Double Free
Weakness ID: 415 (Weakness Variant)Status: Draft
+ Description

Description Summary

The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations.

Extended Description

When a program calls free() twice with the same argument, the program's memory management data structures become corrupted. This corruption can cause the program to crash or, in some circumstances, cause two later calls to malloc() to return the same pointer. If malloc() returns the same value twice and the program later gives the attacker control over the data that is written into this doubly-allocated memory, the program becomes vulnerable to a buffer overflow attack.

+ Alternate Terms
+ Time of Introduction
  • Architecture and Design
  • Implementation
+ Applicable Platforms




+ Common Consequences
Access Control

Doubly freeing memory may result in a write-what-where condition, allowing an attacker to execute arbitrary code.

+ Likelihood of Exploit

Low to Medium

+ Demonstrative Examples

Example 1

The following code shows a simple example of a double free vulnerability.

(Bad Code)
Example Language:
char* ptr = (char*)malloc (SIZE);
if (abrt) {

Double free vulnerabilities have two common (and sometimes overlapping) causes:

  • Error conditions and other exceptional circumstances

  • Confusion over which part of the program is responsible for freeing the memory

Although some double free vulnerabilities are not much more complicated than the previous example, most are spread out across hundreds of lines of code or even different files. Programmers seem particularly susceptible to freeing global variables more than once.

Example 2

While contrived, this code should be exploitable on Linux distributions which do not ship with heap-chunk check summing turned on.

(Bad Code)
Example Language:
#include <stdio.h>
#include <unistd.h>
#define BUFSIZE1 512
#define BUFSIZE2 ((BUFSIZE1/2) - 8)

int main(int argc, char **argv) {
char *buf1R1;
char *buf2R1;
char *buf1R2;
buf1R1 = (char *) malloc(BUFSIZE2);
buf2R1 = (char *) malloc(BUFSIZE2);
buf1R2 = (char *) malloc(BUFSIZE1);
strncpy(buf1R2, argv[1], BUFSIZE1-1);
+ Observed Examples
CVE-2004-0642Double free resultant from certain error conditions.
CVE-2004-0772Double free resultant from certain error conditions.
CVE-2005-1689Double free resultant from certain error conditions.
CVE-2003-0545Double free from invalid ASN.1 encoding.
CVE-2003-1048Double free from malformed GIF.
CVE-2005-0891Double free from malformed GIF.
CVE-2002-0059Double free from malformed compressed data.
+ Potential Mitigations

Phase: Architecture and Design

Choose a language that provides automatic memory management.

Phase: Implementation

Ensure that each allocation is freed only once. After freeing a chunk, set the pointer to NULL to ensure the pointer cannot be freed again. In complicated error conditions, be sure that clean-up routines respect the state of allocation properly. If the language is object oriented, ensure that object destructors delete each chunk of memory only once.

Phase: Implementation

Use a static analysis tool to find double free instances.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfWeakness ClassWeakness Class398Indicator of Poor Code Quality
Seven Pernicious Kingdoms (primary)700
ChildOfCategoryCategory399Resource Management Errors
Development Concepts (primary)699
ChildOfCategoryCategory633Weaknesses that Affect Memory
Resource-specific Weaknesses (primary)631
ChildOfWeakness BaseWeakness Base666Operation on Resource in Wrong Phase of Lifetime
Research Concepts (primary)1000
ChildOfWeakness ClassWeakness Class675Duplicate Operations on Resource
Research Concepts1000
ChildOfCategoryCategory742CERT C Secure Coding Section 08 - Memory Management (MEM)
Weaknesses Addressed by the CERT C Secure Coding Standard (primary)734
PeerOfWeakness BaseWeakness Base123Write-what-where Condition
Research Concepts1000
PeerOfWeakness BaseWeakness Base416Use After Free
Development Concepts699
Research Concepts1000
MemberOfViewView630Weaknesses Examined by SAMATE
Weaknesses Examined by SAMATE (primary)630
PeerOfWeakness BaseWeakness Base364Signal Handler Race Condition
Research Concepts1000
+ Relationship Notes

This is usually resultant from another weakness, such as an unhandled error or race condition between threads. It could also be primary to weaknesses such as buffer overflows.

+ Affected Resources
  • Memory
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
PLOVERDFREE - Double-Free Vulnerability
7 Pernicious KingdomsDouble Free
CLASPDoubly freeing memory
CERT C Secure CodingMEM00-CAllocate and free memory in the same module, at the same level of abstraction
CERT C Secure CodingMEM01-CStore a new value in pointers immediately after free()
CERT C Secure CodingMEM31-CFree dynamically allocated memory exactly once
+ White Box Definitions

A weakness where code path has:

1. start statement that relinquishes a dynamically allocated memory resource

2. end statement that relinquishes the dynamically allocated memory resource

+ Maintenance Notes

It could be argued that Double Free would be most appropriately located as a child of "Use after Free", but "Use" and "Release" are considered to be distinct operations within vulnerability theory, therefore this is more accurately "Release of a Resource after Expiration or Release", which doesn't exist yet.

+ Content History
Submission DateSubmitterOrganizationSource
PLOVERExternally Mined
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigitalExternal
updated Potential Mitigations, Time of Introduction
2008-08-01KDM AnalyticsExternal
added/updated white box definitions
2008-09-08CWE Content TeamMITREInternal
updated Applicable Platforms, Common Consequences, Description, Maintenance Notes, Relationships, Other Notes, Relationship Notes, Taxonomy Mappings
2008-11-24CWE Content TeamMITREInternal
updated Relationships, Taxonomy Mappings
2009-05-27CWE Content TeamMITREInternal
updated Demonstrative Examples
2009-10-29CWE Content TeamMITREInternal
updated Other Notes