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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.

Recommendation

Severity

Likelihood

Remediation Cost

Priority

Level

MEM01-C

High

Unlikely

Low

P9

L2

Automated Detection

Tool

Version

Checker

Description

Astrée
19.04

Supported: Astrée reports usage of invalid pointers.
Axivion Bauhaus Suite

6.9.0

CertC-MEM01Fully implemented
CodeSonar
5.2p0

ALLOC.DF
ALLOC.UAF

Double free
Use after free

Compass/ROSE




Coverity

2017.07

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
9.7.1
484 S, 112 DPartially implemented
Parasoft C/C++test
10.4.2

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

R2019b

Missing reset of a freed pointerPointer free not followed by a reset statement to clear leftover data

Related Vulnerabilities

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

Related Guidelines

SEI CERT C++ Coding StandardVOID MEM01-CPP. Store a valid value in pointers immediately after deallocation
ISO/IEC TR 24772:2013Dangling References to Stack Frames [DCM]
Dangling Reference to Heap [XYK]
Off-by-one Error [XZH]
MITRE CWECWE-415, Double free
CWE-416, Use after free

Bibliography

[Seacord 2013]Chapter 4, "Dynamic Memory Management"
[Plakosh 2005]



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