SnailDoor, the Socketless Backdoor

Imagine a system where users can ssh in, but once logged in cannot create any sockets (or at least, all connections are blocked by a firewall). So you can run code on the system, but can’t create network services for anyone else without giving them your password.

However, there is an instance of Apache running, with home directory hosting enabled. It only supports static files, but surely we can tunnel through this somehow? Enter SnailDoor, which implements a network shell as a crude proof of concept for the technique.

Overview

SnailDoor creates 256 files in the web hosting folder, one for each potential byte. It then records the file access time, and polls all the files a few times a second to see if the access time has changed.

for bf in byteFiles:
    newtime = os.path.getatime(bf.path)
    if( newtime != bf.accesstime ):
        shellBuffer += [bf.byte]

If the access time has changed for a file, SnailDoor adds the corresponding byte to its buffer. It continues polling in a loop until it reads a newline, at which point it executes the buffer as a shell command, and saves the results to output.txt, also in the web hosting folder.

The client can now write a character at a time to the server by making a GET request to http://somewebsite/byte.txt, as follows:

for char in list(cmd):
    filename = str(ord(char)) + ".txt"
    urllib2.urlopen(url + "/" + filename).read()

Caveats

With a trivial implementation, SnailDoor is limited to one-byte-per second in the to-server direction, or 8-baud. This is because file access timestamps are stored in epoch time, which has an accuracy of one second. If multiple files are accessed in a second then each will have the same access time, making the byte order impossible to determine.

However, there are optimizations to stretch this limit. If we create a second set of 256 files we can represent even and odd bytes, increasing bandwidth to 2 bytes per second. Obviously this is an O(n) solution, and with 38400 files we can reach 1200 baud, which is fast enough for a decent interactive shell.

The largest limitation of SnailDoor is that it relies on the accuracy of access timestamps. These are an inherently inefficient part of the filesystem, since they require a write to disk every time a file is read. As a result, many sysadmins disable access time recording, so access time is only updated when a file is written to.

Posted 3/7/17