Wednesday, September 26, 2012


About a year ago I stumbled upon the Over The Wire hacker challenges and started solving the first set of levels (called vortex). Since then, I have been publishing my solutions in my blog. Here is vortex level 7:

The code

int main(int argc, char **argv)
        char buf[58];
        u_int32_t hi;
        if((hi = crc32(0, argv[1], strlen(argv[1]))) == 0xe1ca95ee) {
                strcpy(buf, argv[1]);
        } else {
                printf("0x%08x\n", hi);

The vulnerability exposed in this code is a basic buffer overflow with two subtleties:

  1. The CRC of the buffer must equate to a given value (0xe1ca95ee)
  2. The buffer is rather small (58 bytes)

Manipulating the checksum

The cyclic redundancy check (CRC) computes a check value (or checksum), which is used to detect accidental changes in data, e.g. when transmitting over unreliable communication channels. With this error detecting code, the slightest change (i.e. bit-flip) in the input data results in a very different output pattern. As opposed to cryptographic hash functions like SHA1 or MD5, preimage resistance is not a property of CRC, it is not designed to withstand preimage attacks: given a checksum C, it is not hard to find an input m such that CRC(m) = C. As such, one shouldn't rely on it for integrity checks over insecure channels since it is very easy to manipulate it as is shown in the solution.

To solve this level, I chose to apply the CRC-reversing algorithm described in Reversing CRC – Theory and Practice, which by the way also contains a very nice introduction to CRC. The method consists in appending a 32-bit pattern to the buffer in order to adjust the CRC-remainder to the desired checksum. The same principle is proposed in the suggested lecture, CRC and how to Reverse it. But as opposed to the first approach which uses the inverse of the divisor polynomial, the bit pattern is derived using a system of equations.

Overflowing the buffer

This level contains a classic vulnerability which can easily be exploited to execute arbitrary code: a buffer overflow. The use of the strcpy standard library function to copy a buffer of data to another completely disregards the destination's capacity. If the source buffer is larger than the destination, all bytes will be copied, even though the destination's bound has been exceeded, and in doing so, subsequent structures in memory will be destroyed.

Intercepting the instruction pointer

Depending where the destination buffer is located in the process memory, it may be possible for an attacker to take influence on the program execution flow. In this case, the destination buffer is on the stack. By overflowing the buffer, copying bytes past its bound, the stored eip value will be overwritten. This is a pointer to the next instruction to return to after leaving the current stack frame, i.e. when returning from the current function call. With a meaningful value, it is possible to redirect the execution of the program to any executable location in memory.

Creating a shellcode

The payload we want to execute consists in a small fragment of x86 machine instructions, which perform 2-3 syscalls that allow us to run a shell:

  • geteuid()/setreuid() are used to set the effective user-id. The exploited binary runs with the suid-bit, which means the process is executed in the name of the file owner (the user that has read-privileges for the next level's password file).
  • execve() is called to run /bin/bash.

The original x86/asm code can be found here. Check out the Makefile to see how it is compiled and the raw instruction data is extracted. It is then necessary to encode the data to avoid specific patterns such as \0 bytes. I used metasploit's msfencode tool for this.

Executing arbitrary code

Since the buffer is rather small (58 bytes), it is difficult to dissimulate the malicious payload. An alternative way to include arbitrary data into the process memory is to define an evironment variable containing the data. It will be accessible from the beginning of the stack. The buffer must then overflow the saved eip value to point at the corresponding region in memory. Unfortunately, this address cannot be precisely deduced. Therefore, a common strategy consists in prepending a large number of nop instructions before the shellcode. This extends the landing platform of the target address thus increasing the probability of hitting the shellcode.

The exploit

The finalized exploit is available here. It is a C wrapper which prepares a shellcode and the buffer contents and calls the binary to exploit. I employed following methods from SAR-PR-2006-05 to implement the table driven CRC32 algorithm:

  • make_crc_table()
  • crc32_tabledriven()
  • fix_crc_end()

Since at first the resulting checksum values did not match the ones generated by vortex7 I additionally extracted the CRC32 table from the binary and stored them in crc_table_static. I realized that the vortex7 implementation actually uses 0x00000000 instead of 0xFFFFFFFF for INITXOR and FINALXOR.

The fix_crc_end() function adjusts the buffer such that its checksum eventually results in the desired value 0xe1ca95ee.

make_buffer() creates the data used to overflow the buffer. It contains a repetitive sequence of the target address. It allows to shift the sequence bytewise in order to adjust its alignment. make_payload() generates the buffer which contains the nop sled and the shellcode.

Finally, the wrapper executes vortex7, passing the address buffer as a command line argument and the payload in the environment variables.

The program expects two arguments:

  1. An offset for the target address, relative to the environment pointer taken from the current process (the wrapper).
  2. An alignment index (0-4) used to align the target address in the buffer.

Following arguments worked for me:

$ ./v7_wrapper 0 2
Using address: 0xFFFFD91F
$ whoami

The password for the next level is then retrieved from the password file for the next level:

$ cat /etc/vortex_pass/vortex8 

That's it! If you want to learn more about buffer overflows, I suggest you read Smashing the Stack for Fun and Profit by aleph1, originally posted in Phrack Magazine. Also have a look at my blog where I regularily publish vortex level solutions:

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