YARA · Signature Base (Florian Roth / Nextron)

SIGNATURE_BASE_SUSP_OBFUSC_Indiators_XML_Officedoc_Sep21_1

Detects suspicious encodings in fields used in reference files found in weaponized MS Office documents

Suspicious / huntingSignature Base (Florian Roth / Nextron)Detection Rule License 1.1Compiles with YARA-X

Author: Florian Roth (Nextron Systems)
Date: 2021-09-18
Modified: 2023-12-05
Score / Quality: 65 / 85

What this rule detects

Detects suspicious encodings in fields used in reference files found in weaponized MS Office documents

It defines 3 patterns (3 text, 0 hex, 0 regex).

Summary generated from the rule's metadata and condition.

Rule source

Download .yar

susp-obfusc-indiators-xml-officedoc-sep21-1.yarDetection Rule License 1.1

rule SIGNATURE_BASE_SUSP_OBFUSC_Indiators_XML_Officedoc_Sep21_1 : WINDOWS CVE FILE
{
	meta:
		description = "Detects suspicious encodings in fields used in reference files found in weaponized MS Office documents"
		author = "Florian Roth (Nextron Systems)"
		id = "ffcaf270-f574-5692-90e5-6776c34eb71b"
		date = "2021-09-18"
		modified = "2023-12-05"
		reference = "https://twitter.com/sudosev/status/1439205606129377282"
		source_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/yara/expl_cve_2021_40444.yar#L64-L81"
		license_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/LICENSE"
		hash = "13de9f39b1ad232e704b5e0b5051800fcd844e9f661185ace8287a23e9b3868e"
		hash = "84674acffba5101c8ac518019a9afe2a78a675ef3525a44dceddeed8a0092c69"
		logic_hash = "fc8f0dd02460ab8f8cc6717c66eba51e6ed74881a48e92fd0bf978467dfb40e3"
		score = 65
		quality = 85
		tags = "WINDOWS, CVE, FILE"

	strings:
		$h1 = "<?xml " ascii wide
		$xml_e = "Target=\"&#" ascii wide
		$xml_mode_1 = "TargetMode=\"&#" ascii wide

	condition:
		filesize < 500KB and $h1 and 1 of ( $xml* )
}

Strings (3 patterns)

Identifier	Type	Value	Modifiers
$h1	text	"<?xml "	ascii wide
$xml_e	text	"Target=\"&#"	ascii wide
$xml_mode_1	text	"TargetMode=\"&#"	ascii wide

Condition

filesize < 500KB and $h1 and 1 of ( $xml* )

In plain words (Heuristic)

The file must be smaller than 500KB (a cheap guard that skips large files).
At least 1 patterns in ( $xml* ) must match.
Pattern $h1 must be present.

Test this rule in your browser

Drop a file to scan it with this rule using YARA-X compiled to WebAssembly. Nothing is uploaded.

Drop a file here or click to chooseScanned locally with YARA-X. Files up to 200 MB.

Open in editor

References

https://twitter.com/sudosev/status/1439205606129377282

Sample hashes

Hashes of samples the author lists in the rule's meta (look them up in your threat-intelligence platform).

13de9f39b1ad232e704b5e0b5051800fcd844e9f661185ace8287a23e9b3868e
84674acffba5101c8ac518019a9afe2a78a675ef3525a44dceddeed8a0092c69

Licence & attribution

Author: Florian Roth (Nextron Systems)

Source repository: Signature Base (Florian Roth / Nextron) @ 94a1c48d7a

Licence: Detection Rule License 1.1 (repository licence)

Original rule

Mirrored via YARA Forge 20260927, which prefixes rule names with the source and adds normalised meta fields.

Rule id: ffcaf270-f574-5692-90e5-6776c34eb71b

Tags:WINDOWS CVE FILE

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