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

Static analysis result for SHA-256 5cd3f93797aacf9b…

MALICIOUS

Office (OOXML) / .XLSX

63.9 KB Created: 2021-10-27 10:31:49 UTC Authoring application: Microsoft Excel 12.0000
MD5: a4a611e83f47ee677752ad25f18133fd SHA-1: 184a09b6e21eb35c02b2b4256722a38c8c69140a SHA-256: 5cd3f93797aacf9bf24aff3cac5480616f0189165867ab27793ff558c74dc848
182 Risk Score

Malware Insights

MITRE ATT&CK
T1059.005 Visual Basic T1204.002 Malicious File

The sample contains Excel 4.0 macros that are designed to execute a command. This command, 'mshta C:\ProgramData\fnsfunsfgrgnkjfsgnd.rtf', indicates that the macro attempts to download and execute a file from a URL, likely one of the three provided Discord CDN URLs, and save it as 'fnsfunsfgrgnkjfsgnd.rtf' in the ProgramData directory. The use of XLM macros and the execution of an external file are strong indicators of a downloader or initial access payload.

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/908712469780889674/909803243465482240/NCHBmUluskkilljews.mkv
    • https://cdn.discordapp.com/attachments/908712469780889674/909803328551157790/iScyszkilljews.mkv
    • https://cdn.discordapp.com/attachments/908714245779247107/909805095997304892/BdEOjmNqMQkilljews.mkv

Extracted artifacts 1

Files carved from inside the sample during analysis.

FilenameKindSourceSize
xlm_sheet_00.bin
2ce63efd8990eda872a7b473f437fafc29a8d23d4d49f5e18df8a222b553d722		 xlm-macrosheet OOXML XLM macro sheet: xl/macrosheets/sheet1.bin 3518 bytes

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