Eight bytes to get a shell.

This will be a quick one. Last week was hacklu again. And again it was in the middle of the week. Nothing they can do about that they say, and I believe them of course! Point being I didn’t have time to play properly, I only looked at one challenge. There was one little trick I liked and wanted to share.

Petition Builder is a chalenge presenting itself as a website built using PHP. It’s very simple: a form to submit petitions. A parameter allows to prefil the text you want to submit, and a quick check shows it gives read access any file on the system.

Dumping source code and configuration files gives us a better understanding of the challenge. There is a PHP module that is loaded as a shared object. It has a function array_get_hashes that is called from the PHP script.

It took me a (long) while, but I finally figured out where the vulnerability lay. I’ll admit I didn’t see it reading the C code at first, which I’m truly ashamed for. I had to resort to doing tests to notice I was getting garbage back. From there I fired gdb to see what was up. Turned out some variable in a loop was always used but only set in one of two cases. Because of how PHP works, and the fact that this was used to initialize an array, the object was freed each time. Meaning that the one time it was used without being set it would cause a use after free. Long story short it was really easy to control the contents of a ZVal and gaining control of the execution by hijacking a method call.

call [rax] ; controled rax and rdi pointing to 8 controled bytes.

Afterwards, the organizers told me they expected us to ROP our way out of there. But I wasn’t in the mood of checking all of the available gadgets. With a fairly easy leak and all of PHP and it’s libraries this would have been very doable but it looked so time consuming! I was lazy.

System was in the GOT so calling it with call [rax] is easy enough. rdi also pointed to controled bytes. I didn’t want to search for anything else.

Getting a shell in 8 bytes is easy: 'sh <&3;', or whatever fd is used for the connection. But no, that did not work for apache/php. The developers are smart enough to set the appropriate FD_CLOEXEC flags. Well done!

Something more clever was needed. I didn’t think it was possible to craft a file by appending stuff byte per byte. 'echo a>x;' is one byte too long, and without the ; (or null byte) the filename would be garbage. I was prety sure the cwd wouldn’t even be writable.

The next best thing to get our data on the server was relying on PHP. If you send files using POST it stores them on disk waiting for them to be needed. A check of php.ini confirmed that that they would be stored in the system’s tmp directory: /tmp/php2dz5FZ something like this.

Now, '. /*/*J;' is eight bytes. It took about 20 tries for php to randomly pick 'J' as the last letter while I crossed my fingers that nothing else on the remote system would match *J; it didn’t on mine.

It took a bit longer for me to realise that they had strict firewalls in place preventing my PoC from calling home. Just needed to write all output to a known file and use the website vulnerability to read it.