Buffer Overrun Project, Problem Entering NULL in to stream

This is a discussion on Buffer Overrun Project, Problem Entering NULL in to stream within the C++ Programming forums, part of the General Programming Boards category; Iíve got a strange assignment from a class in which Iím supposed to initiate a buffer overrun to see how ...

  1. #1
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    Buffer Overrun Project, Problem Entering NULL in to stream

    Iíve got a strange assignment from a class in which Iím supposed to initiate a buffer overrun to see how easy it is for someone to take advantage of the gets() function.

    The basic program that Iím given is this:

    Code:
    typedef struct student
    {
    	char name[20];
    	float amount;
    	int other_stuff;
    } person;
    
    int _tmain(int argc, _TCHAR* argv[])
    {
    	person p; // Create a person account (on the stack)
    	p.amount = 10.0; // set amount
    	printf("enter name: ");
    	gets(p.name); // get name (blindly ignoring buffer overrun)
    	printf("Here is a $%6.2f gift for %s\n", p.amount, p.name);
    	getch(); // Wait for user to type <Enter> before quitting
    	return 0;
    }
    I understand that if I write more than 20 characters and the command prompt I will start writing in to the area in memory where ďamountĒ is located so I can change it to anything that I want it to be. We are supposed to set the ďamountĒ to 1,000,000 which Iíve figured out is Ė 0x00 0x24 0x74 0x49 when arranged in memory. I know that I can hold down alt and input the decimal equivalent of those numbers to overwrite memory but I canít get a NULL to work for my first byte.

    In a perfect world I would use this NULL after I enter my name then put in whatever I want up until I get to the amount area in memory and write in alt 000, alt 036, alt 116, alt 073. Unfortunately I canít get alt 000 to register and Iím wondering if there is another way to put a NULL bit in to the stream at a command prompt.

    Does anyone have any ideas?

    ***EDIT***

    I might be wrong on what I need to enter as now looking in my debugger I see a different hex representation for 1,000,000 - 0x00 0x50 0xc3 0x47
    Last edited by Peter5897; 07-10-2006 at 04:52 PM.

  2. #2
    Hardware Engineer
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    gets() should automatcially add the null, when you hit ENTER.


    1,000,000 decimal converts to F4240 hex as an integer. But, when it comes to storing a float, things get complicated, so, the debugger is probably the best way to do it.
    Last edited by DougDbug; 07-10-2006 at 05:16 PM.

  3. #3
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    Yeah but that's the problem, I want a NULL before I hit enter. In an ideal world I'd have a NULL after I enter my name so that it acts as a normal C string and then I would have a NULL in my representation of float 1,000,000 as well.

    IE "Jo BobNULL--------------FLOAT VAL" would be my input at the command line where FLOAT VAL would be whatever characters are needed to make 1,000,000 in floating point arithmetic.
    Last edited by Peter5897; 07-11-2006 at 01:54 AM.

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