Buffer overflow stack8/15/2023 When the stack overflow is exploited, hackers will try to overwrite the return address so that it points at the buffer location with the malicious code injected. If the canary has been altered, the program is terminated with a fatal Stack smashing detected error.Īnother technique used by operating systems is restricting code evaluation on the stack. Before returning the function makes sure that the canary has the same value. A stack canary is just a random integer generated when the function is called. In gcc, for example, if the function allocates a buffer on the stack, an additional so-called stack canary is added. Protection against buffer overflowsĬompilers and operating systems have some techniques to prevent such stack exploits. In order to alter the zero variable we need to represent the number in the little endian form and write the corresponding bytes to the 65, 66, 67 and 68 offsets in in buffer. You changed the zero variable to 13877 (hex: 3635)! For demonstration purposes, we will use the following string as the input: In order to get past the end of the buffer, we need at more than 64 bytes (because the buffer size is 64). => 0x401432 : mov eax,DWORD PTR Īs we can see, 43 zero-characters (ascii code 0x30) was not enough to get to the zero value that we want it to occur. the address calculated with the previous instruction gets saved on the stackĠx0040142d : call 0x403ae8 gets() call by caching it’s value in one of the general-purpose registers.īy disassembling the program with gdb, we get: 0x00401410 : push ebp save the previous ebp registerĠx00401411 : mov ebp,esp initializing ebp of the new stack frameĠx00401413 : and esp,0xfffffff0 memory aligningĠx00401416 : sub esp,0圆0 memory allocation on the stackĠx0040141e : mov DWORD PTR ,0x0 assign to zero The zero variable is marked as volatile to prevent the compiler from optimizing it’s usage, e.g. #include void _attribute_ (( noinline )) fun ( int a, int b, int c )
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