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

Static analysis result for SHA-256 0c38c5a41324970f…

MALICIOUS

Office (OOXML) / .XLSX

78.2 KB Created: 2021-10-27 10:31:49 UTC Authoring application: Microsoft Excel 12.0000
MD5: 265cbfc932d048b81055c362b67f5cc7 SHA-1: 83b134da2aeb48a8ced9ad05707ea2ca6fc2a7df SHA-256: 0c38c5a41324970fbadd32c01a880e9ab3ac73b62fab60df1ed8fbabc141ddd5
182 Risk Score

Malware Insights

MITRE ATT&CK
T1059.005 Visual Basic T1203 Exploitation for Client Execution

The sample contains Excel 4.0 macros that are designed to execute a command. This command uses `mshta` to download and execute a file named `fnsfunsfgrgnkjfsgnd.rtf` from the ProgramData directory. The macros also contain URLs pointing to Discord CDN, likely serving as download locations for the second-stage payload. The execution of `mshta` with a downloaded file indicates a clear intent to download and run a malicious 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/908712469780889674/909803220686217276/tpXVUKkjWmfkilljews.mkv
    • https://cdn.discordapp.com/attachments/908712469780889674/909803337661157376/XkFsKbdJkilljews.mkv
    • https://cdn.discordapp.com/attachments/908712469780889674/909803954207088640/XPvyzkilljews.mkv

Extracted artifacts 1

Files carved from inside the sample during analysis.

FilenameKindSourceSize
xlm_sheet_00.bin
ba5966ed873fb66d57eeaeb7c1f8cfe37e65d57564d35cc153de7e6d3aaf1bc0		 xlm-macrosheet OOXML XLM macro sheet: xl/macrosheets/sheet1.bin 4644 bytes

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