TP-Link http/tftp backdoor

During the analysis of this TP-Link backdoor, I found other issues, which can be handy when analyzing other devices. Finally the following path leads to remote root exec (useful for debugging purposes). Let’s see.
The router allows for ftp connections. But the ftp session is somehow chrooted (ie. one can access only ftp root and USB shared directories):

Let’s try a little trick now. After plugging a USB flash drive into the router we can share a folder from the USB to be available on FTP:

By clicking ‘Save’ I issue an HTTP request, which I can intercept in local http proxy, and modify it like this (ie. path traversal):

After this I can traverse all the filesystem – also in write mode:

But how can I have interactive root-shell? OK, after searching /tmp directory, there is /tmp/samba/smb.conf which can be overwritten. Brief analysis of samba documentation shows many ways of executing external binary. For example:

root preexec (S)

    This is the same as the preexec parameter except that the command is run as root. This is useful for mounting filesystems (such as CDROMs) when a connection is opened.

As you can see, this option (root preexec) apart from CDROM mounting can be used to debug routers  After modification the config looks like this:

/tmp/szel is just a netcat binary (compiled for MIPS architecture) and uploaded by ftp (see the earlier path traversal trick). Now we can try out remote root shell:

Interactive root is nice, but how can it help with locating issues like this? OK, let’s search httpd binary for strings (httpd can be downloaded from the router – for example – using ftp):

Here we can see start_art.html string mentioned in the original disclosure. But how does it work? Let’s check what is going on on the router when start_art.html is launched:

Now it’s clear – 192.168.0.100 is my IP address and nart.out is 777 chmoded and then executed…

Models affected

• TL-WDR4300
• TL-WR743ND (v1.2 v2.0)
• …

History of the bug

• 12.02.2013 – TP-Link e-mailed with details – no response
• 22.02.2013 – TP-Link again e-mailed with details – no response
• 12.03.2013 – public disclosure

Note : Educational use only! We are also not resposible for any potential damages of the devices which are tested for this vulnerability.

Author : –Michał Sajdak (michal.sajdak<at>securitum.pl)
Source : sekurak.pl