Introduction

This series of posts describes how abandoned, partially implemented functionality can be exploited to gain complete, persistent control of Netgear wireless routers. I’ll describe a hidden SOAP method in the UPnP stack that, at first glance, appeared to allow unauthenticated remote firmware upload to the router. After some reverse engineering, it became apparent this functionality was never fully implemented, and could never work properly given a well formed SOAP request and firmware image. If it could work at all, it would be with only the most contrived of inputs.

A Secret Passage to Persistant SOHO Router Pwnage

Almost two years ago plus a house selling, a cross-country move, a house buying, a job change, and a wedding, I downloaded and unpacked the firmware for Netgear’s then-new R6200 wireless router. This was one of Netgear’s first entries into the nascent 802.11ac market. At around US$200 at the time, this device was at the high end of the Netgear lineup. Finding some cool vulnerabilities in some of the newest, swankiest, consumer WiFi gear would make for a neat paper, or at least a good blog post or two.

Need to reverse engineer or exploit a file upload vulnerability in an embedded web server? I added a multipart/form-data class to Bowcaster to help with that. You can have a look here:
https://github.com/zcutlip/bowcaster/blob/master/src/bowcaster/clients/http.py

Here’s some background:
I’ve been reverse engineering how the Netgear R6200 web server parses a new firmware image when you use the firmware update facility in the web interface. Manually browsing to the router’s web interface, then to the firmware update form, then browsing to a firmware file on disk, then clicking “upload” gets really tedious after a few times.

Previously I posted about running and remotely debugging a Netgear UPnP daemon using QEMU and IDA Pro. This time we’ll take on the challenge of running the built-in web server from the Netgear R6200 in emulation.

The httpd daemon is responsible for so much more than the web interface. This daemon is responsible for a silly amount of system management, including configuring firewall rules, managing the samba and ftp file servers, managing attached USB storage, and many other things. And it does all of this management as part of its initialization, which means lots of opportunities to fail or crash when running in emulation as a standalone service.

It’s often the case, when analyzing an embedded device’s firmware, that static analysis isn’t enough. You need to actually execute a binary you’re analyzing in order to see how it behaves. In the world of embedded Linux devices, it’s often fairly easy to put a debugger on the target hardware for debugging. However it’s a lot more convenient if you can run the binary right on your own system and not have to drag hardware around to do your analysis. Enter emulation with QEMU.

A few years ago, when I worked for my previous employer, I put together a proof-of-concept that was to be part of a client demo. I thought it was kind of cool, so I recorded a screencast of it in action. I’ve had the video sitting on my laptop ever since, not really sure what to do with it. I finally decided to post it.

In the video, what you see is a custom exploit script that exploits a buffer overflow in the web interfaces of several D-Link webcams. The neat part is that it tunnels the exploit and callback through each successive webcam. It works basically like this:

Here are some additional resources I may have mentioned in my Infiltrate 2014 presentation.

White Paper: SQL Injection to MIPS Overflows - Part Deux
Slides: SQL Injection to MIPS Overflows - Part Deux

Original white paper from Black Hat USA 2012:
SQL Injections to MIPS Overflows: Rooting SOHO Routers

Proof of Concept Exploit code:
Here’s my Github repository for proof-of-concept exploit code.  In it, you’ll find the exploit code for the Netgear WNDR 3700v3 that I demoed at Infiltrate, among a few others. The white paper is in there as well.
https://github.com/zcutlip/exploit-poc

A grad student emailed me in response to my Netgear auth bypass post.  He’s working on a research project and wanted to know if I knew of any resources or techniques to use emulation for executing and debugging the net-cgi binary in the Netgear firmware.  It turns out I’ve got all the resources to do just that.  I replied with a description of my workspace and some links to resources I use, and, in many cases, have developed.  I thought this might make an interesting blog post, but I don’t really have time to write it up all blog-post-like.  Instead I’ll just paste in my email.  Maybe it’ll be useful to other people as well.

For my presentation at BayThreat, entitled “BT Wireless Routers: Adventures in Reversing and Exploiting”, rather than have one or two or three slides packed with hard to read URLs, I included a single slide with a link to this post.  Here you’ll find links to additional resources that I may have referenced in my talk.

White paper: Reverse Engineering and Exploiting the BT HomeHub 3.0b (pdf)
Slides: BT Wireless Routers: Adventures in Reversing and Exploiting

At the end of my post on the Netgear wndr3700v4’s authentication bugs, I said to expect followup posts. Once the web interface is unlocked, any further bugs that normally require authentication become fair game. Well good news, everyone!!

Previously, I talked about the net-cgi executable in the wndr3700’s firmware. ;net-cgi is a multi-call binary, a little like busybox. As such it has a lot of functionality baked in. One of its more interesting functions is called cmd_ping6(). Here’s what it looks like: