Unrestricted File Upload

Unrestricted File Upload is a security vulnerability where an attacker can upload arbitrary files with attacker-controlled content, potentially to arbitrary locations.

Unrestricted file upload may be used to get:

  • Remote Code Execution

    • Especially in PHP and similar languages, since uploaded files may commonly be parsed by the server if navigated to it.

    • When the web application runs under root and you can use path traversal to specify the directory to upload to. In that case, upload a file to /etc/cron.d in the correct format and it will get executed.

  • Cross-Site Scripting, if you can add or overwrite a file that contains HTML

  • XML External Entities: If the website uses an insecure XML parser, then you might be able to exploit an XXE vulnerability by getting the insecure parser to parse a malicious XML file. In that case, the file upload vulnerability can be used to smuggle a malicious XML file onto the web server for exploitation.

Bypass Mitigations

Bypass MIME Type Check

If the MIME type is checked by the target application, then use Burp Suite to change the MIME type to the correct value.

Bypass Image Validity Check

If the image is checked for validity using an image library, then you can still produce a valid image that has malicious data in embedded in it. This is useful in PHP applications, for example. If you can upload a .php file with PHP code embedded in it, then that might lead to RCE.

This can be done using jhead.

First, clear the headers of an existing myfile.jpg file:

jhead -purejpg myfile.jpg

Add an EXIF comment and write PHP code there:

jhead -ce myfile.jpg

Example code: 

<?php echo "hi"; __halt_compiler();?>

Note: The __halt_compiler() prevents the image bytes from getting executed.

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