CertC-MEM30¶

Do not access freed memory

Required inputs: IR, StaticSemanticAnalysis

Evaluating a pointer-including dereferencing the pointer, using it as an operand of an arithmetic operation, type casting it, and using it as the right-hand side of an assignment-into memory that has been deallocated by a memory management function is undefined behavior. Pointers to memory that has been deallocated are called dangling pointers. Accessing a dangling pointer can result in exploitable vulnerabilities.

According to the C Standard, using the value of a pointer that refers to space deallocated by a call to the free() or realloc() function is undefined behavior. (See undefined behavior 177.)

Reading a pointer to deallocated memory is undefined behavior because the pointer value is indeterminate and might be a trap representation. Fetching a trap representation might perform a hardware trap (but is not required to).

It is at the memory manager's discretion when to reallocate or recycle the freed memory. When memory is freed, all pointers into it become invalid, and its contents might either be returned to the operating system, making the freed space inaccessible, or remain intact and accessible. As a result, the data at the freed location can appear to be valid but change unexpectedly. Consequently, memory must not be written to or read from once it is freed.

Noncompliant Code Example

This example from Brian Kernighan and Dennis Ritchie [ Kernighan 1988] shows both the incorrect and correct techniques for freeing the memory associated with a linked list. In their (intentionally) incorrect example, p is freed before p->next is executed, so that p->next reads memory that has already been freed.

#include <stdlib.h>
 
struct node {
  int value;
  struct node *next;
};
 
void free_list(struct node *head) {
  for (struct node *p = head; p != NULL; p = p->next) {
    free(p);
  }
}

Compliant Solution

Kernighan and Ritchie correct this error by storing a reference to p->next in q before freeing p:

#include <stdlib.h>
 
struct node {
  int value;
  struct node *next;
};
 
void free_list(struct node *head) {
  struct node *q;
  for (struct node *p = head; p != NULL; p = q) {
    q = p->next;
    free(p);
  }
}

Noncompliant Code Example

In this noncompliant code example, buf is written to after it has been freed. Write-after-free vulnerabilities can be exploited to run arbitrary code with the permissions of the vulnerable process. Typically, allocations and frees are far removed, making it difficult to recognize and diagnose these problems.

#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[]) {
  char *return_val = 0;
  const size_t bufsize = strlen(argv[0]) + 1;
  char *buf = (char *)malloc(bufsize);
  if (!buf) {
    return EXIT_FAILURE;
  }
  /* ... */
  free(buf);
  /* ... */
  strcpy(buf, argv[0]);
  /* ... */
  return EXIT_SUCCESS;
}

Compliant Solution

In this compliant solution, the memory is freed after its final use:

#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[]) {
  char *return_val = 0;
  const size_t bufsize = strlen(argv[0]) + 1;
  char *buf = (char *)malloc(bufsize);
  if (!buf) {
    return EXIT_FAILURE;
  }
  /* ... */
  strcpy(buf, argv[0]);
  /* ... */
  free(buf);
  return EXIT_SUCCESS;
}

Noncompliant Code Example

In this noncompliant example, realloc() may free c_str1 when it returns a null pointer, resulting in c_str1 being freed twice. The C Standards Committee's proposed response to Defect Report #400 makes it implementation-defined whether or not the old object is deallocated when size is zero and memory for the new object is not allocated. The current implementation of realloc() in the GNU C Library and Microsoft Visual Studio's Runtime Library will free c_str1 and return a null pointer for zero byte allocations. Freeing a pointer twice can result in a potentially exploitable vulnerability commonly referred to as a double-free vulnerability [ Seacord 2013b].

#include <stdlib.h>
 
void f(char *c_str1, size_t size) {
  char *c_str2 = (char *)realloc(c_str1, size);
  if (c_str2 == NULL) {
    free(c_str1);
  }
}

Compliant Solution

This compliant solution does not pass a size argument of zero to the realloc() function, eliminating the possibility of c_str1 being freed twice:

#include <stdlib.h>

void f(char *c_str1, size_t size) {
  if (size != 0) {
    char *c_str2 = (char *)realloc(c_str1, size);
    if (c_str2 == NULL) {
      free(c_str1);
    }
  }
  else {
    free(c_str1);
  }
 
}

If the intent of calling f() is to reduce the size of the object, then doing nothing when the size is zero would be unexpected; instead, this compliant solution frees the object.

Noncompliant Code Example

In this noncompliant example ( CVE-2009-1364) from libwmf version 0.2.8.4, the return value of gdRealloc (a simple wrapper around realloc() that reallocates space pointed to by im->clip->list) is set to more. However, the value of im->clip->list is used directly afterwards in the code, and the C Standard specifies that if realloc() moves the area pointed to, then the original block is freed. An attacker can then execute arbitrary code by forcing a reallocation (with a sufficient im->clip->count) and accessing freed memory [ xorl 2009].

void gdClipSetAdd(gdImagePtr im, gdClipRectanglePtr rect) {
  gdClipRectanglePtr more;
  if (im->clip == 0) {
   /* ... */
  }
  if (im->clip->count == im->clip->max) {
    more = gdRealloc (im->clip->list,(im->clip->max + 8) *
                      sizeof (gdClipRectangle));
    /*
     * If the realloc fails, then we have not lost the
     * im->clip->list value.
     */
    if (more == 0) return;
    im->clip->max += 8;
  }
  im->clip->list[im->clip->count] = *rect;
  im->clip->count++;

}

Compliant Solution

This compliant solution simply reassigns im->clip->list to the value of more after the call to realloc():

void gdClipSetAdd(gdImagePtr im, gdClipRectanglePtr rect) {
  gdClipRectanglePtr more;
  if (im->clip == 0) {
    /* ... */
  }
  if (im->clip->count == im->clip->max) {
    more = gdRealloc (im->clip->list,(im->clip->max + 8) *
                      sizeof (gdClipRectangle));
    if (more == 0) return;
    im->clip->max += 8;
    im->clip->list = more;
  }
  im->clip->list[im->clip->count] = *rect;
  im->clip->count++;

}

Risk Assessment

Reading memory that has already been freed can lead to abnormal program termination and denial-of-service attacks. Writing memory that has already been freed can additionally lead to the execution of arbitrary code with the permissions of the vulnerable process.

Freeing memory multiple times has similar consequences to accessing memory after it is freed. Reading a pointer to deallocated memory is undefined behavior because the pointer value is indeterminate and might be a trap representation. When reading from or writing to freed memory does not cause a trap, it may corrupt the underlying data structures that manage the heap in a manner that can be exploited to execute arbitrary code. Alternatively, writing to memory after it has been freed might modify memory that has been reallocated.

Programmers should be wary when freeing memory in a loop or conditional statement; if coded incorrectly, these constructs can lead to double-free vulnerabilities. It is also a common error to misuse the realloc() function in a manner that results in double-free vulnerabilities. (See MEM04-C. Beware of zero-length allocations.)

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
MEM30-C	High	Likely	Medium	P18	L1

Related Guidelines

Taxonomy	Taxonomy item	Relationship
CERT C Secure Coding Standard	MEM01-C. Store a new value in pointers immediately after free()	Prior to 2018-01-12: CERT: Unspecified Relationship
CERT C	MEM50-CPP. Do not access freed memory	Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013	Dangling References to Stack Frames [DCM]	Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TR 24772:2013	Dangling Reference to Heap [XYK]	Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961	Accessing freed memory [accfree]	Prior to 2018-01-12: CERT: Unspecified Relationship
ISO/IEC TS 17961	Freeing memory multiple times [dblfree]	Prior to 2018-01-12: CERT: Unspecified Relationship
MISRA C:2012	Rule 18.6 (required)	Prior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11	CWE-416, Use After Free	2017-07-07: CERT: Exact
CWE 2.11	CWE-672	2017-07-07: CERT: Rule subset of CWE

Bibliography

[ ISO/IEC 9899:2011]	7.22.3, "Memory Management Functions"
[ Kernighan 1988]	Section 7.8.5, "Storage Management"
[ OWASP Freed Memory]
[ MIT 2005]
[ Seacord 2013b]	Chapter 4, "Dynamic Memory Management"
[ Viega 2005]	Section 5.2.19, "Using Freed Memory"
[ VU#623332]
[ xorl 2009]	CVE-2009-1364: LibWMF Pointer Use after free()

Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/rules/memory-management-mem/mem30-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key	Text	Severity	Disabled
double_free	Dynamic memory released here was already released earlier	None	False
possible_double_free	Dynamic memory released here possibly already released earlier	None	False
possible_use_after_free	Dynamic memory possibly used after it was previously released	None	False
use_after_free	Dynamic memory used after it was previously released	None	False

Options¶

This rule shares the following common options: exclude_in_macros, exclude_messages_in_system_headers, excludes, extend_exclude_to_macro_invocations, includes, justification_checker, languages, post_processing, provider, report_at, severity
The following places define options that affect this rule: Stylechecks, Analysis-GlobalOptions

functions_with_ignored_deallocators¶

functions_with_ignored_deallocators : set[str] = set()

Set of functions (given by their qualified name) where all deallocators are ignored. For these functions, the check will never report a use-after-free. It will also assume that these functions never create freed pointers, neither by return value, out param, nor by modifying global state.

report_freed_this_at_call¶

report_freed_this_at_call : bool = False

This option controls findings when a freed pointer is used in C++ to call a non-static member function. When set to true, the use at the call is directly reported. When false, the analysis waits for an actual dereference (of the this-pointer then) inside the callee, and only reports those.

report_read_pointer_args_in_calls_to_undefined¶

report_read_pointer_args_in_calls_to_undefined : bool = True

Report when freed pointers are passed to undefined (external) functions.

resources¶

resources

Type: set[str]

Default: {'C++ArrayHeapMemory', 'C++HeapMemory', 'CudaAsyncMemory', 'CudaDeviceMemory', 'CudaDriverAsyncMemory', 'CudaHostMemory', 'CudaManagedMemory', 'FileHandle', 'HeapMemory', 'UniquePtrHeapMemory'}

Set of resources to be checked (selection of rules in the Resources group).

witness_paths¶

witness_paths : bool = True

Whether witness paths should be determined and included in the issue.

Axivion Suite 7.12.0

Navigation

CertC-MEM30¶

Noncompliant Code Example

Compliant Solution

Noncompliant Code Example

Compliant Solution

Noncompliant Code Example

Compliant Solution

Noncompliant Code Example

Compliant Solution

Risk Assessment

Related Guidelines

Bibliography

Options¶

functions_with_ignored_deallocators¶

report_freed_this_at_call¶

report_read_pointer_args_in_calls_to_undefined¶

resources¶

witness_paths¶