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YARA · Signature Base (Florian Roth / Nextron)

SIGNATURE_BASE_EXPL_MAL_Maldoc_OBFUSCT_MHTML_Sep21_1

Detects suspicious office reference files including an obfuscated MHTML reference exploiting CVE-2021-40444

ExploitSignature 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
90 / 85

What this rule detects

Detects suspicious office reference files including an obfuscated MHTML reference exploiting CVE-2021-40444

It defines 2 patterns (2 text, 0 hex, 0 regex).

Summary generated from the rule's metadata and condition.

Rule source

Download .yar
expl-mal-maldoc-obfusct-mhtml-sep21-1.yarDetection Rule License 1.1
rule SIGNATURE_BASE_EXPL_MAL_Maldoc_OBFUSCT_MHTML_Sep21_1 : CVE_2021_40444 FILE
{
	meta:
		description = "Detects suspicious office reference files including an obfuscated MHTML reference exploiting CVE-2021-40444"
		author = "Florian Roth (Nextron Systems)"
		id = "781cfd61-d5ac-58e5-868f-dbd2a2df3500"
		date = "2021-09-18"
		modified = "2023-12-05"
		reference = "https://twitter.com/decalage2/status/1438946225190014984?s=20"
		source_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/yara/expl_cve_2021_40444.yar#L27-L41"
		license_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/LICENSE"
		hash = "84674acffba5101c8ac518019a9afe2a78a675ef3525a44dceddeed8a0092c69"
		logic_hash = "11a73572970d2d85d308330119a2c5243f2848ae78a861decdb0cdbde0d9d1c2"
		score = 90
		quality = 85
		tags = "CVE-2021-40444, FILE"

	strings:
		$h1 = "<?xml " ascii wide
		$s1 = "109;&#104;&#116;&#109;&#108;&#58;&#104;&#116;&#109;&#108" ascii wide

	condition:
		filesize < 25KB and all of them
}

Strings (2 patterns)

IdentifierTypeValueModifiers
$h1text"<?xml "ascii wide
$s1text"109;&#104;&#116;&#109;&#108;&#58;&#104;&#116;&#109;&#108"ascii wide

Condition

filesize < 25KB and all of them

In plain words (Heuristic)

  • The file must be smaller than 25KB (a cheap guard that skips large files).
  • All patterns in the rule must match.

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References

Sample hashes

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

  • 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: 781cfd61-d5ac-58e5-868f-dbd2a2df3500

Tags:CVE_2021_40444FILE

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