The pitfalls of allowing file uploads on your website

http://blog.detectify.com/post/86298380233/the-pitfalls-of-allowing-file-uploads-on-your-website

by BogdanCalin • 12 years ago

100 30 12 years ago

blog.detectify.com

tantalor2• 12 years ago

Should clarify: "The pitfalls of hosting user-uploaded files on your website"

Hosting user-uploaded files on a separate domain would probably solve this problem.

rpedela• 12 years ago

How does simply using a different domain protect against malware?

teraflop2• 12 years ago

As the article explains, the problem is that SWF files hosted on one domain can execute in the security context of that domain, even when embedded in a page on a completely different site. So allowing attacker-controlled uploads makes any credentials on that domain, such as session cookies and CSRF tokens, vulnerable. If the SWF is hosted on a domain with no sensitive credentials, this particular problem goes away.

meowface4• 12 years ago

It depends on the attack you're trying to prevent against.

The blog post in the OP solely discusses XSS vulnerabilities that are introduced by unrestricted file uploads. There are numerous other issues that can occur from arbitrary file uploads (malware hosting, arbitrary code execution if it's PHP, phishing), but to prevent against a user content ever reaching sensitive data via XSS, placing all user data on a separate domain is pretty much your best bet.

pimlottc• 12 years ago

Should clarify: "The pitfalls of hosting user-uploaded files on your website"

Indeed. Nothing about this applies to sites that accept uploads for internal use (e.g. parsed as input data).

ChuckMcM• 12 years ago

The bottom line is this, if users can upload something to your site, and then your site will show that thing to other users before you have a chance to figure out if its a problem, then your site will be exploited by bad actors.

For a long time an out of the box server installation would include anonymous ftp access. Of course nothing is quite so attractive as a 'free' place to dump and retrieve stuff. It was kind of like setting up a warez/malware camera trap.

jacquesm• 12 years ago

Even if it does not show anything to other users, just having the wrong extension can already bite you badly.

Uploading php files instead of images has been used to gain access to machines. Anything that gets stored as a file on the filesystem of the destination machine is a huge risk. All it takes is one little misconfiguration somewhere else and you're wide open.

teach2• 12 years ago

This is only if you subsequently give them a link to what they uploaded, correct?

I have a site that allows uploads (students turning in Java files) but the files are just stored in a folder on the server that isn't in the web-served path. They can't see the file again once uploaded. I assume (and I think rightly) that there's no security risk in my case.

meowface4• 12 years ago

Not to mention that many Apache configurations will use mod_mime, which by default enables multiple extensions.

So if someone uploads a file called `image.php.jpg`, the file is executed by Apache as PHP code. And obviously verifying the MIME type or even the content of the file won't help you here, since you can just write a JPEG header and then throw in `<?php system("..."); ?>` after it.

Even when you think you're safe based on what you'd consider to be obvious assumptions ("the file extension is whatever comes up after the last period"), there are weird things like this that might bite you.

nkurz• 12 years ago

and then your site will show that thing to other users

I think this is worth emphasizing more than the article does. The problem is just as much with the after-the-fact direct access as with the upload. Given the wide variety of illegal things you will quickly end up hosting and the amount of traffic this will generate, cross site scripting attacks may not be your top concern.

callmeed• 12 years ago

Hold-on, doesn't using a

    Content-Disposition: attachment; filename=”image.jpg”

header mean you can no longer display the image in your service? Won't browsers treat it as a file download? Most services that allow image uploads do so because the images will get displayed on a page? (that's what I do)

Most services seem to be moving file uploads to S3 (or similar services) these days, so I'm not sure this advice is really helpful. To take that a step further, my preference now is to upload directly to S3 and bypass my app server altogether. At least in Rails, it's fairly easy to setup.

fransr• 12 years ago

Problem is that many tend to use S3 but bind a subdomain to it. S3 does not validate the content of those files, so combined with a [wildcard].domain.com crossdomain.xml and you're still as vulnerable as per above.

Some also restricts so that different filetypes on S3 will be served as Inline content, but that will just save you from XSS, and not the CSRF leakage. It's still suprisingly common with a crossdomain.xml restricted to [wildcard].domain.com.

elchief• 12 years ago

I'm pretty sure you can use Apache Tika to check the actual content type of a file too. Either way, I hate flash.

staunch• 12 years ago

So if you allow file uploads or printing arbitrary user data in your service, you should always verify the contents as well as sending a Content-Disposition header where applicable.

The idea that you can "verify the contents" is pretty much just wrong. You actually have to parse the files and write out your own known-safe version. It's a real pain in the butt to do that correctly and securely across a wide variety of file types.

Even parsing arbitrary user uploads with something like ImageMagick is probably exploitable, simply because those libraries weren't designed to handle hostile input.

meowface4• 12 years ago

This isn't too related to what the blog post was discussing, but just to give an example of how you're right:

If a PHP page is allowing file uploads and only verifies the content of the data, but nothing else, then no protection is offered against arbitrary code execution. It's easy to craft a JPEG header and then place `<?php ... ?>` right after it; you could even append it to a valid JPEG body, too.

rhizome• 12 years ago

Isn't it a reasonable workaround to scan the file for content-type and make the file available once it passes the criteria for the upload? Find a php file uploaded with an extension of .jpg? "Sorry, there was a problem with your file. Please try again."

ubernostrum• 12 years ago

And then you run into polyglot files which are valid for multiple types...

tehwebguy• 12 years ago

A nice way to achieve this with Rails is to upload straight to S3 and then use Paperclip to get, verify and process the file.

By uploading straight to S3 you also get a faster upload (than, say, Heroku) and server separation.

rebel• 12 years ago

So out of curiosity, what would be the easiest way to securely accept file uploads? Taking into account all of the possible malicious attacks.

BogdanCalin• 12 years ago

Host them on a separate domain like Google and Github does (eg. Google uses googleusercontent.com and Github uses githubusercontent.com).

tom_jones• 12 years ago

Nice post, just goes to show the value of properly validating uploads!

The pitfalls of allowing file uploads on your website

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