Office (OOXML) / .XLSX malware analysis — malicious · 6313cd1c4143ca67

MALICIOUS

Office (OOXML) / .XLSX

167.0 KB Created: 2021-10-27 10:31:49 UTC Authoring application: Microsoft Excel 12.0000

MD5: 1fa201b16f16b64958c8438d1f245c8c SHA-1: c869b6deb00b7217eff1dd591eef1e9df466e406 SHA-256: 6313cd1c4143ca671cc2524dbc4d07834810fd0d18aecc6e4bec976f8b3b8829

120 Risk Score

Malware Insights

MITRE ATT&CK

T1059.005 Visual Basic T1203 Exploitation for Client Execution

The file contains Excel 4.0 macros, which are known to be used for malicious purposes. The macros reassemble a command-line execution string, specifically 'mshta C:\ProgramData\excel.rtf', indicating an attempt to download and execute a second-stage payload from the specified path. This technique is often used to bypass security controls and deliver further malware.

Heuristics 2

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.

Extracted artifacts 1

Files carved from inside the sample during analysis.

Filename	Kind	Source	Size
`xlm_sheet_00.bin` `0af856f77bbd59757b49fd7692efd321a817931dd16a42a1a03b8d6f1fc44f6e`	xlm-macrosheet	OOXML XLM macro sheet: xl/macrosheets/sheet1.bin	397095 bytes

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