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

Static analysis result for SHA-256 fc3359643e9b6461…

MALICIOUS

Office (OOXML) / .XLSX

26.4 KB Created: 2006-09-16 00:00:00 UTC Authoring application: Microsoft Excel 14.0300
MD5: f9a4904a85989b541da3e6a9b9c75c12 SHA-1: a6164cb3eb7f057003711170704c6f53dc9a909d SHA-256: fc3359643e9b6461f007ab1ea5ad6fb28597f25bd9896d83541e17b6ae8304b6
182 Risk Score

Malware Insights

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

The file contains Excel 4.0 macros, which are known to be used for malicious purposes. The macros contain strings such as URLDownloadToFileA, DownloadToFileA, and ShellExecuteA, indicating an intent to download and execute a second-stage payload from the URL https://structuredwiring.nrsecsecuritysystems.com/ds/3.gif. The presence of these WinAPI strings and the embedded URL strongly suggests a downloader attack pattern.

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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