CWE-761 Variante Incomplet

Free of Pointer not at Start of Buffer

This vulnerability occurs when a program incorrectly frees a memory pointer that no longer points to the beginning of the allocated heap buffer, often due to pointer arithmetic.

Définition

What is CWE-761?

This vulnerability occurs when a program incorrectly frees a memory pointer that no longer points to the beginning of the allocated heap buffer, often due to pointer arithmetic.
This issue typically happens when you allocate memory using functions like `malloc()`, `calloc()`, or `realloc()`, and then later modify the pointer—for example, by incrementing it to traverse a data structure. When you later pass this offset pointer to `free()`, the memory manager cannot correctly identify the original memory block's metadata, leading to heap corruption. This corruption can cause immediate crashes, unpredictable behavior, or even create opportunities for attackers to manipulate program data or execution flow. To prevent this, always ensure you free the exact pointer returned by the allocation function, or use a separate tracking variable to preserve the original starting address.
Impact réel

Real-world CVEs caused by CWE-761

  • CVE-2019-11930

    function "internally calls 'calloc' and returns a pointer at an index... inside the allocated buffer. This led to freeing invalid memory."

Comment les attaquants l'exploitent

Parcours de l'attaquant étape par étape

  1. 1

    In this example, the programmer dynamically allocates a buffer to hold a string and then searches for a specific character. After completing the search, the programmer attempts to release the allocated memory and return SUCCESS or FAILURE to the caller. Note: for simplification, this example uses a hard-coded "Search Me!" string and a constant string length of 20.

  2. 2

    However, if the character is not at the beginning of the string, or if it is not in the string at all, then the pointer will not be at the start of the buffer when the programmer frees it.

  3. 3

    Instead of freeing the pointer in the middle of the buffer, the programmer can use an indexing pointer to step through the memory or abstract the memory calculations by using array indexing.

  4. 4

    This code attempts to tokenize a string and place it into an array using the strsep function, which inserts a \0 byte in place of whitespace or a tab character. After finishing the loop, each string in the AP array points to a location within the input string.

  5. 5

    Since strsep is not allocating any new memory, freeing an element in the middle of the array is equivalent to free a pointer in the middle of inputstring.

Exemple de code vulnérable

Vulnerable C

In this example, the programmer dynamically allocates a buffer to hold a string and then searches for a specific character. After completing the search, the programmer attempts to release the allocated memory and return SUCCESS or FAILURE to the caller. Note: for simplification, this example uses a hard-coded "Search Me!" string and a constant string length of 20.

Vulnérable C 
#define SUCCESS (1)
  #define FAILURE (0)
  int contains_char(char c){
  		char *str;
  		str = (char*)malloc(20*sizeof(char));
  		strcpy(str, "Search Me!");
  		while( *str != NULL){
  				if( *str == c ){
```
/* matched char, free string and return success */* 
  						free(str);
  						return SUCCESS;}
  				
  				 */* didn't match yet, increment pointer and try next char */* 
  				
  				str = str + 1;}
  		
  		 */* we did not match the char in the string, free mem and return failure */* 
  		
  		free(str);
  		return FAILURE;}
Exemple de code sécurisé

Secure C

Instead of freeing the pointer in the middle of the buffer, the programmer can use an indexing pointer to step through the memory or abstract the memory calculations by using array indexing.

Sécurisé C 
#define SUCCESS (1)
  #define FAILURE (0)
  int cointains_char(char c){
  		char *str;
  		int i = 0;
  		str = (char*)malloc(20*sizeof(char));
  		strcpy(str, "Search Me!");
  		while( i < strlen(str) ){
  				if( str[i] == c ){
```
/* matched char, free string and return success */* 
  						free(str);
  						return SUCCESS;}
  				
  				 */* didn't match yet, increment pointer and try next char */* 
  				
  				i = i + 1;}
  		
  		 */* we did not match the char in the string, free mem and return failure */* 
  		
  		free(str);
  		return FAILURE;}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Liste de contrôle de prévention

How to prevent CWE-761

  • Implementation When utilizing pointer arithmetic to traverse a buffer, use a separate variable to track progress through memory and preserve the originally allocated address for later freeing.
  • Implementation When programming in C++, consider using smart pointers provided by the boost library to help correctly and consistently manage memory.
  • Architecture and Design Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, glibc in Linux provides protection against free of invalid pointers.
  • Architecture and Design Use a language that provides abstractions for memory allocation and deallocation.
  • Testing Use a tool that dynamically detects memory management problems, such as valgrind.
Signaux de détection

How to detect CWE-761

SAST High

Exécuter une analyse statique (SAST) sur le code source à la recherche du motif non sécurisé dans le flux de données.

DAST Moderate

Exécuter des tests de sécurité applicative dynamique (DAST) contre le point de terminaison en ligne.

Runtime Moderate

Surveiller les journaux runtime pour détecter des traces d'exception inhabituelles, des entrées malformées ou des tentatives de contournement d'autorisation.

Code review Moderate

Revue de code : signaler tout nouveau code qui traite les entrées de cette surface sans utiliser les helpers du framework validés.

Correction automatique Plexicus

Plexicus détecte automatiquement CWE-761 et ouvre une PR de correction en moins de 60 secondes.

Codex Remedium analyse chaque commit, identifie cette faiblesse précise et livre une pull request prête à être relue avec le correctif. Pas de tickets. Pas de transferts.

Obtenir une démo Essayer Plexicus gratuitement
Questions fréquentes

Frequently asked questions

Qu'est-ce que CWE-761 ?

This vulnerability occurs when a program incorrectly frees a memory pointer that no longer points to the beginning of the allocated heap buffer, often due to pointer arithmetic.

Quelle est la gravité de CWE-761 ?

MITRE n'a pas publié de note de probabilité d'exploitation pour cette faiblesse. Traitez-la comme un impact moyen jusqu'à ce que votre modèle de menace prouve le contraire.

Quels langages ou plateformes sont affectés par CWE-761 ?

MITRE n'a pas spécifié les plateformes affectées pour ce CWE — il peut s'appliquer à la plupart des stacks applicatives.

Comment puis-je prévenir CWE-761 ?

When utilizing pointer arithmetic to traverse a buffer, use a separate variable to track progress through memory and preserve the originally allocated address for later freeing. When programming in C++, consider using smart pointers provided by the boost library to help correctly and consistently manage memory.

Comment Plexicus détecte et corrige CWE-761 ?

Le moteur SAST de Plexicus reconnaît la signature de flux de données de CWE-761 à chaque commit. Lorsqu'une correspondance est trouvée, notre agent Codex Remedium ouvre une PR de correction avec le code corrigé, les tests et un résumé d'une ligne pour le relecteur.

Où puis-je en savoir plus sur CWE-761 ?

MITRE publie la définition canonique à https://cwe.mitre.org/data/definitions/761.html. Vous pouvez également consulter la documentation OWASP et NIST pour des conseils adjacents.

Faiblesses associées

Weaknesses related to CWE-761

CWE-763 Parent

Release of Invalid Pointer or Reference

This vulnerability occurs when a program tries to free a memory resource back to the system but uses an incorrect deallocation method or…

CWE-762 Frère

Mismatched Memory Management Routines

This vulnerability occurs when a program uses incompatible functions to allocate and free memory. For example, freeing memory with a…

Ressources

Further reading

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