Saturday 6 December 2014

Reading local files from Facebook's server (fixed)


Recently I found a vulnerability in Facebook which allowed me to read local files from Facebook's servers. The vulnerable part of Facebook was their Careers resume uploader, located at every job offer, for example this one.

You can upload any extension there, so I decided to upload a .php file :-) - one can always hope. Of course, it was not executed nor could I get file path, but contents of the file were returned, after being base64 encoded. Next thing I tried was to name my file /etc/passwd, "file:///etc/passwd" and couple others. None of these worked.

Couple tries later I uploaded a zipped .php file, and the response contained unzipped, base64'd contents of .php. If you read Facebook's "Bounty hunter's guide" you will know where this leads.
The guide describes how one researcher uploaded zip with symlink to /etc/passwd, and couple steps later Facebook returned few lines of /etc/passwd.

I have done exactly the same, so:
1. create a symlink to /etc/passwd (or any other file you want to read)
ln -s /etc/passwd link
2. zip the created link while preserving symlinks:
zip --symlinks link
3. upload as your resume, system will unzip it
4. the response to POST will have details of (whole) /etc/passwd or other file. 

Here is a screenshot of response containing /etc/passwd:

I have speculated about symlinking a directory, but never tried it. Neal from Facebook thought that this might get me contents of files from the directory, but not necessarily filenames. We shall never know. 

Here is a timeline of the bug report:
Nov 30, 2014  09:45 - vulnerability reported
Nov 30, 2014  17:58 - reply from Facebook's security (Neal) saying they cannot reproduce bug
Nov 30, 2014  18:08 - update from Neal, they can reproduce it
Nov 30, 2014  19:10 - temporary fix has been pushed, disabling resume uploads
Dec 01, 2014 ~23:00 - more permanent fix pushed, now server no longer responds with contents of uploaded resume (Emrakul)
Dec 05, 2014  18:15 - bounty awarded (Neal).
Dec 05, 2014 ~19:00 - objection about reward sent to Facebook's team
Dec 06, 2014 ~23:30 - Neal from Facebook explains this is actually a third party system they run

I'd like to ramble a bit about the award for this bug. When I found it, it looked like a critical bug that could allow me to read parts of Facebook's source. It turned out to actually be a third party software they used to analyse uploaded resumes, therefore I could not actually access any part of Facebook's internals. The network this system is hosted on was pretty locked down, too.

Basically, this is a bug that looks really critical, but is much lower severity. It is an exact opposite to a low severity bug I found some time ago that turned out to be more dangerous.

Lessons learned:

  • Read all write-ups you can get your hands on. I would never think of uploading the zip with symlinks if I did not read Facebook's blog. Philippe has a good list of Facebook bugs here. 
  • Some bugs are not what they look like :-)
Huge thanks to researcher who first used ZIP with symlinks, and to Facebook for blogging about it, and their security team for being awesome as always, fixing the bug in 10 hours on Sunday.


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