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33 <p> </p>
34 <h1>Break the Ice — Hardware CTF</h1>
35
36<h2>SecureLayer7’s hardware CTF at Nullcon ’19, Goa</h2>
37
38<p>Earlier this month at Nullcon Goa, we had the chance to attempt a hardware CTF challenge designed by the folks at <a href="https://securelayer7.net">SecureLayer7</a>. We weren’t able to solve it during the period of 2 days that we had (we had talks and parties to be at), but the SL7 guys were kind enough to let us keep the hardware and solve it back at home. Which we did, otherwise this write-up wouldn’t have happened :)</p>
39
40<h3>The Hardware</h3>
41
42<p>So what’s this cryptic “hardware” I keep mentioning, you wonder? It’s an ESP8266 board — better known as a <strong>NodeMCU</strong>. Here’s a picture.</p>
43
44<p><img src="1*cWpvtbXan4LjdJBldelW-g.jpeg" alt="" /></p>
45
46<p>Oh, and it came with a pretty OLED display too. So the obvious task at hand was to connect the display to the board. A quick search, and we found an (ever helpful) <a href="https://www.instructables.com/id/Interface-LCD-Using-NodeMCU/">Instructables</a> link with the image down below.</p>
47
48<p><img src="1*1avLAYsHDTGU-JS3e6oVrA.jpeg" alt="Not the same display, but it works" /><em>Not the same display, but it works</em></p>
49
50<p>Mind you, we struggled quite a bit at this seemingly trivial step, but hey we’re CS students ;)</p>
51
52<p>On connecting the device via USB, the board spins up a wireless hotspot called “Device-6”.</p>
53
54<p><img src="1*wJ3ZY2EskoSSfvCjliP_jQ.png" alt="" /></p>
55
56<p>We tried to connect to this, but it was password protected. We’ll get back to it later.</p>
57
58<h3>Flash dump analysis</h3>
59
60<p>During one of the many web searches I made with regard to this board, an interesting tool showed up — <a href="https://github.com/espressif/esptool">esptool</a>. A Python utility to communicate with the ESP8266. Wonderful.</p>
61
62<p>This tool allows us to do a bunch of operations on the board, but what we’re actually interested in is reading the flash. After looking up the syntax for it, we arrived at:</p>
63
64<pre><code>› sudo ./esptool.py -p /dev/ttyUSB0 -b 460800 read_flash 0 0x400000 flash_contents.bin
65Serial port /dev/ttyUSB0
66Connecting....
67Detecting chip type... ESP8266
68Chip is ESP8266EX
69Features: WiFi
70MAC: 84:f3:eb:05:83:1e
71Uploading stub...
72Running stub...
73Stub running...
74Changing baud rate to 460800
75Changed.
764194304 (100 %)
774194304 (100 %)
78Read 4194304 bytes at 0x0 in 100.8 seconds (333.0 kbit/s)...
79Hard resetting via RTS pin...
80</code></pre>
81
82<p>The command is fairly easy to understand, the <code>-p</code> flag denotes the serial port of our device, <code>-b</code> specifies the Baud rate and <code>read_flash</code>, well, reads the flash starting at <code>0</code> till <code>0x400000</code> which is 4MB.
83We faced a lot of trouble here, since we kept reading only upto 2MB. Why? Because that’s what the command on the Internet said.</p>
84
85<p>Anyway, we have our flash dumped into a file <code>flash_contents.bin</code>.</p>
86
87<p>We then decided to run <code>strings</code> on the flash binary and peruse through the thousands of lines it had. Brilliant right? It was, actually. We found a bunch of interesting strings, along with what we guessed to be the wireless hotspot’s password. Spoiler alert: it was.</p>
88
89<p><img src="1*5Hc-_XYFw-4_hw3iZpfqkQ.png" alt="The entire dump was 6000+ lines. Did we actually do this D:" /><em>The entire dump was 6000+ lines. Did we actually do this D:</em></p>
90
91<p>The go-to utility to (actually) analyze binaries is <code>binwalk</code>. The <code>-e</code> flag extracts the known file types it recognizes within the binary.</p>
92
93<pre><code>› binwalk -e flash_contents.bin
94
95DECIMAL HEXADECIMAL DESCRIPTION
96--------------------------------------------------------------------------------
97283960 0x45538 Unix path: /root/.arduino15/packages/esp8266/hardware/esp8266/2.5.0/libraries/ESP8266WiFi/src/include/DataSource.h
98289387 0x46A6B HTML document footer
99291156 0x47154 HTML document header
100291296 0x471E0 Unix path: /root/.arduino15/packages/esp8266/hardware/esp8266/2.5.0/cores/esp8266/abi.cpp
1013145728 0x300000 Squashfs filesystem, little endian, version 4.0, compression:gzip, size: 139733 bytes, 10 inodes, blocksize: 131072 bytes, created: 2019-02-25 09:14:19
102</code></pre>
103
104<p>We see a squashfs filesystem here. <code>binwalk</code> creates a directory in your current path containing all the files and folders it managed to extract. <code>cd</code>ing into our squashfs folder, we see this:</p>
105
106<p><img src="1*VsEzd8PSYMIUwjBLNFFetA.png" alt=":O" /><em>:O</em></p>
107
108<p>Oooh yes. <code>cat</code>ting the file, we see:</p>
109
110<pre><code>› cat 1/Hidden.txt
111
112######################################### Hints :) ########################################
113
114---telnet server on esp
115
116--Hunt the key to get MQTT creds
117 --
118--MQTT box
119
120--Publish the correct message to get ^FLAG^
121
122<<<<<<<<<<<<<<<< PUBLISH..... DISPLAY.... SUBMIT.... :) >>>>>>>>>>>>>>>>>>>>>>
123</code></pre>
124
125<p>Looking inside the directory named <code>2</code>, we see another dir <code>3</code> containing a JPEG image and a file telling us about steganography.</p>
126
127<p><img src="1*68k1Y6IoK0XTCPTQRn_0fw.png" alt="" /></p>
128
129<p>And the final directory <code>4</code> had nothing in it but a file with the string <code>flag</code>. Probably to show up as a false positive in the <code>strings</code> output of the flash dump.</p>
130
131<h3>Connecting to “Device-6”</h3>
132
133<p>The first file we came across, containing the hints, mentioned a <code>telnet</code> server running on the board. But how do we reach it? Yep, via the wireless hotspot it exposes — “Device-6”. We authenticated using the PSK we found earlier.
134On doing so, we’re prompted with a captive portal:</p>
135
136<p><img src="1*XelmAgITUw-9aZc26meUDQ.png" alt="" /></p>
137
138<p>A few things can be done here, configure WiFi on the board, view some info about the board, and reset it. Let’s connect the ESP to our own SSID — like a mobile hotstpot.</p>
139
140<p><img src="1*oQcTNKOFGphPbX50K2pmlg.png" alt="" /></p>
141
142<p>Once that’s done, we should see the “Device-6” SSID disappear, indicating that the board is now connected to our own wireless hotstpot. Another thing we notice is the board lights up, and so does our display!</p>
143
144<p><img src="1*lzKOxEkzJqo8TNI4WckmOg.png" alt="That’s so sad. Alexa play Despacito." /><em>That’s so sad. Alexa play Despacito.</em></p>
145
146<h3>The telnet server</h3>
147
148<p>Once our host machine and the ESP are on the same network, we can <code>nmap</code> our subnet to find our ESP’s IP.</p>
149
150<p><img src="1*lPNqoIFmNfxfabdt4sqYSQ.png" alt="nmap scan report" /><em>nmap scan report</em></p>
151
152<p>We see an <code>http</code> server running, which was obviously the captive portal, and our <code>telnet</code> server on port 23.</p>
153
154<pre><code>› telnet 192.168.43.223
155Trying 192.168.43.223...
156Connected to 192.168.43.223.
157Escape character is '^]'.
158Press Enter & sumbit your key :)
159somekey
160Wrong Key!!!
161</code></pre>
162
163<p>On connecting, we see a prompt asking for a key. And no, ‘sumbit’ was spelt that way ;)</p>
164
165<p>Where could this key possibly be? Well, the only unexplored part of this CTF so far is the image file we came across before. So… steganography.</p>
166
167<p>Although you won’t need it, I downloaded this Docker image for cracking stego — <a href="https://hub.docker.com/r/dominicbreuker/stego-toolkit/">stego-toolkit</a>. We then tossed the image under a bunch of steganography detection and breaking tools, but to no avail.</p>
168
169<p>After a good while <code>steghide</code> gave us something:</p>
170
171<pre><code>› steghide extract -sf 10071856.jpg
172Enter passphrase:
173</code></pre>
174
175<p>This took <em>really</em> long for us to figure but the password was the name of the image file itself. Urgh. On entering the password, we get a <code>keys.txt</code> file. Here’s what it looked like:</p>
176
177<pre><code>So you guessed the password i think...
178
179Nice!!!
180
181Key is somewhere hidden in this strings ...
182
183XH}<
184TJJ*
185Y#pU
186<g?/N
187gr[i}5
188>+h1
189...snip...
190jlW8B
191yjbm
192M4%'
193tx;ZzL
1943 k]
195wPUf'rc
196)Pz#
1970AwN\
198Lgr:J2
199!H9u
2004bSVy
201(*-C
202nOf2E\
203
204Aaaaaand key is not guessable ....
205
206WARNING:Manual checking for correct key might take you 2 days to complete the challange!!
207</code></pre>
208
209<p>Nearly 600 lines of gibberish. We guessed that one of these strings had to be they key for our <code>telnet</code> session. We tried to automate it, but the <code>telnet</code> session was very unstable. So being the madmen we were, we did it manually. We had all the time in the world. Off we went, copy/pasting the keys in batches of 5… and it worked.</p>
210
211<p><img src="1*vY84DrSpJU1H4c9pSvoB5Q.png" alt="yeet" /><em>yeet</em></p>
212
213<p>As the hint file mentioned, we had to connect to an MQTT instance somewhere and publish something for the flag. So this is what they were talking about.</p>
214
215<p>For those out-of-the-loop, <a href="https://en.wikipedia.org/wiki/MQTT">MQTT</a> is the protocol used in IoT basec client-server interactions, among other things. Go read about it if you want to understand the next bit.</p>
216
217<h3>Capturing the flag</h3>
218
219<p>To interact with the MQTT server, we’ll be using the <a href="https://mosquitto.org">Mosquitto</a> client. We then use the credentials and attempt to “publish” a message:</p>
220
221<pre><code>› mosquitto_pub -h 'm16.cloudmqtt.com' -p 17551 -t 'inTopic/web/test' -u 'hchzbuhr' -P 'Sz4plHnlVnHc' -m '(^.^)'
222</code></pre>
223
224<p><img src="1*W_iVf3vDf4UaelycMbvPvw.png" alt="UwU" /></p>
225
226<p>After messing around with this for quite a bit (as is evident from the screen behind), we tried sending the string ‘flag’ as our message and… <em>dramatic pause</em> we got what you’d expect.</p>
227
228<p><img src="1*sO9vDtGgGjejxklF46gTlg.jpeg" alt="We were 10 days late, mind you" /><em>We were 10 days late, mind you</em></p>
229
230<h3>Conclusion</h3>
231
232<p>This was our first time playing a hardware CTF, and to be honest, there wasn’t <em>much</em> of “hacking” involved — at least by the word’s textbook definition. A lot of guesswork too, which made some parts of it excruciatingly painful to figure out. But all things considered, it was probably the most fun CTF I’ve played yet. Here’s a shoutout to the folks at SL7 for making this CTF <em>and</em> letting us keep the ESP :)</p>
233
234<p>That’s it. The end.</p>
235
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