YARA · Signature Base (Florian Roth / Nextron)

SIGNATURE_BASE_WEBSHELL_ASPX_Proxyshell_Sep21_1

Detects webshells dropped by ProxyShell exploitation based on their file header (must be PST) and base64 decoded request

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

Author: Tobias Michalski
Date: 2021-09-17
Modified: 2025-11-03
Score / Quality: 75 / 85

What this rule detects

Detects webshells dropped by ProxyShell exploitation based on their file header (must be PST) and base64 decoded request

It defines 1 patterns (1 text, 0 hex, 0 regex).

Summary generated from the rule's metadata and condition.

Rule source

Download .yar

webshell-aspx-proxyshell-sep21-1.yarDetection Rule License 1.1

rule SIGNATURE_BASE_WEBSHELL_ASPX_Proxyshell_Sep21_1 : FILE
{
	meta:
		description = "Detects webshells dropped by ProxyShell exploitation based on their file header (must be PST) and base64 decoded request"
		author = "Tobias Michalski"
		id = "d0d23e17-6b6a-51d1-afd9-59cc2404bcd8"
		date = "2021-09-17"
		modified = "2025-11-03"
		reference = "Internal Research"
		source_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/yara/expl_proxyshell.yar#L66-L80"
		license_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/LICENSE"
		hash = "219468c10d2b9d61a8ae70dc8b6d2824ca8fbe4e53bbd925eeca270fef0fd640"
		logic_hash = "233ec15dff8da5f2beaa931eb06849aa37e548947c1068d688a1695d977605d8"
		score = 75
		quality = 85
		tags = "FILE"

	strings:
		$s = ".FromBase64String(Request["

	condition:
		uint32( 0 ) == 0x4e444221 and any of them
}

Strings (1 patterns)

Identifier	Type	Value	Modifiers
$s	text	".FromBase64String(Request["

Condition

uint32( 0 ) == 0x4e444221 and any of them

In plain words (Heuristic)

At least one pattern in the rule must match.

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Sample hashes

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

219468c10d2b9d61a8ae70dc8b6d2824ca8fbe4e53bbd925eeca270fef0fd640

Licence & attribution

Author: Tobias Michalski

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: d0d23e17-6b6a-51d1-afd9-59cc2404bcd8

Tags:FILE