Office (OOXML) / .XLSX malware analysis — malicious · 5f9172d1193e0f8a

MALICIOUS

Office (OOXML) / .XLSX

292.2 KB Created: 2006-09-16 00:00:00 UTC Authoring application: Microsoft Excel 14.0300

MD5: c52615f1bb93f23b2ac1d632517b2db9 SHA-1: e0d5a0bd5949619c60b0e180fdf47913c34aaaa9 SHA-256: 5f9172d1193e0f8aabcbc6dff60474a0f9f7914a96424ddcffb26a66e685031b

242 Risk Score

Malware Insights

MITRE ATT&CK

T1059.005 Visual Basic T1204.002 Malicious File T1105 Ingress Tool Transfer

The file contains Excel 4.0 macros, which are known to be used for malicious purposes. These macros are designed to download and execute a secondary payload from the URL http://nickseth.com/ds/12.gif. The use of WinAPI functions like DownloadToFileA and ShellExecuteA within the macros indicates a clear intent to fetch and run external code.

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 http://nickseth.com/ds/12.gif

Extracted artifacts 1

Files carved from inside the sample during analysis.

Filename	Kind	Source	Size
`xlm_sheet_00.bin` `1ce9db3f9b489afd3f5c5403f079ac3b17c262c60073e800d0829403d24a8117`	xlm-macrosheet	OOXML XLM macro sheet: xl/macrosheets/intlsheet1.bin	197394 bytes

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