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

Static analysis result for SHA-256 0d0cce91ec054bbd…

MALICIOUS

Office (OOXML) / .XLSX

114.5 KB Created: 2021-02-03 15:28:44 UTC Authoring application: Microsoft Excel 16.0300
MD5: 4975bca10aec34429b817f9361ce1a02 SHA-1: ef4b95a59beb70b51aabca792de72610fb1d5477 SHA-256: 0d0cce91ec054bbd534a683608e7ac4a6d1137f0cfcadb055dba9c8be1945dc4
180 Risk Score

Malware Insights

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

The file contains Excel 4.0 macros, identified by critical heuristics indicating disguised relationships and reassembled payloads. The macros are designed to execute 'regsvr32 -s ', which is commonly used to download and execute a second-stage payload from a remote source. This indicates a downloader or dropper functionality.

Heuristics 3

  • 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.
  • Excel 4.0 macro sheet stored under disguised package path critical OOXML_XLM_DISGUISED_RELATIONSHIP
    OOXML package declares an xlMacrosheet relationship whose target is outside the canonical xl/macrosheets/ path. Excel follows the relationship type, while path-only scanners can miss the macro execution surface.
  • 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
3fd2c5ac9f1339b006d4b8558a66e92632e93690860205c61803635b864cb6a6		 xlm-macrosheet OOXML XLM macro sheet: xl/xls/sheet1.bin 1007414 bytes

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