WEBSHELL_ASP_Scan_Writable

rule WEBSHELL_ASP_Scan_Writable
{
    meta:
        description = "ASP webshell searching for writable directories (to hide more webshells ...)"
        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 = "2023-04-05"
        hash = "2409eda9047085baf12e0f1b9d0b357672f7a152"
        hash = "af1c00696243f8b062a53dad9fb8b773fa1f0395631ffe6c7decc42c47eedee7"

        id = "1766e081-0591-59ab-b546-b13207764b4d"
    strings:
        $scan1 = "DirectoryInfo" nocase fullword wide ascii
        $scan2 = "GetDirectories" nocase fullword wide ascii
        $scan3 = "Create" nocase fullword wide ascii
        $scan4 = "File" nocase fullword wide ascii
        $scan5 = "System.IO" nocase fullword wide ascii
        // two methods: check permissions or write and delete:
        $scan6 = "CanWrite" nocase fullword wide ascii
        $scan7 = "Delete" nocase fullword wide ascii


        $sus1 = "upload" nocase fullword wide ascii
        $sus2 = "shell" nocase wide ascii
        $sus3 = "orking directory" nocase fullword wide ascii
        $sus4 = "scan" nocase 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:
        filesize < 10KB and (
        (
            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 6 of ( $scan* ) and any of ( $sus* )
}