Security Threats With Uploads

Security threats with uploads

First of all, realize that uploading a file means that the user is giving you a lot of data in various formats, and that the user has full control over that data. That's even a concern for a normal form text field, file uploads are the same and a lot more. The first rule is: Don't trust any of it.

What you get from the user with a file upload:

• the file data
• a file name
• a MIME type

These are the three main components of the file upload, and none of it is trustable.

1. Do not trust the MIME type in $_FILES['file']['type']. It's an entirely arbitrary, user supplied value.

2. Don't use the file name for anything important. It's an entirely arbitrary, user supplied value. You cannot trust the file extension or the name in general. Do not save the file to the server's hard disk using something like 'dir/' . $_FILES['file']['name']. If the name is '../../../passwd', you're overwriting files in other directories. Always generate a random name yourself to save the file as. If you want you can store the original file name in a database as meta data.

3. Never let anybody or anything access the file arbitrarily. For example, if an attacker uploads a malicious.php file to your server and you're storing it in the webroot directory of your site, a user can simply go to example.com/uploads/malicious.php to execute that file and run arbitrary PHP code on your server.

   • Never store arbitrary uploaded files anywhere publicly, always store them somewhere where only your application has access to them.

   • Only allow specific processes access to the files. If it's supposed to be an image file, only allow a script that reads images and resizes them to access the file directly. If this script has problems reading the file, it's probably not an image file, flag it and/or discard it. The same goes for other file types. If the file is supposed to be downloadable by other users, create a script that serves the file up for download and does nothing else with it.

   • If you don't know what file type you're dealing with, detect the MIME type of the file yourself and/or try to let a specific process open the file (e.g. let an image resize process try to resize the supposed image). Be careful here as well, if there's a vulnerability in that process, a maliciously crafted file may exploit it which may lead to security breaches (the most common example of such attacks is Adobe's PDF Reader).

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To address your specific questions:

[T]o check even the size of these images I have to store them in my /tmp folder. Isn't it risky?

No. Just storing data in a file in a temp folder is not risky if you're not doing anything with that data. Data is just data, regardless of its contents. It's only risky if you're trying to execute the data or if a program is parsing the data which can be tricked into doing unexpected things by malicious data if the program contains parsing flaws.

Of course, having any sort of malicious data sitting around on the disk is more risky than having no malicious data anywhere. You never know who'll come along and do something with it. So you should validate any uploaded data and discard it as soon as possible if it doesn't pass validation.

What if a prankster gives me a url and I end up downloading an entire website full of malware?

It's up to you what exactly you download. One URL will result at most in one blob of data. If you are parsing that data and are downloading the content of more URLs based on that initial blob that's your problem. Don't do it. But even if you did, well, then you'd have a temp directory full of stuff. Again, this is not dangerous if you're not doing anything dangerous with that stuff.

What security issues appear when users can upload their own files?

Your first line of defense will be to limit the size of uploaded files, and kill any transfer that is larger than that amount.

File extension validation is probably a good second line of defense. Type validation can be done later... as long as you aren't relying on the (user-supplied) mime-type for said validation.

Why file extension validation? Because that's what most web servers use to identify which files are executable. If your executables aren't locked down to a specific directory (and most likely, they aren't), files with certain extensions will execute anywhere under the site's document root.

File extension checking is best done with a whitelist of the file types you want to accept.

Once you validate the file extension, you can then check to verify that said file is the type its extension claims, either by checking for magic bytes or using the unix file command.

I'm sure there are other concerns that I missed, but hopefully this helps.

Security issues in accepting image uploads

Some things I learned recently from a web security video:

• The nuclear option is to serve all uploaded content from a separate domain which only serves static content - all features are disabled and nothing important is stored there.
• Considering processing images through imagemagick etc. to strip out funny business.
• For an example of what you are up against, look up GIFAR, a technique that puts a GIF and Java JAR in the same file.

php uploading a file security issues?

Resizing the images sounds like a good idea. Don't forget that some real images are uploaded with php code just to use them in any local include vulnerability.

What is the Security risk behind allowing a user to upload an HTML file to a Web Server?

Pretty minimal if any. It's HTML so you're not executing anything on the server. If it's absolutely, positively only viewed by the uploader then there's no XSS or CSRF attack vectors of any value.

Your biggest risk is probably in the upload function and ensuring there's no malicious payload that could execute on the server side.

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