Pre-requisite: GDB (Step by Step Introduction)
A BufferOverflow often occurs when the content inside the defined variable is copied to another variable without doing Bound Checks or considering the size of the buffer. Let’s analyze buffer overflow with the help GNU Debugger (GDB) which is inbuilt every Linux system.
The motive of this exercise is to get comfortable with debugging code and understand how does buffer overflow works in action.
- Step 1
Let’s compile this code with the following flags :
gcc overflow.c -o overflow -fno-stack-protector -z execstack -no-pie
The above code is going to create a compiled binary that disables various stack protections
-z execstack : Disables Executable Stack -fno-stack-protector : Disables Stack Canaries -no-pie : Disables Position Independent Executables
- Step 2
Now that stack protections are disabled we can load the code in GDB by typing
- Step 3
Once the code is open we can look at the functions that are inside the binary by using typing
We can see there’s a gets call which is being used which is vulnerable in nature as it doesn’t do any bound checks.
- Step 4
- Step 5
Let’s put a breakpoint by typing
b * main+39
so that we can analyze the content of stack when the program hits the breakpoint.
- Step 6
to run the code and input any number of A’s as we already know from the code above.
Let’s input 63 A’s and 78 A’s and see the change in the result.
- Step 7
You can use python code to print A’s by typing after leaving the GDB.
python -c "print 'A' * 63"
- Step 8
Let’s try the whole process again and this time let’s input any number of A’s let’s say 78.
A cool way to do this can be
python -c "print 'A' * 78" | ./overflow
As we can see once the overflow occurs it changes the variable because of memory being leaked on the stack and changing values of variables
- Step 9
Let’s check the stack which it over writes, so we have to set a break point at
and then we can type
x : eXamine 20s : 20 values in string $rsp : for register RSP (Stack Pointer)
Hence we can see how 78 A’s are written on the stack and are overflowing the memory.
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