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CWE-761 Variante Incompleto
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.
Definição
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.
Impacto no mundo real
Real-world CVEs caused by CWE-761
-
function "internally calls 'calloc' and returns a pointer at an index... inside the allocated buffer. This led to freeing invalid memory."
Como os atacantes a exploram
Trajeto do atacante passo a passo
- 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
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
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
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
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.
Exemplo de código vulnerável
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.
#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;} Exemplo de código seguro
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.
#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.
Lista de verificação de prevenção
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.
Sinais de deteção
How to detect CWE-761
Executar testes dinâmicos de segurança de aplicações (DAST) contra o endpoint em execução.
Monitorizar os registos em tempo de execução para traços de exceção invulgares, input malformado ou tentativas de contornar a autorização.
Revisão de código: sinalizar qualquer novo código que trate input desta superfície sem usar os ajudantes validados do framework.
CWE-761
Don't catalog this weakness. Prove it's reachable.
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Perguntas frequentes O que é o CWE-761?
Qual a gravidade do CWE-761?
Que linguagens ou plataformas são afetadas pelo CWE-761?
Como posso prevenir o CWE-761?
Como é que o Plexicus deteta e corrige o CWE-761?
Onde posso saber mais sobre o CWE-761?
Frequently asked questions
O que é o 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.
Qual a gravidade do CWE-761?
A MITRE não publicou uma classificação de probabilidade de exploração para esta fraqueza. Trate-a como impacto médio até o seu modelo de ameaças provar o contrário.
Que linguagens ou plataformas são afetadas pelo CWE-761?
A MITRE não especificou as plataformas afetadas por este CWE — pode aplicar-se à maioria das stacks de aplicações.
Como posso prevenir o 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.
Como é que o Plexicus deteta e corrige o CWE-761?
O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-761 em cada commit. Quando é encontrada uma correspondência, o nosso agente Codex Remedium abre um PR de correção com o código corrigido, testes e um resumo de uma linha para o revisor.
Onde posso saber mais sobre o CWE-761?
A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/761.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.
Fraquezas relacionadas
Weaknesses related to CWE-761
CWE-763 Pai
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 Irmão
Mismatched Memory Management Routines
This vulnerability occurs when a program uses incompatible functions to allocate and free memory. For example, freeing memory with a…
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Pronto para validar o que importa.
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