Buffer overflow to jump to a part of code

My teacher gave me some code and I have to run it and make it jump to the admin section using a buffer overflow. I cann开发者_如何学JAVAot modify the source code. Could someone explain how I could jump to the admin method using a buffer overflow? I'm running it on ubuntu 8.10 and it was compiled with an older version of gcc so the overflow will work.

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Without being able to see the code, on a general level you need to design inputs to the function that will overwrite the return address (or another address to which control will be transferred by the function) on the stack.

At a guess, the code has a fixed length character buffer and copies values from a function parameter into that buffer without validating that the length does not exceed the length of the buffer.

You need to make a note of what the stack layout looks like for your application (running it under a debugger may well be the quickest way to do this) to work out where the address you need to override is, then put together a string to overwrite this with the address of the admin function you need to call.

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You can always get the asm output of it (I forgot how right now... brainfart) and see where the buffer you want to overflow is being used/read and check it's length. Next you want to calculate how far you need to overflow it so that you either replace the next instruction with a JMP (address of admin code) or change a JMP address to that of the admin section. 0xE8 is the jump opcode for x86 if you need it since you want to overwrite the binary data of the instruction with your own.