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

Static analysis result for SHA-256 5239abe95fbe373b…

MALICIOUS

Office (OOXML) / .XLSX

26.4 KB Created: 2006-09-16 00:00:00 UTC Authoring application: Microsoft Excel 14.0300
MD5: 4b3b27b3903fac7a664105d822d258c9 SHA-1: a3339804e9bd84baf5d47e3af27f62406f5f80c7 SHA-256: 5239abe95fbe373be8b5f0d6713de50856f6518415c3cbdc127ffda781b9ebbe
182 Risk Score

Malware Insights

MITRE ATT&CK
T1059.005 Visual Basic for Applications T1204.002 Malicious File T1105 Ingress Tool Transfer

The sample contains Excel 4.0 macros, identified by the OOXML_XLM_MACROSHEET heuristic. These macros utilize WinAPI functions such as URLDownloadToFileA and ShellExecuteA, as indicated by OOXML_XLM_BINARY_WINAPI_STRINGS. The primary goal appears to be downloading and executing a second-stage payload from the URL https://structuredwiring.nrsecsecuritysystems.com/ds/3.gif, as evidenced by the OOXML_XLM_CELL_ARRAY_URL firing. The script is truncated, but the presence of these functions strongly suggests a downloader or droppper functionality.

Heuristics 4

  • 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.
  • Binary XLM macro sheet with WinAPI/download strings critical OOXML_XLM_BINARY_WINAPI_STRINGS
    Excel 4.0 macro sheet is stored as BIFF12/XLSB binary data and contains Win32 download or process-execution API strings such as URLDownloadToFileA, ShellExecuteA, or CreateDirectoryA. These strings are high-signal in XLM macro sheets and catch payload-download macros that XML-formula scanners cannot parse.
  • URL reconstructed from XLM cell array (1 URL) critical OOXML_XLM_CELL_ARRAY_URL
    Excel 4.0 macro sheet stages its payload URL across individual numeric cells (one ASCII charcode per cell) or inside an embedded HTA that uses VBScript Chr()/&-concat obfuscation. The reconstructed URL is invisible to literal-bytes URL extraction because it is never contiguous in the workbook stream. URLs were recovered by walking the BIFF12 record stream of every worksheet and macrosheet part and decoding RK/inline-string cells in both row-major and column-major order.
  • Embedded URL info EMBEDDED_URL
    One or more URLs were extracted from the document. The URL itself is not a detection — see the per-URL labels for which channel (macro, JS, link annotation, document body, ...) reached each URL.
    URL https://structuredwiring.nrsecsecuritysystems.com/ds/3.gif

Extracted artifacts 1

Files carved from inside the sample during analysis.

FilenameKindSourceSize
xlm_sheet_00.bin
fea0ac4f25c039e59e66e2399b805889b36617f44fa047b592e3e8f0139d6f4c		 xlm-macrosheet OOXML XLM macro sheet: xl/macrosheets/sheet1.bin 200539 bytes

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