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

Static analysis result for SHA-256 0cfe792c1c4ab4c5…

MALICIOUS

Office (OOXML) / .XLSX

291.1 KB Created: 2006-09-16 00:00:00 UTC Authoring application: Microsoft Excel 14.0300
MD5: 804366110a6fb52f714ac60480293885 SHA-1: 0d67222633001a0f78c5047b92484b1e807a8a35 SHA-256: 0cfe792c1c4ab4c53672aacfdf025b2591ead50a8380ee919a149e4c19a7974d
242 Risk Score

Malware Insights

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

The file contains Excel 4.0 macros, identified by critical heuristics indicating the presence of WinAPI and download strings. The macros reconstruct a URL, 'https://asimmistry.in/ds/121120.gif', which is likely used to download and execute a second-stage payload. The use of XLM macros and WinAPI calls for execution points to a downloader or droppper functionality.

Heuristics 5

  • 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.
  • 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 (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://asimmistry.in/ds/121120.gif

Extracted artifacts 1

Files carved from inside the sample during analysis.

FilenameKindSourceSize
xlm_sheet_00.bin
e39f99cce4c08293316c530f9ffbec398f6b293f40fe8f01025c72a98bb0721c		 xlm-macrosheet OOXML XLM macro sheet: xl/macrosheets/intlsheet1.bin 197215 bytes

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