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Microsoft Defender
How to prevent efficiently Defender from considering a given VBS script as containing a threat
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<blockquote data-quote="Andy Ful" data-source="post: 935562" data-attributes="member: 32260"><p>I made some tests and found some bugs in the way how Defender manages threats via Security Center. Some scripts are blocked by AMSI-based models without any alert, even from the scripting engine.</p><p>Now I use a simple test-script:</p><p>[CODE]urlsource = "https://sample-videos.com/img/Sample-jpg-image-100kb.jpg"</p><p>Set WshShell = WScript.CreateObject("WScript.Shell")</p><p>targetfolder = WshShell.ExpandEnvironmentStrings("%userprofile%\\Downloads\\2021\\")</p><p>downloadpath = targetfolder & "temp.jpg"</p><p></p><p>Set fso = WScript.CreateObject("Scripting.FileSystemObject")</p><p>If not fso.FolderExists(targetfolder) Then</p><p> fso.CreateFolder(targetfolder)</p><p>End If</p><p></p><p>Set XHR = CreateObject("Msxml2.ServerXMLHTTP")</p><p>On Error Resume Next</p><p>With XHR</p><p>' .setTimeouts 15000, 15000, 15000, 15000</p><p> .Open "GET", urlsource, False</p><p> .Send</p><p>End With</p><p></p><p>Set Stream = CreateObject("ADODB.Stream")</p><p>With Stream</p><p> .Type = 1</p><p> .Open</p><p> .Write XHR.ResponseBody</p><p> .SaveToFile downloadpath, 2</p><p> .Close</p><p>End With</p><p>Set Stream = Nothing</p><p>Set XHR = Nothing</p><p>Set fso = Nothing</p><p></p><p>WshShell.run """C:\Windows\notepad.exe"""</p><p>Set WshShell = Nothing[/CODE]</p><p></p><p>It simply downloads the JPG from URL (different from yours) to the folder %userprofile%\Downloads\2021\, and runs Notepad. If the JPG is downloaded but Notepad is not executed then it is the sign of AMSI-based post-execution detection.</p><p></p><p>After some tests, I can say that I do not like the way how Defender manages these detections. They were introduced in the year 2020 and will require many improvements.<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite111" alt=":(" title="Frown :(" loading="lazy" data-shortname=":(" /></p><p>Anyway, I did not see that Avast would use post-execution AMSI-based detections. If I correctly remember it only uses AMSI for pre-execution behavior detections. Defender script detection without AMSI-based post-execution detections is similar to Avast and can be stronger after enabling Defender ASR rules.</p></blockquote><p></p>
[QUOTE="Andy Ful, post: 935562, member: 32260"] I made some tests and found some bugs in the way how Defender manages threats via Security Center. Some scripts are blocked by AMSI-based models without any alert, even from the scripting engine. Now I use a simple test-script: [CODE]urlsource = "https://sample-videos.com/img/Sample-jpg-image-100kb.jpg" Set WshShell = WScript.CreateObject("WScript.Shell") targetfolder = WshShell.ExpandEnvironmentStrings("%userprofile%\\Downloads\\2021\\") downloadpath = targetfolder & "temp.jpg" Set fso = WScript.CreateObject("Scripting.FileSystemObject") If not fso.FolderExists(targetfolder) Then fso.CreateFolder(targetfolder) End If Set XHR = CreateObject("Msxml2.ServerXMLHTTP") On Error Resume Next With XHR ' .setTimeouts 15000, 15000, 15000, 15000 .Open "GET", urlsource, False .Send End With Set Stream = CreateObject("ADODB.Stream") With Stream .Type = 1 .Open .Write XHR.ResponseBody .SaveToFile downloadpath, 2 .Close End With Set Stream = Nothing Set XHR = Nothing Set fso = Nothing WshShell.run """C:\Windows\notepad.exe""" Set WshShell = Nothing[/CODE] It simply downloads the JPG from URL (different from yours) to the folder %userprofile%\Downloads\2021\, and runs Notepad. If the JPG is downloaded but Notepad is not executed then it is the sign of AMSI-based post-execution detection. After some tests, I can say that I do not like the way how Defender manages these detections. They were introduced in the year 2020 and will require many improvements.:( Anyway, I did not see that Avast would use post-execution AMSI-based detections. If I correctly remember it only uses AMSI for pre-execution behavior detections. Defender script detection without AMSI-based post-execution detections is similar to Avast and can be stronger after enabling Defender ASR rules. [/QUOTE]
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