Security threats with uploads

I am allowing users to upload files to my server. What possible security threats do I face and how can I eliminate them?

Let's say I am allowing users to upload images to my server either from their system or from net. Now to check even the size of these images I have to store them in my /tmp folder. Isn't it risky? How can I minimize the risk?

Also let's say I am using wget to download the images from the link that the users upload in my form. I first have to save those files in my server to check if they actually are images. Also what if a prankster gives me a URL and I end up downloading an entire website full of malware?

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).

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.

1 simple scenario will be : If you use a upload interface where there are no restrictions about the type of files allowed for upload then an attacker can upload a PHP or .NET file with malicious code that can lead to a server compromise.

refer: http://www.acunetix.com/websitesecurity/upload-forms-threat.htm Above link discusses the common issues

also refer: http://php.net/manual/en/features.file-upload.php

Here are some of them:

• When a file is uploaded to the server, PHP will set the variable $_FILES[‘uploadedfile’][‘type’] to the mime-type provided by the web browser the client is using. However, a file upload form validation cannot depend on this value only. A malicious user can easily upload files using a script or some other automated application that allows sending of HTTP POST requests, which allow him to send a fake mime-type.

• It is almost impossible to compile a list that includes all possible extensions that an attacker can use. E.g. If the code is running in a hosted environment, usually such environments allow a large number of scripting languages, such as Perl, Python, Ruby etc, and the list can be endless.

  A malicious user can easily bypass such check by uploading a file called “.htaccess”, which contains a line of code similar to the below: AddType application/x-httpd-php .jpg

There are common rules to avoid general issues with files upload:

• Store uploaded files not under your website root folder - so users won't be able to rewrite your application files and directly access uploaded files (for example in /var/uploads while your app is in /var/www).
• Store sanitated files names in database and physical files give name of file hash value (this also resolves issue of storing files duplicates - they'll have equal hashes).

• To avoid issues with filesystem in case there are too many files in /var/uploads folder, consider to store files in folders tree like that:

  file hash = 234wffqwdedqwdcs -> store it in /var/uploads/23/234wffqwdedqwdcs

  common rule: /var/uploads/<first 2 hash letters>/<hash>

• install nginx if you haven't done its already - it serves static like magic and its 'X-Accel-Redirect' header will allow you to serve files with permissions being checked first by custom script