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

Static analysis result for SHA-256 10f82bd85569975b…

MALICIOUS

Office (OOXML) / .XLSX

74.4 KB Created: 2021-10-27 10:31:49 UTC Authoring application: Microsoft Excel 12.0000
MD5: 117910439e0a28e1cad934f210bad0a8 SHA-1: 29404e3e2a0c19cbb6666ec939f1a9bee3d1c4f0 SHA-256: 10f82bd85569975b336d53cb310f97a9220896dfd439ca4d0ba083a561613c03
182 Risk Score

Malware Insights

MITRE ATT&CK
T1059.005 Visual Basic T1203 Exploitation for Client Execution T1566.001 Spearphishing Attachment

The sample is an Excel file containing Excel 4.0 macros, which is a critical finding. These macros are designed to reconstruct and execute a command using 'mshta C:\ProgramData\excel.rtf', and they also contain multiple URLs pointing to Discord CDN. This indicates the file's primary purpose is to download and execute a second-stage payload, likely a malicious executable disguised as an audio file.

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.
  • 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.
  • URL reconstructed from XLM cell array (3 URLs) 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://cdn.discordapp.com/attachments/905044663310884887/905081080200196096/GiHzElGQEqFjqK.mp3
    • https://cdn.discordapp.com/attachments/905044663310884887/905079283905593425/WGjAYGyqcZeghy.mp3
    • https://cdn.discordapp.com/attachments/905044663310884887/905081303760777216/ElHnDAWjbJuW.mp3

Extracted artifacts 1

Files carved from inside the sample during analysis.

FilenameKindSourceSize
xlm_sheet_00.bin
ba273d85cdf177447041da505fa46c56e9866b7780c4bc64d83f497571ceea25		 xlm-macrosheet OOXML XLM macro sheet: xl/macrosheets/sheet1.bin 114381 bytes

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