YARA · Signature Base (Florian Roth / Nextron)

SIGNATURE_BASE_APT_WEBSHELL_Tiny_Webshell

Detects WebShell Injection

APT / threat actorSignature Base (Florian Roth / Nextron)Detection Rule License 1.1Compiles with YARA-X

Author: Markus Neis,Swisscom
Date: 2021-03-05
Modified: 2023-12-05
Score / Quality: 75 / 85

What this rule detects

Detects WebShell Injection

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

Summary generated from the rule's metadata and condition.

Rule source

Download .yar

apt-webshell-tiny-webshell.yarDetection Rule License 1.1

rule SIGNATURE_BASE_APT_WEBSHELL_Tiny_Webshell : APT HAFNIUM WEBSHELL FILE
{
	meta:
		description = "Detects WebShell Injection"
		author = "Markus Neis,Swisscom"
		id = "aa2fcecc-4c8b-570d-a81a-5dfb16c04e05"
		date = "2021-03-05"
		modified = "2023-12-05"
		reference = "https://www.volexity.com/blog/2021/03/02/active-exploitation-of-microsoft-exchange-zero-day-vulnerabilities/"
		source_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/yara/apt_hafnium.yar#L67-L82"
		license_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/LICENSE"
		hash = "099c8625c58b315b6c11f5baeb859f4c"
		logic_hash = "9309f9b57353b6fe292048d00794699a8637a3e6e429c562fb36c7e459003a3b"
		score = 75
		quality = 85
		tags = "APT, HAFNIUM, WEBSHELL, FILE"

	strings:
		$x1 = "<%@ Page Language=\"Jscript\" Debug=true%>"
		$s1 = "=Request.Form(\""
		$s2 = "eval("

	condition:
		filesize < 300 and all of ( $s* ) and $x1
}

Strings (3 patterns)

Identifier	Type	Value
$x1	text	"<%@ Page Language=\"Jscript\" Debug=true%>"
$s1	text	"=Request.Form(\""
$s2	text	"eval("

Condition

filesize < 300 and all of ( $s* ) and $x1

In plain words (Heuristic)

The file must be smaller than 300 (a cheap guard that skips large files).
All patterns in ( $s* ) must match.
Pattern $x1 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.

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References

https://www.volexity.com/blog/2021/03/02/active-exploitation-of-microsoft-exchange-zero-day-vulnerabilities/

Sample hashes

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

099c8625c58b315b6c11f5baeb859f4c

Licence & attribution

Author: Markus Neis,Swisscom

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: aa2fcecc-4c8b-570d-a81a-5dfb16c04e05

Tags:APT HAFNIUM WEBSHELL FILE

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