This detection identifies active ASP webshells capable of intercepting and analyzing local network traffic to exfiltrate sensitive data or pivot within the environment. Proactively hunting for this behavior in Azure Sentinel is critical because these webshells often operate with low visibility on standard endpoints, allowing adversaries to maintain persistent access while silently monitoring internal communications before triggering broader alerts.
rule WEBSHELL_ASP_Sniffer
{
meta:
description = "ASP webshell which can sniff local traffic"
license = "Detection Rule License 1.1 https://github.com/Neo23x0/signature-base/blob/master/LICENSE"
author = "Arnim Rupp (https://github.com/ruppde)"
reference = "Internal Research"
score = 75
date = "2021/03/14"
modified = "2023-07-05"
hash = "1206c22de8d51055a5e3841b4542fb13aa0f97dd"
hash = "60d131af1ed23810dbc78f85ee32ffd863f8f0f4"
hash = "c3bc4ab8076ef184c526eb7f16e08d41b4cec97e"
hash = "ed5938c04f61795834751d44a383f8ca0ceac833"
id = "b5704c19-fce1-5210-8185-4839c1c5a344"
strings:
$sniff1 = "Socket(" wide ascii
$sniff2 = ".Bind(" wide ascii
$sniff3 = ".SetSocketOption(" wide ascii
$sniff4 = ".IOControl(" wide ascii
$sniff5 = "PacketCaptureWriter" fullword wide ascii
//strings from private rule capa_asp
$tagasp_short1 = /<%[^"]/ wide ascii
// also looking for %> to reduce fp (yeah, short atom but seldom since special chars)
$tagasp_short2 = "%>" wide ascii
// classids for scripting host etc
$tagasp_classid1 = "72C24DD5-D70A-438B-8A42-98424B88AFB8" nocase wide ascii
$tagasp_classid2 = "F935DC22-1CF0-11D0-ADB9-00C04FD58A0B" nocase wide ascii
$tagasp_classid3 = "093FF999-1EA0-4079-9525-9614C3504B74" nocase wide ascii
$tagasp_classid4 = "F935DC26-1CF0-11D0-ADB9-00C04FD58A0B" nocase wide ascii
$tagasp_classid5 = "0D43FE01-F093-11CF-8940-00A0C9054228" nocase wide ascii
$tagasp_long10 = "<%@ " wide ascii
// <% eval
$tagasp_long11 = /<% \w/ nocase wide ascii
$tagasp_long12 = "<%ex" nocase wide ascii
$tagasp_long13 = "<%ev" nocase wide ascii
// <%@ LANGUAGE = VBScript.encode%>
// <%@ Language = "JScript" %>
// <%@ WebHandler Language="C#" class="Handler" %>
// <%@ WebService Language="C#" Class="Service" %>
// <%@Page Language="Jscript"%>
// <%@ Page Language = Jscript %>
// <%@PAGE LANGUAGE=JSCRIPT%>
// <%@ Page Language="Jscript" validateRequest="false" %>
// <%@ Page Language = Jscript %>
// <%@ Page Language="C#" %>
// <%@ Page Language="VB" ContentType="text/html" validaterequest="false" AspCompat="true" Debug="true" %>
// <script runat="server" language="JScript">
// <SCRIPT RUNAT=SERVER LANGUAGE=JSCRIPT>
// <SCRIPT RUNAT=SERVER LANGUAGE=JSCRIPT>
// <msxsl:script language="JScript" ...
$tagasp_long20 = /<(%|script|msxsl:script).{0,60}language="?(vb|jscript|c#)/ nocase wide ascii
$tagasp_long32 = /<script\s{1,30}runat=/ wide ascii
$tagasp_long33 = /<SCRIPT\s{1,30}RUNAT=/ wide ascii
// avoid hitting php
$php1 = "<?php"
$php2 = "<?="
// avoid hitting jsp
$jsp1 = "=\"java." wide ascii
$jsp2 = "=\"javax." wide ascii
$jsp3 = "java.lang." wide ascii
$jsp4 = "public" fullword wide ascii
$jsp5 = "throws" fullword wide ascii
$jsp6 = "getValue" fullword wide ascii
$jsp7 = "getBytes" fullword wide ascii
$perl1 = "PerlScript" fullword
//strings from private rule capa_asp_input
// Request.BinaryRead
// Request.Form
$asp_input1 = "request" fullword nocase wide ascii
$asp_input2 = "Page_Load" fullword nocase wide ascii
// base64 of Request.Form(
$asp_input3 = "UmVxdWVzdC5Gb3JtK" fullword wide ascii
$asp_xml_http = "Microsoft.XMLHTTP" fullword nocase wide ascii
$asp_xml_method1 = "GET" fullword wide ascii
$asp_xml_method2 = "POST" fullword wide ascii
$asp_xml_method3 = "HEAD" fullword wide ascii
// dynamic form
$asp_form1 = "<form " wide ascii
$asp_form2 = "<Form " wide ascii
$asp_form3 = "<FORM " wide ascii
$asp_asp = "<asp:" wide ascii
$asp_text1 = ".text" wide ascii
$asp_text2 = ".Text" wide ascii
condition:
(
(
any of ( $tagasp_long* ) or
// TODO : yara_push_private_rules.py doesn't do private rules in private rules yet
any of ( $tagasp_classid* ) or
(
$tagasp_short1 and
$tagasp_short2 in ( filesize-100..filesize )
) or (
$tagasp_short2 and (
$tagasp_short1 in ( 0..1000 ) or
$tagasp_short1 in ( filesize-1000..filesize )
)
)
) and not (
(
any of ( $perl* ) or
$php1 at 0 or
$php2 at 0
) or (
( #jsp1 + #jsp2 + #jsp3 ) > 0 and ( #jsp4 + #jsp5 + #jsp6 + #jsp7 ) > 0
)
)
)
and (
any of ( $asp_input* ) or
(
$asp_xml_http and
any of ( $asp_xml_method* )
) or
(
any of ( $asp_form* ) and
any of ( $asp_text* ) and
$asp_asp
)
)
and filesize < 30KB and all of ( $sniff* )
}This YARA rule can be deployed in the following contexts:
This rule contains 42 string patterns in its detection logic.
Here are 5 specific false positive scenarios for the “ASP webshell which can sniff local traffic” detection rule, along with recommended filters and exclusions:
Scenario: Automated Health Check & Monitoring Scripts
svc_solarwinds, svc_prtg) and filter out file paths located in dedicated monitoring directories (e.g., C:\Program Files\SolarWinds\Agent\HealthCheck\).Scenario: Scheduled Backup and Integrity Verification Jobs
Veeam.Backup.Agent). Additionally, exclude file hashes of known legitimate backup agents and restrict the rule to only trigger on files created within the last 24 hours in web root directories.Scenario: Internal Load Balancer Health Probes