Formbook Office (OOXML) / .XLSX malware analysis — malicious · f5c3bea5b81c221b

MALICIOUS

Office (OOXML) / .XLSX

586.8 KB Created: 2006-09-16 00:00:00 UTC Authoring application: Microsoft Excel 12.0000

MD5: 5de2e8bdb620804fd22d76f1e9fedf6e SHA-1: 942ce29cd8138a1594ee416debf753d8eaa71528 SHA-256: f5c3bea5b81c221bc8737bd8489154745c8d6644d7d19484218151f9a1c1f656

210 Risk Score

Malware Insights

Formbook · confidence 95%

MITRE ATT&CK

T1204 Malicious Link T1204.002 Malicious File T1566 Phishing T1566.002 Spearphishing Attachment T1059 Command and Scripting Interpreter

The file is an Excel document containing an embedded OLE object, specifically exploiting a vulnerability in the Equation Editor (CVE-2018-0798). This exploit is known to deliver malware, and ClamAV detection confirms it as a Formbook downloader. The presence of hidden sheets further suggests an attempt to conceal malicious activity.

Heuristics 7

Equation Editor OLE object high CVE related OLE_EQUATION_EDITOR

Embedded OLE object xl/embeddings/oleObject1.bin contains the Equation Editor CLSID, the legacy component exploited by CVE-2017-11882, CVE-2018-0802, and CVE-2018-0798.
CVE-2018-0798 — anomalous Equation Editor native stream high CVE likely CVE_2018_0798_EQUATION_NATIVE_ANOMALY

Embedded Equation Editor OLE data contains anomalous native stream bytes consistent with a CVE-2018-0798-style Equation Editor exploit. This is treated as likely CVE evidence because the Equation object is malformed and payload-like, but it does not match the exact public matrix-overflow byte signature.
ClamAV: Doc.Downloader.Formbook-bc97c1e0c33c3c93-9951465-0 critical CLAMAV_DETECTION

ClamAV detected this file as malware: Doc.Downloader.Formbook-bc97c1e0c33c3c93-9951465-0
Equation Editor object carries payload-like Ole10Native stream high OLE_EQUATION_OLE10NATIVE_PAYLOAD_ANOMALY

Embedded OLE object declares the Equation Editor CLSID but stores a large high-entropy Ole10Native stream with malformed package sizing. This is an exploit-shaped Equation/OLE payload container seen in malicious OOXML samples. It is not assigned to a specific CVE unless the MTEF/Equation Native primitive also matches.
Embedded OLE object medium OOXML_OLE_OBJECT

Document contains an embedded OLE object
Hidden worksheet (hidden) low OOXML_HIDDEN_SHEET

Excel workbook contains 1 hidden sheet(s) — hidden sheets are commonly used to conceal macro code, staging data, or intermediate payload construction
Suspicious extracted artifact info EXTRACTED_FILE_STATIC_TRIAGE

One or more files extracted from inside this sample matched static suspicious-content checks such as script obfuscation, encoded payload blobs, packed data, or execution/download terms.

Extracted artifacts 3

Files carved from inside the sample during analysis.

Filename	Kind	Source	Size
`ooxml_oleobject_00.bin` `40c382521dbb5c3a9f5008773eb8cc789cec3891702f3b3c00062d5e8ab45713`	ooxml-ole-object	OOXML embedded OLE part: xl/embeddings/oleObject1.bin	412160 bytes
Detection ClamAV: No threats found Obfuscation or payload: likely Carved artifact entropy is 8.00, consistent with packed or encrypted content.
`ooxml_oleobject_00_ole10native_00.bin` `64fc0ab3bf6b5087984e9c2dfdd43f443fcae760b06368197cd5cdd0bf5f0de6`	ole-package	OOXML xl/embeddings/oleObject1.bin Ole10Native stream: oLe10NatIve	406296 bytes
Detection ClamAV: No threats found Obfuscation or payload: likely Carved artifact entropy is 8.00, consistent with packed or encrypted content.
`emf_00.emf` `03b22f8ad84430f5c1064c38d88f66f2a224bf97ddc82a21aab379c6078b917d`	ooxml-emf	OOXML EMF part: xl/media/image2.emf	1099960 bytes

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