CTF-pwn-tips
Catalog
- Overflow
- Find string in gdb
- Binary Service
- Find specific function offset in libc
- Find '/bin/sh' or 'sh' in library
- Leak stack address
- Fork problem in gdb
- Secret of a mysterious section - .tls
- Predictable RNG(Random Number Generator)
- Make stack executable
- Use one-gadget-RCE instead of system
- Hijack hook function
- Use printf to trigger malloc and free
- Use execveat to open a shell
Overflow
Assume that: char buf[40]
and signed int num
scanf
-
scanf("%s", buf)
%s
doesn't have boundary check.- pwnable
-
scanf("%39s", buf)
%39s
only takes 39 bytes from the input and puts NULL byte at the end of input.- useless
-
scanf("%40s", buf)
- At first sight, it seems reasonable.(seems)
- It takes 40 bytes from input, but it also puts NULL byte at the end of input.
- Therefore, it has one-byte-overflow.
- pwnable
-
scanf("%d", num)
- Used with
alloca(num)
- Since
alloca
allocates memory from the stack frame of the caller, there is an instructionsub esp, eax
to achieve that. - If we make num negative, it will have overlapped stack frame.
- E.g. Seccon CTF quals 2016 cheer_msg
- Since
- Use num to access some data structures
- In most of the time, programs only check the higher bound and forget to make num unsigned.
- Making num negative may let us overwrite some important data to control the world!
- Used with
gets
-
gets(buf)
- No boundary check.
- pwnable
-
fgets(buf, 40, stdin)
- It takes only 39 bytes from the input and puts NULL byte at the end of input.
- useless
read
read(stdin, buf, 40)
- It takes 40 bytes from the input, and it doesn't put NULL byte at the end of input.
- It seems safe, but it may have information leak.
- leakable
E.g.
memory layout
0x7fffffffdd00: 0x4141414141414141 0x4141414141414141
0x7fffffffdd10: 0x4141414141414141 0x4141414141414141
0x7fffffffdd20: 0x4141414141414141 0x00007fffffffe1cd
-
If there is a
printf
orputs
used to output the buf, it will keep outputting until reaching NULL byte. -
In this case, we can get
'A'*40 + '\xcd\xe1\xff\xff\xff\x7f'
. -
fread(stdin, buf, 1, 40)
- Almost the same as
read
. - leakable
- Almost the same as
strcpy
Assume that there is another buffer: char buf2[60]
-
strcpy(buf, buf2)
- No boundary check.
- It copies the content of buf2(until reaching NULL byte) which may be longer than
length(buf)
to buf. - Therefore, it may happen overflow.
- pwnable
-
strncpy(buf, buf2, 40)
- It copies 40 bytes from buf2 to buf, but it won't put NULL byte at the end.
- Since there is no NULL byte to terminate, it may have information leak.
- leakable
strcat
Assume that there is another buffer: char buf2[60]
-
strcat(buf, buf2)
- Of course, it may cause overflow if
length(buf)
isn't large enough. - It puts NULL byte at the end, it may cause one-byte-overflow.
- In some cases, we can use this NULL byte to change stack address or heap address.
- pwnable
- Of course, it may cause overflow if
-
strncat(buf, buf2, n)
- Almost the same as
strcat
, but with size limitation. - pwnable
- E.g. Seccon CTF quals 2016 jmper
- Almost the same as
Find string in gdb
In the problem of SSP, we need to find out the offset between argv[0]
and the input buffer.
gdb
- Use
p/x ((char **)environ)
in gdb, and the address of argv[0] will be theoutput - 0x10
E.g.
(gdb) p/x (char **)environ
$9 = 0x7fffffffde38
(gdb) x/gx 0x7fffffffde38-0x10
0x7fffffffde28: 0x00007fffffffe1cd
(gdb) x/s 0x00007fffffffe1cd
0x7fffffffe1cd: "/home/naetw/CTF/seccon2016/check/checker"
gdb peda
- Use
searchmem "/home/naetw/CTF/seccon2016/check/checker"
- Then use
searchmem $result_address
gdb-peda$ searchmem "/home/naetw/CTF/seccon2016/check/checker"
Searching for '/home/naetw/CTF/seccon2016/check/checker' in: None ranges
Found 3 results, display max 3 items:
[stack] : 0x7fffffffe1cd ("/home/naetw/CTF/seccon2016/check/checker")
[stack] : 0x7fffffffed7c ("/home/naetw/CTF/seccon2016/check/checker")
[stack] : 0x7fffffffefcf ("/home/naetw/CTF/seccon2016/check/checker")
gdb-peda$ searchmem 0x7fffffffe1cd
Searching for '0x7fffffffe1cd' in: None ranges
Found 2 results, display max 2 items:
libc : 0x7ffff7dd33b8 --> 0x7fffffffe1cd ("/home/naetw/CTF/seccon2016/check/checker")
[stack] : 0x7fffffffde28 --> 0x7fffffffe1cd ("/home/naetw/CTF/seccon2016/check/checker")
Binary Service
Normal:
ncat -vc ./binary -kl 127.0.0.1 $port
With specific library in two ways:
ncat -vc 'LD_PRELOAD=/path/to/libc.so ./binary' -kl 127.0.0.1 $port
ncat -vc 'LD_LIBRARY_PATH=/path/of/libc.so ./binary' -kl 127.0.0.1 $port
After this, you can connect to binary service by command nc localhost $port
.
Find specific function offset in libc
If we leaked libc address of certain function successfully, we could use get libc base address by subtracting the offset of that function.
Manually
readelf -s $libc | grep ${function}@
E.g.
$ readelf -s libc-2.19.so | grep system@
620: 00040310 56 FUNC GLOBAL DEFAULT 12 __libc_system@@GLIBC_PRIVATE
1443: 00040310 56 FUNC WEAK DEFAULT 12 system@@GLIBC_2.0
Automatically
- Use pwntools, then you can use it in your exploit script.
E.g.
from pwn import *
libc = ELF('libc.so')
system_off = libc.symbols['system']
Find '/bin/sh' or 'sh' in library
Need libc base address first
Manually
objdump -s libc.so | less
then search 'sh'strings -tx libc.so | grep /bin/sh
Automatically
- Use pwntools
E.g.
from pwn import *
libc = ELF('libc.so')
...
sh = base + next(libc.search('sh\x00'))
binsh = base + next(libc.search('/bin/sh\x00'))
Leak stack address
constraints:
- Have already leaked libc base address
- Can leak the content of arbitrary address
There is a symbol environ
in libc, whose value is the same as the third argument of main
function, char **envp
.
The value of char **envp
is on the stack, thus we can leak stack address with this symbol.
(gdb) list 1
1 #include <stdlib.h>
2 #include <stdio.h>
3
4 extern char **environ;
5
6 int main(int argc, char **argv, char **envp)
7 {
8 return 0;
9 }
(gdb) x/gx 0x7ffff7a0e000 + 0x3c5f38
0x7ffff7dd3f38 <environ>: 0x00007fffffffe230
(gdb) p/x (char **)envp
$12 = 0x7fffffffe230
0x7ffff7a0e000
is current libc base address0x3c5f38
is offset ofenviron
in libc
This manual explains details about environ
.
Fork problem in gdb
When you use gdb to debug a binary with fork()
function, you can use the following command to determine which process to follow (The default setting of original gdb is parent, while that of gdb-peda is child.):
set follow-fork-mode parent
set follow-fork-mode child
Alternatively, using set detach-on-fork off
, we can then control both sides of each fork. Using inferior X
where X
is any of the numbers that show up for info inferiors
will switch to that side of the fork. This is useful if both sides of the fork are necessary to attack a challenge, and the simple follow
ones above aren't sufficient.
Secret of a mysterious section - .tls
constraints:
- Need
malloc
function and you can malloc with arbitrary size - Arbitrary address leaking
We make malloc
use mmap
to allocate memory(size 0x21000 is enough). In general, these pages will be placed at the address just before .tls
section.
There is some useful information on .tls
, such as the address of main_arena
, canary
(value of stack guard), and a strange stack address
which points to somewhere on the stack but with a fixed offset.
Before calling mmap:
7fecbfe4d000-7fecbfe51000 r--p 001bd000 fd:00 131210 /lib/x86_64-linux-gnu/libc-2.24.so
7fecbfe51000-7fecbfe53000 rw-p 001c1000 fd:00 131210 /lib/x86_64-linux-gnu/libc-2.24.so
7fecbfe53000-7fecbfe57000 rw-p 00000000 00:00 0
7fecbfe57000-7fecbfe7c000 r-xp 00000000 fd:00 131206 /lib/x86_64-linux-gnu/ld-2.24.so
7fecc0068000-7fecc006a000 rw-p 00000000 00:00 0 <- .tls section
7fecc0078000-7fecc007b000 rw-p 00000000 00:00 0
7fecc007b000-7fecc007c000 r--p 00024000 fd:00 131206 /lib/x86_64-linux-gnu/ld-2.24.so
7fecc007c000-7fecc007d000 rw-p 00025000 fd:00 131206 /lib/x86_64-linux-gnu/ld-2.24.so
After call mmap:
7fecbfe4d000-7fecbfe51000 r--p 001bd000 fd:00 131210 /lib/x86_64-linux-gnu/libc-2.24.so
7fecbfe51000-7fecbfe53000 rw-p 001c1000 fd:00 131210 /lib/x86_64-linux-gnu/libc-2.24.so
7fecbfe53000-7fecbfe57000 rw-p 00000000 00:00 0
7fecbfe57000-7fecbfe7c000 r-xp 00000000 fd:00 131206 /lib/x86_64-linux-gnu/ld-2.24.so
7fecc0045000-7fecc006a000 rw-p 00000000 00:00 0 <- memory of mmap + .tls section
7fecc0078000-7fecc007b000 rw-p 00000000 00:00 0
7fecc007b000-7fecc007c000 r--p 00024000 fd:00 131206 /lib/x86_64-linux-gnu/ld-2.24.so
7fecc007c000-7fecc007d000 rw-p 00025000 fd:00 131206 /lib/x86_64-linux-gnu/ld-2.24.so
Predictable RNG(Random Number Generator)
When the binary uses the RNG to make the address of important information or sth, we can guess the same value if it's predictable.
Assuming that it's predictable, we can use ctypes which is a build-in module in Python.
ctypes allows calling a function in DLL(Dynamic-Link Library) or Shared Library.
Therefore, if binary has an init_proc like this:
srand(time(NULL));
while(addr <= 0x10000){
addr = rand() & 0xfffff000;
}
secret = mmap(addr,0x1000,PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_ANONYMOUS ,-1,0);
if(secret == -1){
puts("mmap error");
exit(0);
}
Then we can use ctypes to get the same value of addr.
import ctypes
LIBC = ctypes.cdll.LoadLibrary('/path/to/dll')
LIBC.srand(LIBC.time(0))
addr = LIBC.rand() & 0xfffff000
Make stack executable
Use one-gadget-RCE instead of system
constraints:
- Have libc base address
- Write to arbitrary address
Almost every pwnable challenge needs to call system('/bin/sh')
in the end of the exploit, but if we want to call that, we have to manipulate the parameters and, of course, hijack some functions to system
. What if we can't manipulate the parameter?
Use one-gadget-RCE!
With one-gadget-RCE, we can just hijack .got.plt
or something we can use to control eip to make program jump to one-gadget, but there are some constraints that need satisfying before using it.
There are lots of one-gadgets in libc. Each one has different constraints but those are similar. Each constraint is about the state of registers.
E.g.
- ebx is the address of
rw-p
area of libc - [esp+0x34] == NULL
How can we get these constraints? Here is an useful tool one_gadget !!!!
So if we can satisfy those constraints, we can get the shell more easily.
Hijack hook function
constraints:
- Have libc base address
- Write to arbitrary address
- The program uses
malloc
,free
orrealloc
.
By manual:
The GNU C Library lets you modify the behavior of
malloc
,realloc
, andfree
by specifying appropriate hook functions. You can use these hooks to help you debug programs that use dynamic memory allocation, for example.
There are hook variables declared in malloc.h and their default values are 0x0
.
__malloc_hook
__free_hook
- ...
Since they are used to help us debug programs, they are writable during the execution.
0xf77228e0 <__free_hook>: 0x00000000
0xf7722000 0xf7727000 rw-p mapped
Let's look into the src of malloc.c. I will use __libc_free
to demo.
void (*hook) (void *, const void *) = atomic_forced_read (__free_hook);
if (__builtin_expect (hook != NULL, 0))
{
(*hook)(mem, RETURN_ADDRESS (0));
return;
}
It checks the value of __free_hook
. If it's not NULL, it will call the hook function first. Here, we would like to use one-gadget-RCE. Since hook function is called in the libc, the constraints of one-gadget are usually satisfied.
Use printf to trigger malloc and free
Look into the source of printf, there are several places which may trigger malloc. Take vfprintf.c line 1470 for example:
#define EXTSIZ 32
enum { WORK_BUFFER_SIZE = 1000 };
if (width >= WORK_BUFFER_SIZE - EXTSIZ)
{
/* We have to use a special buffer. */
size_t needed = ((size_t) width + EXTSIZ) * sizeof (CHAR_T);
if (__libc_use_alloca (needed))
workend = (CHAR_T *) alloca (needed) + width + EXTSIZ;
else
{
workstart = (CHAR_T *) malloc (needed);
if (workstart == NULL)
{
done = -1;
goto all_done;
}
workend = workstart + width + EXTSIZ;
}
}
We can find that malloc
will be triggered if the width field is large enough.(Of course, free
will also be triggered at the end of printf if malloc
has been triggered.) However, WORK_BUFFER_SIZE is not large enough, since we need to go to else block. Let's take a look at __libc_use_alloca
and see what exactly the minimum size of width we should give.
/* Minimum size for a thread. We are free to choose a reasonable value. */
#define PTHREAD_STACK_MIN 16384
#define __MAX_ALLOCA_CUTOFF 65536
int __libc_use_alloca (size_t size)
{
return (__builtin_expect (size <= PTHREAD_STACK_MIN / 4, 1)
|| __builtin_expect (__libc_alloca_cutoff (size), 1));
}
int __libc_alloca_cutoff (size_t size)
{
return size <= (MIN (__MAX_ALLOCA_CUTOFF,
THREAD_GETMEM (THREAD_SELF, stackblock_size) / 4
/* The main thread, before the thread library is
initialized, has zero in the stackblock_size
element. Since it is the main thread we can
assume the maximum available stack space. */
?: __MAX_ALLOCA_CUTOFF * 4));
}
We have to make sure that:
size > PTHREAD_STACK_MIN / 4
size > MIN(__MAX_ALLOCA_CUTOFF, THREAD_GETMEM(THREAD_SELF, stackblock_size) / 4 ?: __MAX_ALLOCA_CUTOFF * 4)
- I did not fully understand what exactly the function - THREAD_GETMEM do, but it seems that it mostly returns 0.
- Therefore, the second condition is usually
size > 65536
More details:
conclusion
- The minimum size of width to trigger
malloc
&free
is 65537 most of the time. - If there is a Format String Vulnerability and the program ends right after calling
printf(buf)
, we can hijack__malloc_hook
or__free_hook
withone-gadget
and use the trick mentioned above to triggermalloc
&free
then we can still get the shell even there is no more function call or sth afterprintf(buf)
.
Use execveat to open a shell
When it comes to opening a shell with system call, execve
always pops up in mind. However, it's not always easily available due to the lack of gadgets or others constraints.
Actually, there is a system call, execveat
, with following prototype:
int execveat(int dirfd, const char *pathname,
char *const argv[], char *const envp[],
int flags);
According to its man page, it operates in the same way as execve
. As for the additional arguments, it mentions that:
If pathname is absolute, then dirfd is ignored.
Hence, if we make pathname
point to "/bin/sh"
, and set argv
, envp
and flags
to 0, we can still get a shell whatever the value of dirfd
.