the sample during analysis.

Filename	Kind	Source	Size
`xlm_sheet_00.bin` `19c3fa1c7c84b52da9ae7d1d1155e1d23ced704bedbcfc3ae03bc2224cfb1a4a`	xlm-macrosheet	OOXML XLM macro sheet: xl/macrosheets/intlsheet1.bin	1775 bytes

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