The importance of securing file upload

On a recent vulnerability assessment, I discovered an unsecured file upload and was challenged by the client.

"Was the file uploaded to the webroot, where it was executable?"

"Well, no." I said. "I don't know where it was uploaded to, but there weren't the necessary protections in place, I can assure you that."

"What can happen if it isn't executable by a web user, then?"

Oh, let me count the ways.

What we are talking about

Allowing file upload from a web page is a common activity. Everything form web email clients to interoffice management systems allow for file upload. Profile pictures are uploaded form client machines. Data is upload to support websites. Anytime you click a Browse button and look at your local hard drive from a web browser, you are uploading a file.

Keep in mind, file upload by itself isn’t a security risk. The web developer is taking advantage of built-in browser functionality, and the site itself doesn’t have any insight into your PC’s internals. File upload is governed by RFC 1867 and has been around for a long time. The communication is one way.

The code for a file upload just has to invoke an enctype of multipart/form-data, and that gives us an input type of ‘file’ as seen in this example snippet:

<html>
<head></head>
<body>
<form action="http://localhost/handlefile.pl"
enctype="multipart/form-data" method="post">
<p>
Please select a file:<br>
<input type="file" name="datafile" size="40">
</p>
<div>
<input type="submit" value="Send">
</div>
</form>
</body>
</html>

If the upload isn’t intrinsically bad, then what is? Well, what you upload can be bad – aiming to exploit something later, when the file is accessed.

The classic case – upload to the web root

The case that my clients were interested in is the classic problem. Back in the day, we used to let users upload files into the web directory. So, for instance, you might be able to save an image directly to the /img/ directory of the web server.

How is that a problem? If I upload a backdoor shell, I can call it with http://yourserver/image/mywebshell.php and you are pwned. Anytime I can write and execute my own code on your server, it is a bad thing. I think we can all agree on that. As a community of people that didn’t want our servers hacked, we stopped allowing upload of files to the web root, and the problem (largely) went away.

But the problem of unrestricted file upload didn’t go away. There are still a number of things that pentesters and bad guys both can do to get your server under their control. Here we’ll look at a handful of them.

Uploading something evil for a user to find later

The first, most common and probably most dangerous act is most likely the uploading of malware. Often, business and social sites both allow for uploading of files for other user’s use. If there is insufficient protection on the file upload, an attacker just uploads evilmalware.docx.exe and waits until the target opens the file. When the file is downloaded, the .exe extension isn’t visible to the user (if on Windows) and bang, we got them.

This attack isn’t much different from phishing by sending email messages. Since the site is trusted by the user, however, the malware executable might have a little more change of getting executed.

Mitigation is fairly straightforward. First, in a Windows environment, check extensions. If you are expecting a docx file, rename the file with the extension. Whitelist the extensions you expect. Second, run a pattern analyzing virus scanner on your server. There are a number of products that are designed to be run on web servers for just this case.

Taking advantage of parsers

A far more subtle attack is to directly affect the server by attacking file handlers on the host machine itself. Adobe PDF, Word and images are all common targets of these kinds of attacks.

What happens, is some flaw in the underlying system – say a PDF parser that reads a PDH form and uses the information to correctly forward the document – is exploited by a malware author. Then documents can be created that exploit this flaw and uploaded somewhere where the attacker knows they will be opened.

Let’s talk about just one of these, as an example. MS13-096 is a Microsoft security advisory that warns us that TIFF images could cause a buffer overflow in Windows. This overflow, then, could be used to execute arbitrary code on the user’s context. Remember what we said before: anytime I can write an execute my code on your machine, bad things will happen.

All of that said, doesn’t’ take a lot of technical expertise to write an exploit for that? I mean, you would need to make an image that exploited the flaw, safely put it into a Word document, then write more code to be injected into the overflow, and then have something that takes advantage of whatever we ran.

Well, the answer to that is yes, it is hard to write. So enter Metasploit. This penetration testing tool has a framework for the enablement, encoding, delivery and combination of exploits. Specifically, this one vulnerability can be baked in with already existing tools to deliver, run and get a remote shell using this exploit.

In fact, one already has.

Stealing hashes with UNC pathed templates

Not everything that Metasploit does revolves around a flaw in a software system. Sometimes you can use Metasploit to take advantage of the way software is SUPPOSED to work. For example, did you know it was possible to build a word document that uses a template that is on a UNC path? Neither did I!

Of course, in order to use that feature, Word will have to send it’s NTLM session hashes to the share. That’s only a problem if we have a listener collecting them on the other end of the wire. That’s exactly what Metasploit will do for us.

For this particular example, I use the word_unc_injector Metasploit module. This module allows me to create a Word document that calls to a template at an address of my choosing; I usually use an AWS instance for the target. This example is done in my local lab.

Once the document is made, I try and upload it into the site, as shown in Figure 1. Since it is expecting a Word document, and it got a Word document, we are in good shape.

Uplolading the sample file

Now we wait. When the user opens the file, Word will call out to the template as seen in Figure 2.

The Word opening dialog calling the network share

If it doesn’t find it, it just gives up after a while. Meanwhile, though, the NTFS session hashes are bring stores by my ‘template’ server, like Figure 3.

Woah Hashes

So what do we do about this and other document parser problems? Honestly, there isn’t much you can do. General security principles are best here:

  • Whitelist so you only are allowing the document types you want uploaded.
  • Don’t process files on the server.
  • Run virus scanners on all workstations, and the web server.

DoS with massive files

Sometimes installing or stealing things isn’t the best path at all. Sometimes, I just want to make your server freak out and crash, so I can see the error messages. I keep a Digital Download of Disney’s Brave on my testing box just for that purpose – 3.4 gigabyte file to upload a few times, just to see how your box handles it. Usually it isn’t good.

Only the beginning

These core categories of attacks are the most common, but they are 1) not everything and 2) much broader than presented here. There are several dozen parser exploits in Metasploit, and those are just the ones that are deemed worth the effort. It is much safer to just follow slightly more-strict-than-usual security policy when it comes to file upload and save yourself the trouble later.

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Bill Sempf

Husband. Father. Pentester. Secure software composer. Brewer. Lockpicker. Ninja. Insurrectionist. Lumberjack. All words that have been used to describe me recently. I help people write more secure software.

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