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

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Do not attempt to assess an object outside of its lifetime.  Attempting to do so is undefined behavior and lead to an exploitable vulnerability. (See also undefined behavior 9 in Appendix J of the C Standard.)

Noncompliant Code Example (

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Differing Storage Durations)

In this noncompliant code example, the address of local a variable c_str with automatic storage duration 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 (

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Same Storage Durations)

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

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

Compliant Solution (Differing

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Storage Durations)

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.

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In this noncompliant code sample, the function init_array() returns a pointer to a local stack variablecharacter array with automatic storage duration, which could be accessed by the caller:

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Some compilers generate a diagnostic message when a pointer to an object with automatic variable storage duration 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.)

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In this noncompliant code sampleexample, 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(void) {
  char *ptr;
  squirrel_away(&ptr);
  /* ptr is live but invalid here */
}

Compliant Solution (Output Parameter)

The In this compliant solution, the variable local has static storage duration, so ptr is live and valid in the function ; consequently ptr can be used to reference the local array within the rodent() function:

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

void rodent(void) {
  char *ptr;
  squirrel_away(&ptr);
  /* ptr is livevalid andin validthis herescope */
}

Risk Assessment

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

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