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https://github.com/naetw/ctf-pwn-tips

Here record some tips about pwn. Something is obsoleted and won't be updated. Sorry about that.
https://github.com/naetw/ctf-pwn-tips

ctf pwn tips

Last synced: 12 days ago
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Here record some tips about pwn. Something is obsoleted and won't be updated. Sorry about that.

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CTF-pwn-tips

===========================



# Catalog

* [Overflow](#overflow)

* [Find string in gdb](#find-string-in-gdb)

* [Binary Service](#binary-service)

* [Find specific function offset in libc](#find-specific-function-offset-in-libc)

* [Find '/bin/sh' or 'sh' in library](#find-binsh-or-sh-in-library)

* [Leak stack address](#leak-stack-address)

* [Fork problem in gdb](#fork-problem-in-gdb)

* [Secret of a mysterious section - .tls](#secret-of-a-mysterious-section---tls)

* [Predictable RNG(Random Number Generator)](#predictable-rngrandom-number-generator)

* [Make stack executable](#make-stack-executable)

* [Use one-gadget-RCE instead of system](#use-one-gadget-rce-instead-of-system)

* [Hijack hook function](#hijack-hook-function)

* [Use printf to trigger malloc and free](#use-printf-to-trigger-malloc-and-free)

* [Use execveat to open a shell](#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 instruction `sub esp, eax` to achieve that.

        * If we make num negative, it will have overlapped stack frame.

        * E.g. [Seccon CTF quals 2016 cheer_msg](https://github.com/ctfs/write-ups-2016/tree/master/seccon-ctf-quals-2016/exploit/cheer-msg-100)

    * 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!



### 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` or `puts` 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(buf, 1, 40, stdin)`

    * Almost the same as `read`.

    * **leakable**



### 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)` && `memcpy(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**



* `strncat(buf, buf2, n)`

    * Almost the same as `strcat`, but with size limitation.

    * **pwnable**

    * E.g. [Seccon CTF quals 2016 jmper](https://github.com/ctfs/write-ups-2016/tree/master/seccon-ctf-quals-2016/exploit/jmper-300)



## Find string in gdb



In the problem of [SSP](http://j00ru.vexillium.org/blog/24_03_15/dragons_ctf.pdf), 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 the `output - 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](https://github.com/longld/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](https://github.com/Gallopsled/pwntools), then you can use it in your exploit script.



E.g.



```python

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](https://github.com/Gallopsled/pwntools)



E.g.



```python

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 

2       #include 

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 :       0x00007fffffffe230

(gdb) p/x (char **)envp

$12 = 0x7fffffffe230

```



* `0x7ffff7a0e000` is current libc base address

* `0x3c5f38` is offset of `environ` in libc



This [manual](https://www.gnu.org/software/libc/manual/html_node/Program-Arguments.html) 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](https://docs.python.org/2/library/ctypes.html) 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:



```c

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.



```python

import ctypes

LIBC = ctypes.cdll.LoadLibrary('/path/to/dll')

LIBC.srand(LIBC.time(0))

addr = LIBC.rand() & 0xfffff000

```



## Make stack executable



* [link1](http://radare.today/posts/defeating-baby_rop-with-radare2/)

* [link2](https://sploitfun.wordpress.com/author/sploitfun/)

* Haven't read yet orz



## 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](http://j00ru.vexillium.org/blog/24_03_15/dragons_ctf.pdf)!



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](https://github.com/david942j/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` or `realloc`.



By manual:



> The GNU C Library lets you modify the behavior of `malloc`, `realloc`, and `free` 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](https://code.woboq.org/userspace/glibc/malloc/malloc.c.html#2917) of malloc.c. I will use `__libc_free` to demo.



```c

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](https://code.woboq.org/userspace/glibc/stdio-common/vfprintf.c.html#1470) for example:



```c

#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.



```c



/* 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:



1. `size > PTHREAD_STACK_MIN / 4`

2. `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:



* [__builtin_expect](https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html)

* [THREAD_GETMEM](https://code.woboq.org/userspace/glibc/sysdeps/x86_64/nptl/tls.h.html#_M/THREAD_GETMEM)



### 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` with `one-gadget` and use the trick mentioned above to trigger `malloc` & `free` then we can still get the shell even there is no more function call or sth after `printf(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:



```c

int execveat(int dirfd, const char *pathname,

             char *const argv[], char *const envp[],

             int flags);

```



According to its [man page](http://man7.org/linux/man-pages/man2/execveat.2.html), 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`.