I am working on a 32-bit binary which reads an input from the user and uses that input as a format string for printf.
I need to overwrite a specific address with a single byte.
The issue is that I am not able to overwrite the address with the expected value.
I use the following approach and I need help to understand why the incorrect value is being written at the chosen memory address.
Memory address to be overwritten: 0xaabbccdd
$ echo -n $(python -c 'print "\xdd\xcc\xbb\xaa" + "%x" * 6') | ./bin �̻�ffffd0a81814ffffd34556557000aabbccdd
So, I know that when I enter %x, 6 times, the address that I want to overwrite will be popped from the stack. So, using the 6th %x, I can interact with this memory address.
To read the contents of 0xaabbccdd, I would do:
$ echo -n $(python -c 'print "\xdd\xcc\xbb\xaa" + "%x" * 5 + "%s"') | ./bin �̻�ffffd0a81814ffffd34556557000aabbccdd
Now, I want to write 0x18 to the address: 0xaabbccdd.
0x18 = 24 (in decimal).
If I use %x 5 times, then the number of bytes written by printf are:
4 bytes -> corresponding to address: 0xaabbccdd 5 DWORDs from the stack = 5 * 4 = 20 bytes
so, %n should write (20 + 4) = 24 bytes at the memory address 0xaabbccdd with the below format string:
echo -n $(python -c 'print "\xdd\xcc\xbb\xaa" + "%x" * 5 + "%n') | ./bin
Instead, it overwrites the address with the value, 0x20.
I am not able to understand, why those extra 2 bytes?
%n is supposed to write the number of bytes printed by printf so far.