Reverse Engineering Notes: Heap Grooming

 Uninitialized data access in C can also occur when a program reads or writes to memory that has not been properly initialized or allocated, such as with dynamically allocated memory on the heap. Heap grooming is a technique that can be used to intentionally manipulate the heap in order to cause vulnerabilities in a program.

Here's an example of how heap grooming can lead to uninitialized data access in C:

c

#include <stdio.h>
#include <stdlib.h>

int main() {
    char *buffer = (char *)malloc(10);
    char *secret = (char *)malloc(10);
    printf("Enter a secret: ");
    gets(secret);  // Unsafe function that can write beyond secret boundaries
    strcpy(buffer, secret);  // Copy secret to buffer
    printf("You entered: %s\n", buffer);
    free(buffer);
    free(secret);
    return 0;
}

In this example, the program declares two character pointers buffer and secret, and dynamically allocates 10 bytes of memory for each of them using the malloc function. It then reads a secret string from the user using the unsafe gets function, and copies the secret string to the buffer using the strcpy function. Finally, the program prints the contents of the buffer, and frees the memory allocated for both buffer and secret.

However, this program is vulnerable to heap grooming attacks because the size of the allocated memory for secret is not checked, and the strcpy function does not validate the size of the destination buffer. An attacker can craft an input string that is longer than the size of the secret buffer, and include a specific pattern of values that will overwrite adjacent memory locations on the heap. For example, the attacker might fill the first few bytes of the input string with a specific pattern that will overwrite the memory location just after the secret buffer, which is the location where buffer is stored.

In this way, heap grooming can be used to cause uninitialized data access and other security vulnerabilities in C programs. Here is a diagram that illustrates the memory layout of the program after a heap grooming attack:

lua

   +--------------------------+
   |      Input String        |
   +--------------------------+
   | Secret |                 |
   +--------+----------------+
   |   A    |    B    |  C  | Memory just after Secret
   +--------+----------------+
   |   0x41414141             | A's overwritten value
   +--------------------------+
   |   0x42424242             | B's overwritten value
   +--------------------------+
   |   0x43434343             | C's overwritten value
   +--------------------------+
   | Buffer |                 |
   +--------+----------------+
   |    Secret's contents     |
   +--------------------------+

In this diagram, the input string from the attacker is represented by the "Input String" box at the top of the diagram. The secret buffer is represented by the "Secret" box, which has a size of 10 bytes. The three variables A, B, and C are adjacent to the secret buffer on the heap, and are overwritten with specific values that were included in the input string. The buffer pointer is stored just after the secret buffer, and is overwritten with the value of variable A.

When the strcpy function is called, it copies the contents of the secret buffer to the buffer buffer, but also overwrites the adjacent memory locations on the heap with the specific pattern of values included in the input string. This can cause uninitialized data access and other security vulnerabilities, depending on the contents of the overwritten memory locations.