Use of Invariant Value in Dynamically Changing Context

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

The following code is an example of an internal hard-coded password in the back-end:

int VerifyAdmin(char *password) {

  if (strcmp(password, "Mew!")) {

    printf("Incorrect Password!\n");
    return(0)

  }
  printf("Entering Diagnostic Mode...\n");
  return(1);

}

int VerifyAdmin(String password) {
  if (!password.equals("Mew!")) {
    return(0)
  }
  //Diagnostic Mode
  return(1);
}

Every instance of this program can be placed into diagnostic mode with the same password. Even worse is the fact that if this program is distributed as a binary-only distribution, it is very difficult to change that password or disable this "functionality."

Example Two

This code assumes a particular function will always be found at a particular address. It assigns a pointer to that address and calls the function.

int (*pt2Function) (float, char, char)=0x08040000;
int result2 = (*pt2Function) (12, 'a', 'b');
// Here we can inject code to execute.

The same function may not always be found at the same memory address. This could lead to a crash, or an attacker may alter the memory at the expected address, leading to arbitrary code execution.

See Also