donfanning/mips-cve-2014-9222Lets have fun by digging into a Zyxel router firmware and MIPS Arch
Tutorials are for informational and educational purposes only. I believe that ethical hacking, information security and cyber security should be familiar subjects to anyone using digital information and computers. I believe that it is impossible to defend yourself from hackers without knowing how hacking is done.
$ IDA Pro +7.0 (you can use Radar2 since IDA is quite expensive)
$ Router running Rompager 4.07 : there're immense number of them such as TP-Link TD-W8951*, ZyXEL P-660R, Billion BiPAC 51**/52**
$ UART to USB
The misfortune cookie vulnerability has been around for a while but still lacking an analysis which illustrate the techinical details of the vulnerability in public. According to Check Point the affected software is the embedded web server RomPager from AllegroSoft. Internet-wide scans suggest RomPager is likely the most popular web server software in the world with respect to number of available endpoints. RomPager is typically embedded in the firmware released with the device. This specific vulnerability was introduced to the code base in 2002. Moreover, the devices exposed are the one using RomPager services with versions before 4.34 (and specifically 4.07).
For this tutorial I'll be using a ZXHN H108L that uses Rompager 4.07
HTTP/1.1 401 Unauthorized
WWW-Authenticate: Basic realm="ZXHN H108L"
Content-Type: text/html
Transfer-Encoding: chunked
Server: RomPager/4.07 UPnP/1.0
EXT:
Now lets use the serial port, first we must determine the Baud rate
$ sudo stty -F /dev/ttyUSB0
OUT :
speed 115200 baud; line = 0;
-brkint -imaxbel
device
I use minicom, you can use anything from screen to HyperTerminal, but I prefer this one for its stability ;)
$ sudo minicom -s
+-----------------------------------------------------------------------+
| A - Serial Device : /dev/modem |
| B - Lockfile Location : /var/lock |
| C - Callin Program : |
| D - Callout Program : |
| E - Bps/Par/Bits : 115200 8N1 |
| F - Hardware Flow Control : No |
| G - Software Flow Control : No |
| |
| Change which setting? |
+-----------------------------------------------------------------------+
| Screen and keyboard |
| Save setup as dfl |
| Save setup as.. |
| Exit |
| Exit from Minicom |
+--------------------------+
Now we're good to go:
$ sudo minicom -D /dev/ttyUSB0 -b 115200
Bootbase Version: VTC_SPI1.26 | 2012/12/26 16:00:00
RAM: Size = 8192 Kbytes
Found SPI Flash 2MiB GD25Q16 at 0xbfc00000
SPI Flash Quad Enable
Turn off Quad Mode
RAS Version: ZXHN H108LV4.0.0a_ZRQ_MA
System ID: $2.12.162.0(G94.BY.2)3.20.19.0| 2014/03/21 20140321_v006 | 2014
Press any key to enter debug mode within 3 seconds.
..........
To show possible commands:
$ ATHE
======= Debug Command Listing =======
AT just answer OK
ATHE print help
ATBAx change baudrate. 1:38.4k, 2:19.2k, 3:9.6k 4:57.6k 5:115.2k
ATENx,(y) set BootExtension Debug Flag (y=password)
ATSE show the seed of password generator
ATTI(h,m,s) change system time to hour:min:sec or show current time
ATDA(y,m,d) change system date to year/month/day or show current date
ATDS dump RAS stack
ATDT dump Boot Module Common Area
ATDUx,y dump memory contents from address x for length y
ATRBx display the 8-bit value of address x
ATRWx display the 16-bit value of address x
ATRLx display the 32-bit value of address x
ATGO(x) run program at addr x or boot router
ATGR boot router
ATGT run Hardware Test Program
ATRTw,x,y(,z) RAM test level w, from address x to y (z iterations)
ATSH dump manufacturer related data in ROM
ATDOx,y download from address x for length y to PC via XMODEM
ATTD download router configuration to PC via XMODEM
ATUR upload router firmware to flash ROM
< press any key to continue >
This firmware has a set of hidden commands that are very useful to our dynamic analysis, the problem is that we can't see the registers nor set breakpoints but hopefully with these hidden commands we'd have that features, in order to see and use them we need to use ATEN. However, this command require a password, so what we can do now? let reverse engineer the firmware to find the password generator's algorithm.
(Static analysis) Let see the firmware file first:
$ binwalk ras
binwalk1
$ dd if=ras of=rom1.lzma bs=1 skip=85043 count=66696
$ dd if=ras of=rom2.lzma bs=1 skip=350259 count=3350556
$ lzma -d <rom1.lzma> rom1
$ lzma -d <rom2.lzma> rom2
Let see now the signature in the extracted files:
$ binwalk -Y rom1
DECIMAL HEXADECIMAL DESCRIPTION
--------------------------------------------------------------------------------
0 0x0 MIPS executable code, 32/64-bit, big endian, at least 1250 valid instructions
string is a great tool to get an idea of what a binary file is or what it contains, let see if we can find the password algorithm :
$ strings rom1 | grep -i aten
$ strings rom2 | grep -i aten
Apparently, we can't see it because the firmware is still compressed, which means we must dump it from the router using the debugging mode:
$ ATDO bfc00000, 1f0000
xmodem
The first thing that comes in mind is to start looking for a dispatch table. Why? just intuition, the only alternative is if-elif but it'll be a long set of them, in order to check for the validity of the password provided by the user, and here's what I got:
ROM:800158A0 CmdTable: .word unk_80015C20 # DATA XREF: sub_8000E3D8+4Co
ROM:800158A0 # sub_8000E3D8+90o ...
ROM:800158A4 .word sub_8000E6D0
ROM:800158A8 .byte 0
ROM:800158A9 .byte 0
ROM:800158AA .byte 0
ROM:800158AB .byte 0
ROM:800158AC .word aJustAnswerOk # " just answer OK"
ROM:800158B0 .word aAthe # "ATHE"
ROM:800158B4 .word CMD_SHOW_HELP_ATHE
ROM:800158B8 .byte 0
ROM:800158B9 .byte 0
ROM:800158BA .byte 0
ROM:800158BB .byte 0
ROM:800158BC .word aPrintHelp # " print help"
ROM:800158C0 .word aAtba # "ATBA"
ROM:800158C4 .word CMD_ATBA
...
ROM:800158D0 .word aAten # "ATEN"
ROM:800158D4 .word CMD_ATEN
Keep in mind that MIPS word is 4 bytes, now let see the CMD_ATEN:
ROM:8000FF84 lw $a0, 8($s0)
ROM:8000FF88 jal sub_80013654
ROM:8000FF8C li $a1, 0
ROM:8000FF90 jal passwd_generator
and now: passwd_generator