Today we will discuss about arbitrary file uploads, a less common vulnerability, but one of the most powerful out there.
Less common? Why?
Because any platform with sane developers will validate the content type and the file extension of any file they interact with.
But today I want to introduce you with a new validation attitude that I now advise everyone to use – validate the content itself.
If you are expecting an image to be uploaded – expect the content to be suited as a content of an image. If you are expecting an html file – sanitize it and make sure no script tags exist. There are plenty of libraries and tools to do that.
With that being said, a lot of companies and developers I have came across with said to me: “Why should we make all that effort when we can just host the uploaded files on a CDN (Content Delivery Network)?”.
Well, today’s story is just about that.
Freelancer.com was my first interaction with this new attack vector. They allow images to be uploaded as profile and cover pictures, DOC and DOCX files to be uploaded as resumes and far more.
But they also have a nice feature where users can write articles and publish them for others to read.
This form supports images uploads, and those images, as you can imagine, are being uploaded to a CDN (https://cdn.f-cdn.com/).
f-cdn.com is of course the official CDN of Freelancer INC.
The developers here probably assumed that since images are being uploaded to a CDN, no sanitation and content validation should be done – The form does not support HTML – so XSS cannot be done, and the CDN does not allows php files (or any other like it) to be accessed – just download them.
Well, this attitude allowed me to be able to upload a .exe file to the official CDN of Freelancer INC.
Can you imagine a scenario where a hacker will host malicious viruses on the CDN of an official company? I hope you do.
Far more, there is no size limit – any size of a file can be hosted on the CDN (although it may take time to upload it and the connection may be refused eventually).
So remember – using CDNs is not always the best option. Yes, XSSes will not affect users (because of a different domains) and yes, shells and RCEs will not endanger your server directly. But mis-validating the type of files that your CDN hosts might cause your company to a great lost.
Keep that in mind.