CODEWITHSUNDEEP
cbufferbuffer-overflow
user1973108
user1973108

Reputation:

Buffer overflow in C with gets

I am very new to C and as a class assignment my instructor wanted us to play with buffer overflows. I found the following online as an example and I can't figure out how to use it!

#include <stdio.h>
char temp[32];

unsigned int setThis=1;

printf("Enter your temp: \n");
fgets(temp, 34, stdin); //Takes a 34 buffer size when temp can only be 32
printf("Value of you setThis: %d", setThis);

So my question is, how do i set "setThis" to a certain variable? Any help is appreciated, BeastlyJman.

Upvotes: 0

Views: 433

Answers (1)

user3386109
user3386109

Reputation: 34839

There's no guaranteed way to do it, but typically variables are put on the stack such that the first variable is last in memory. So if you declare setThis before temp[32], then setThis will be at the end of the temp array, and you can overwrite it.

But as I said, there's no guarantee that's what the compiler will do. You should really check the assembly code that the compiler generates to see where temp and setThis are located.

Also, you can save yourself some typing if you reduce the size of temp to temp[8] and then pass 10 to fgets. To cause an overflow, you need to type more characters than the buffer can hold.

Upvotes: 0

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