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Now that you understand the dynamics of the stack in this function, it becomes easy to see that only two unique stack addresses are being referenced in this function. The parameter is accessed in the first line (and it looks like the function only takes one parameter), and the beginning of the local variable area in the other three accesses. The function starts by copying a string whose pointer was passed as the first parameter to a local variable (whose size we know is 100 bytes). This is exactly where the potential stack overflow lies. strcpy has no idea how big a buffer has been reserved for the copied string and will keep on copying until it encounters the null terminator in the source string or until the program crashes. If a string longer than 100 bytes is fed to this function, strcpy will essentially overwrite whatever follows the local string variable in the stack. In this particular function, this would be the function s return address. Overwriting the return address is a sure way of gaining control of the system. The classic exploit for this kind of overflow bug is to feed this function with a string that essentially contains code and to carefully place the pointer to that code in the position where strcpy is going to be overwriting the return address. One thing that makes this process slightly more complicated than it initially seems is that the entire buffer being fed to the function can t contain any zero bytes (except for one at the end), because that would cause strcpy to stop copying. There are several simple patterns to look for when searching for a stack overflow vulnerability in a program. The first thing is probably to look at a function s stack size. Functions that take large buffers such as strings or other data and put it on the stack are easily identified because they tend to have huge local variable regions in their stack frames. This can be identified by looking for a SUB ESP instruction at the very beginning of the function. Functions that store large buffers on the stack will usually subtract ESP by a fairly large number. Of course, in itself a large stack size doesn t represent a problem. Once you ve located a function that has a conspicuously large stack space, the next step is to look for places where a pointer to the beginning of that space is used. This would typically be a LEA instruction that uses an operand such as [EBP 0x200], or [ESP 0x200], with that constant being near or equal to the specific size of the stack space allocated. The trick at this point is to make sure the code that s accessing this block is properly aware of its size. It s not easy, but it s not impossible either.
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The C runtime library string-manipulation routines have historically been the reason for quite a few vulnerabilities. Most programmers nowadays know better than to leave such doors wide open, but it s still worthwhile to learn to identify calls to these functions while reversing. The problem is that some
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compilers treat these functions as intrinsic, meaning that the compiler automatically inserts their implementation into the calling function (like an inline function) instead of calling the runtime library implementation. Here is the same vulnerable launch function from before, except that both string-manipulation calls have been compiled into the function.
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7!launch: 00401060 mov 00401064 lea 00401068 sub 0040106b sub 0040106d lea 00401070 mov 00401072 mov 00401075 inc 00401076 test 00401078 jnz 0040107a push 0040107b lea 0040107f dec 00401080 mov 00401083 inc 00401084 test 00401086 jnz 00401088 mov 0040108d mov 00401093 lea 00401097 mov 00401099 push 0040109a mov 0040109d call 004010a2 add 004010a5 pop 004010a6 add 004010a9 ret eax,[esp+0x4] edx,[esp-0x64] esp,0x64 edx,eax ecx,[ecx] cl,[eax] [edx+eax],cl eax cl,cl 7!launch+0x10 (00401070) edi edi,[esp+0x4] edi al,[edi+0x1] edi al,al 7!launch+0x20 (00401080) eax,[7!'string (00408128)] cl,[7!'string +0x4 (0040812c)] edx,[esp+0x4] [edi],eax edx [edi+0x4],cl 7!system (00401102) esp,0x4 edi esp,0x64
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It is safe to say that regardless of intrinsic string-manipulation functions, any case where a function loops on the address of a stack-variable such as the one obtained by the lea edx,[esp-0x64] in the preceding function is worthy of further investigation.
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