Securing Systems with chroot

Tuesday, 4 February 2003, 11:27 AM EST

One popular technique crackers use to compromise machines is exploiting buffer overflows. Buffer overflows are programming bugs which often plague software written with the C language, which makes such mistakes easy to make.

So what happens? The programmer has expected the user to supply a DOS filename, which should fit in a 13 character buffer. Unfortunately, he has failed to validate user input. If the user types a filename which is 25 bytes long, scanf() will copy all the 25 bytes in the buffer, which is only 13 bytes long. The extraneous data will overwrite memory locations near the buffer.

Local variables such as the buffer used in this example are allocated on a memory area called the stack. The processor maintains the stack, using a register known as the stack pointer to keep track of the memory location of the top of the stack. The stack pointer moves with every function call, and some or all of the following data is pushed on the stack: the current contents of the CPU's registers, the arguments to the function, and a placeholder for the function's return value. Additionally, a placeholder for local variables is created.

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