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An object has a storage duration that determines its lifetime. There are four storage durations: static, thread, automatic, and allocated.

According to the C Standard, subclause 6.2.4, paragraph 2 [ISO/IEC 9899:2011],

The lifetime of an object is the portion of program execution during which storage is guaranteed to be reserved for it. An object exists, has a constant address, and retains its last-stored value throughout its lifetime. If an object is referred to outside of its lifetime, the behavior is undefined. The value of a pointer becomes indeterminate when the object it points to reaches the end of its lifetime.

Attempting to access an object outside of its lifetime can result in undefined behavior and lead to an exploitable vulnerability. (See also undefined behavior 9 in Appendix J of the C Standard.)

Noncompliant Code Example (Static Variables)

In this noncompliant code example, the address of local variable c_str is assigned to the variable p, which has static storage duration. The assignment itself is valid, but it is invalid for c_str to go out of scope while p holds its address, as happens at the end of dont_do_this().

#include <stdio.h>
 
const char *p;
void dont_do_this(void) {
  const char c_str[] = "This will change";
  p = c_str; /* Dangerous */
}

void innocuous(void) {
  printf("%s\n", p);
}

int main(void) {
  dont_do_this();
  innocuous();
  return 0;
}

Compliant Solution (Similar Scope)

In this compliant solution, p is declared with the same scope as c_str, preventing p from taking on an indeterminate value outside of this_is_OK():

void this_is_OK(void) {
  const char c_str[] = "Everything OK";
  const char *p = c_str;
  /* ... */
}
/* p is inaccessible outside the scope of string c_str */

Alternatively, both p and c_str could be declared with static scope.

Compliant Solution (Differing Scope)

If it is necessary for p to be defined with static storage duration but c_str with a more limited duration, then p can be set to NULL before c_str is destroyed. This practice prevents p from taking on an indeterminate value, although any references to p must check for NULL.

const char *p;
void is_this_OK(void) {
  const char c_str[] = "Everything OK?";
  p = c_str;
  /* ... */
  p = NULL;
}

Noncompliant Code Example (Return Values)

In this noncompliant code sample, the function init_array() returns a pointer to a local stack variable, which could be accessed by the caller:

char *init_array(void) {
  char array[10];
  /* Initialize array */
  return array;
}

Some compilers generate a diagnostic message when a pointer to an automatic variable is returned from a function, as in this example. Compile your code at high warning levels and resolve any diagnostic messages. (See MSC00-C. Compile cleanly at high warning levels.)

Compliant Solution (Return Values)

The solution, in this case, depends on the intent of the programmer. If the intent is to modify the value of array and have that modification persist outside of the scope of init_array(), the desired behavior can be achieved by declaring array elsewhere and passing it as an argument to init_array():

#include <stddef.h>
void init_array(char *array, size_t len) {
  /* Initialize array */
  return;
}

int main(int argc, char *argv[]) {
  char array[10];
  init_array(array, sizeof(array) / sizeof(array[0]));
  /* ... */
  return 0;
}

Noncompliant Code Example (Output Parameter)

In this noncompliant code sample, the function squirrel_away() stores a pointer to local stack variable local into a location pointed to by function parameter ptr_param. Upon the return of squirrel_away() the pointer ptr_param points to a variable that has an expired lifetime.

void squirrel_away(char **ptr_param) {
  char local[10];
  /* Initialize array */
  *ptr_param = local;
}

void rodent() {
  char *ptr;
  squirrel_away(&ptr);
  /* ptr is live but invalid here */
}

Compliant Solution (Output Parameter)

The variable local has static storage duration, so ptr is live and valid in the function rodent():

char local[10];
 
void squirrel_away(char **ptr_param) {
  /* Initialize array */
  *ptr_param = local;
}

void rodent() {
  char *ptr;
  squirrel_away(&ptr);
  /* ptr is live and valid here */
}

Risk Assessment

Referencing an object outside of its lifetime can result in an attacker being able to run arbitrary code.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography