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

Static analysis result for SHA-256 73553bf99cefa188…

MALICIOUS

Office (OOXML) / .XLSX

83.6 KB Created: 2021-10-27 10:31:49 UTC Authoring application: Microsoft Excel 12.0000
MD5: 0a69381d2a07d9f0f02f63676fb6d4a0 SHA-1: 3097a4ca6d8c5162746a556c4fad112f51705f86 SHA-256: 73553bf99cefa188cb0cce3c7c164749adeb79b9573dc29d9b8ada4105c3530d
182 Risk Score

Malware Insights

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

The file contains Excel 4.0 macros that are configured to execute a command-line payload, mshta C:\ProgramData\excel.rtf. This payload is likely responsible for downloading and executing a second-stage payload from one of the three provided Discord URLs. The use of Excel 4.0 macros and the execution of an external command strongly suggest a malicious intent, likely for initial compromise via spearphishing.

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/905044663310884887/905080887631286272/UvDhPSqFHfAZ.mp3
    • https://cdn.discordapp.com/attachments/905044663310884887/905079625967878164/rQLknl.mp3
    • https://cdn.discordapp.com/attachments/905044663310884887/905080434889728050/jvBdNqGaERzpd.mp3

Extracted artifacts 1

Files carved from inside the sample during analysis.

FilenameKindSourceSize
xlm_sheet_00.bin
ebd6c4b94c99a0f88f4e2e1ade6f676ccb2b6b9e3f4accc77080c5b4b0b57fdd		 xlm-macrosheet OOXML XLM macro sheet: xl/macrosheets/sheet1.bin 136320 bytes

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