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Unrestricted file upload

Unrestricted file upload


Unrestricted File Upload is a major risk for applications, resulting in system takeover, file system overload, client-side attacks, or defacement. This vulnerability can be caused by insecure file metadata validation, like HTTP Multi-part fields or insecure content and size validation, like uploading unauthorized file formats.


Recommendation to limit and prevent unrestricted file upload:

  • The file types allowed to be uploaded should be restricted to only those that are necessary for business functionality.
  • Never accept a filename and its extension directly without having a whitelist filter.
  • The application should perform filtering and content-checking of any files uploaded to the server. Files should be thoroughly scanned and validated before being made available to other users. If in doubt, the file should be discarded.
  • It is necessary to have a list of only permitted extensions on the web application. And file extensions can be selected from the list. For instance, it can be a “select case” syntax (in the case of having VBScript) to choose the file extension regarding the actual file extension.
  • All control and Unicode characters should be removed from the filenames and extensions. Also, the special characters such as ;, :, >, <, / ,\, additional ., *, %, $, and so on should be discarded as well. If it is applicable and there is no need to have Unicode characters, it is highly recommended only to accept Alpha-Numeric characters and only one dot as an input for the file name and the extension; in which the file name and also the extension should not be empty at all (regular expression: [a-zA-Z0-9]{1,200}\.[a-zA-Z0-9]{1,10}).
  • Limit the filename length. For instance, the maximum length of the name of a file plus its extension should be less than 255 characters (without any directory) in an NTFS partition.
  • It is recommended to use an algorithm to determine the filenames. For instance, a filename can be an MD5 hash of the name of the file plus the date of the day.
  • Uploaded directory should not have any execute permission, and all the script handlers should be removed from these directories.
  • Limit the file size to a maximum value to prevent denial of service attacks (on file space or another web application’s functions, such as the image resizer).
  • Restrict small files as they can lead to denial of service attacks. So, the minimum size of files should be considered.
  • Use Cross-Site Request Forgery protection methods.
  • Prevent overwriting a file in case of having the same hash for both.
  • Use a virus scanner on the server (if applicable). Or, if the contents of files are not confidential, a free virus scanner website can be used. In this case, the file should be stored with a random name and without any extension on the server first. Then, after the virus checking (uploading to a free virus scanner website and getting the result), it can be renamed to its specific name and extension.
  • Use the POST method instead of PUT or GET
  • Log users’ activities. However, the logging mechanism should be secured against log forgery and code injection.
  • In case of having compressed file extract functions, contents of the compressed file should be checked one by one as a new file.
  • If possible, consider saving the files in a database rather than on the filesystem.
  • If files should be saved in a filesystem, consider using an isolated server with a different domain to serve the uploaded files.
  • File uploaders should be only accessible to authenticated and authorized users if possible.
  • Write permission should be removed from files and folders other than the upload folders. The upload folders should not serve any
  • Ensure that configuration files such as .htaccess or web.config cannot be replaced using file uploaders. Ensure that appropriate settings are available to ignore the .htaccess or web.config files if uploaded in the upload directories.
  • Ensure that files with double extensions (e.g., file.php.txt) cannot be executed, especially in Apache.
  • Ensure that unauthorized users cannot access uploaded files.
  • Adding the Content-Disposition: Attachment and X-Content-Type-Options: nosniff headers to the response of static files will secure the website against Flash or PDF-based cross-site content-hijacking attacks. It is recommended that this practice be performed for all of the files that users need to download in all the modules that deal with a file download. Although this method does not fully secure the website against attacks using Silverlight or similar objects, it can mitigate the risk of using Adobe Flash and PDF objects, especially when uploading PDF files is permitted.
  • Flash/PDF (crossdomain.xml) or Silverlight (clientaccesspolicy.xml) cross-domain policy files should be removed if they are not in use and there is no business requirement for Flash or Silverlight applications to communicate with the website.
  • Browser caching should be disabled for the crossdomain.xml and clientaccesspolicy.xml files. This enables the website to easily update the file or restrict access to the Web services if necessary. Once the client access policy file is checked, it remains in effect for the browser session, so the impact of non-caching to the end-user is minimal. This can be raised as a low or informational risk issue based on the content of the target website and the security and complexity of the policy file(s).
  • CORS headers should be reviewed to only be enabled for static or publicly accessible data. Otherwise, the Access-Control-Allow-Origin header should only contain authorized addresses. Other CORS headers such as Access-Control-Allow-Credentials should only be used when they are required. Items within the CORS headers such as “Access-Control-Allow-Methods” or Access-Control-Allow-Headers should be reviewed and removed if they are not required.