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YARA · Signature Base (Florian Roth / Nextron)

SIGNATURE_BASE_WEBSHELL_ASPX_Proxyshell_Aug21_2

Detects webshells dropped by ProxyShell exploitation based on their file header (must be PST), size and content

WebshellSignature Base (Florian Roth / Nextron)Detection Rule License 1.1Compiles with YARA-X
Author
Florian Roth (Nextron Systems)
Date
2021-08-13
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), size and content

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-aug21-2.yarDetection Rule License 1.1
rule SIGNATURE_BASE_WEBSHELL_ASPX_Proxyshell_Aug21_2 : FILE
{
	meta:
		description = "Detects webshells dropped by ProxyShell exploitation based on their file header (must be PST), size and content"
		author = "Florian Roth (Nextron Systems)"
		id = "a351a466-695e-570e-8c7f-9c6c0534839c"
		date = "2021-08-13"
		modified = "2025-11-03"
		reference = "https://www.bleepingcomputer.com/news/microsoft/microsoft-exchange-servers-are-getting-hacked-via-proxyshell-exploits/"
		source_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/yara/expl_proxyshell.yar#L35-L48"
		license_url = "https://github.com/Neo23x0/signature-base/blob/94a1c48d7ab499879287ff611dfe7f9c56376030/LICENSE"
		logic_hash = "4ede197d482f0a9e553ba857b5049e7b7405e3df92460e19418fa0653c844982"
		score = 75
		quality = 85
		tags = "FILE"

	strings:
		$s1 = "Page Language=" ascii nocase

	condition:
		uint32( 0 ) == 0x4e444221 and filesize < 2MB and $s1
}

Strings (1 patterns)

IdentifierTypeValueModifiers
$s1text"Page Language="ascii nocase

Condition

uint32( 0 ) == 0x4e444221 and filesize < 2MB and $s1

In plain words (Heuristic)

  • The file must be smaller than 2MB (a cheap guard that skips large files).
  • Pattern $s1 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

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: a351a466-695e-570e-8c7f-9c6c0534839c

Tags:FILE

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