Ciscox notes (Anthony C. Zboralski Gaius)
Research is being done on a useless Cisco 1600 with 4 megs of flash running IOS 11.1.
Recently after writting my first cisco warez (tunnelx), I told myself hey we need to find a way to inject arbitrary code, poke and peek at the memory
on a cisco, hide interfaces, route-maps, Access-lists.
Let's look around:
scep#show proc
CPU utilization for five seconds: 10%/4%; one minute: 14%; five minutes: 14%
PID QTy PC Runtime (ms)Invoked uSecsStacks TTY Process
1 M* 0 1248107 11663 2204/4000 1 Virtual Exec
2 Lst802DF1634668313110760 1760/2000 0 Check heaps
3 Cwe801D5DE01 0 1736/2000 0 Pool Manager
4 Mst8058B2002 0 1708/2000 0 Timers
5 Lwe80BFD4A 24 46 521 1448/2000 0 ARP Input
6 Mwe81F78F0414000 1744/2000 0 SERIAL A'detect
7 Lwe80D935A414000 1656/2000 0 Probe Input
8 Mwe80D8CD601 0 1744/2000 0 RARP Input
9 Hwe80CA966 80 89 898 3116/4000 0 IP Input
10 Mwe80F41BA 1632249 1348/2000 0 TCP Timer
11 Lwe80F5EB8832666 3244/4000 0 TCP Protocols
12 Mwe813785E 80177 451 1588/2000 0 CDP Protocol
13 Mwe80D577001 0 1620/2000 0 BOOTP Server
14 Mwe81112C0 1356 1522 890 1592/2000 0 IP Background
15 Lsi81212980 25 0 1792/2000 0 IP Cache Ager
16 Cwe80237BE01 0 1748/2000 0 Critical Bkgnd
17 Mwe802365A 1252400 1476/2000 0 Net Background
18 Lwe804E82E 1644000 1192/2000 0 Logger
19 Msp80456DE 80 149353 1728/2000 0 TTY Background
20 Msp802345C 20 149413 1800/2000 0 Per-Second Jobs
21 Msp80233F2 68 149445 1488/2000 0 Net Periodic
22 Hwe80234DC414000 1724/2000 0 Net Input
23 Msp8023482772 25 30880 1800/2000 0 Per-minute Jobs
24 Lwe8109834422000 3620/4000 0 IP SNMP
25 Mwe815CE0801 0 1712/2000 0 SNMP Traps
26 ME 811805A0 26 0 1892/2000 0 IP-RT Background
27 ME 803B0F8 32 112909 2760/4000 2 Virtual Exec
now you can even dump the memory with 'show memory'. Good but there isn't a write memory command, too bad. Maybe not...
I started looking for undocumented and hidden commands and found quite a bunch of them.
Among all the stupid hidden command, the best candidate for taking full control of the cisco is 'gdb'.
The IOS gdb command offers three subcommands:
gdb
debug PID
examine PID
kernel
the kernel subcommand works only on the console.
However 'examine' and 'debug' works perfectly; the debug subcommand is a bit tricky to use though.
scep#gdb debug 27
oops..
Ok grab a copy of gdb-4.18 and try to compile a version for your cisco.
mkdir m68k-cisco
../configure --target m68k-cisco
make
if you have a mips based cisco, just s/m68k/mips64/ the above 4 lines.
now type make install and you should have a m68-cisco-gdb binary in your path.
fire# m68k-cisco-gdb
GNU gdb 4.18
Copyright 1998 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.Type "show warranty" for details.
This GDB was configured as "--host=i686-pc-Linux-gnu --target=m68k-cisco".
(cisco-68k-gdb)
my cisco 1600 is connected to /dev/ttyS0,
scepen
PassWord:
scep#gdb debug 18
scep#
As you can see it bails out if you hit return. while examine works it seems.
scep#gdb examine 18
now the console seems locked.
go back to our gdb-4.18 source tree and check out gdb/remote.c which contains a nice documentation of the gdb remote communication protocol.
added.
IOS gdbserver implementation
Don't get too excited, IOS gdbserver supports only a limited subset of those commands. I'll grab a binary of IOS 12 and check if new commands were added.
I didn't have to test every command by hand.. let's just say I havereliable sources and I know that in IOS 11.2-8 (hum hum), the following commands are supported:
RequestPacket
read registersg
write regsGXX..XXEach byte of register data
is described by two hex digits.
Registers are in the internal order
for GDB, and the bytes in a register
are in the same order the machine uses.
read memmAA..AA,LLLLAA..AA is address, LLLL is length.
write memMAA..AA,LLLL:XX..XX
AA..AA is address,
LLLL is number of bytes,
XX..XX is data
continuecAA.AAAA..AA is address to resume
IF AA..AA is omitted
resume at same address.
stepsAA..AAAA..AA is address to resume
If AA..AA is omitted,
resume at same address.
kill requestk
last signal?Reply the current reason for stopping.
This is the same reply as is generated
for step or cont : SAA where AA is the
signal number.
toggle debugdtoggle debug flag (see 386 & 68k stubs)
All other commands will be ignored... too bad 'search' isn't implemented.
The protocol is simple, quoting remote.c comments:
A debug packet whose contents are <data is encapsulated for transmission in the form.
$ <data # CSUM1 CSUM2
<data must be ASCII alphanumeric and cannot include characters
'$' or '#'.If <data starts with two characters followed by
':', then the existing stubs interpret this as a sequence number.
CSUM1 and CSUM2 are ascii hex representation of an 8-bit checksum of <data, the most significant nibble is sent first.
the hex digits 0-9,a-f are used.
Before trying to make gdb work i wrote a little program that computed the right checksum:
#include <stdio.h
unsigned char const hexchars[] = "0123456789abcdef";
char tohexchar (unsigned char c)
{
c &= 0x0f;
return(hexchars[c]);
}
int main(int argc, char **argv)
{
unsigned char checksum;
int count;
char *command;
char ch;
if (argc <= 1)
exit(1);
printf("gdb protocol command: ");
command = argv[1];
putchar ('$');
checksum = count = 0;
while ((ch = command[count]))
{
putchar(ch);
checksum += ch;
count++;
}
putchar('#');
putchar(tohexchar(checksum 4));
putchar(tohexchar(checksum));
putchar(' ');
}
./gdbproto g
gdb protocol command: $g#67
now paste that on the prompt and you get register output:
scep
