Dangling pointers can lead to exploitable double-free and access-freed-memory vulnerabilities. A simple yet effective way to eliminate dangling pointers and avoid many memory-related vulnerabilities is to set pointers to NULL after they are freed or to set them to another valid object.

Noncompliant Code Example

In this noncompliant code example, the type of a message is used to determine how to process the message itself. It is assumed that message_type is an integer and message is a pointer to an array of characters that were allocated dynamically. If message_type equals value_1, the message is processed accordingly. A similar operation occurs when message_type equals value_2. However, if message_type == value_1 evaluates to true and message_type == value_2 also evaluates to true, then message is freed twice, resulting in a double-free vulnerability.

char *message;
int message_type;

/* Initialize message and message_type */

if (message_type == value_1) {
  /* Process message type 1 */
  free(message);
}
/* ...*/
if (message_type == value_2) {
   /* Process message type 2 */
  free(message);
}

Compliant Solution

Calling free() on a null pointer results in no action being taken by free(). Setting message to NULL after it is freed eliminates the possibility that the message pointer can be used to free the same memory more than once.

char *message;
int message_type;

/* Initialize message and message_type */

if (message_type == value_1) {
  /* Process message type 1 */
  free(message);
  message = NULL;
}
/* ... */
if (message_type == value_2) {
  /* Process message type 2 */
  free(message);
  message = NULL;
}

Exceptions

MEM01-C-EX1: If a nonstatic variable goes out of scope immediately following the free(), it is not necessary to clear its value because it is no longer accessible.

void foo(void) {
  char *str;
  /* ... */
  free(str);
  return;
}

Risk Assessment

Setting pointers to NULL or to another valid value after memory is freed is a simple and easily implemented solution for reducing dangling pointers. Dangling pointers can result in freeing memory multiple times or in writing to memory that has already been freed. Both of these problems can lead to an attacker executing arbitrary code with the permissions of the vulnerable process.

Automated Detection

Tool	Version	Checker	Description
Astrée			Supported: Astrée reports usage of invalid pointers.
Axivion Bauhaus Suite		CertC-MEM01	Fully implemented
CodeSonar		ALLOC.DF ALLOC.UAF	Double free Use after free
Compass/ROSE
Coverity		USE_AFTER_FREE	Can detect the specific instances where memory is deallocated more than once or read/written to the target of a freed pointer
LDRA tool suite		484 S, 112 D	Partially implemented
Parasoft C/C++test		CERT_C-MEM01-a CERT_C-MEM01-b CERT_C-MEM01-c CERT_C-MEM01-d	Do not use resources that have been freed Always assign a new value to an expression that points to deallocated memory Always assign a new value to global or member variable that points to deallocated memory Always assign a new value to parameter or local variable that points to deallocated memory
Parasoft Insure++			Detects dangling pointers at runtime
Polyspace Bug Finder		CERT C: Rec. MEM01-C	Checks for missing reset of a freed pointer (rec. fully covered)

Related Vulnerabilities

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

Related Guidelines

Bibliography