Fuzz testing (fuzzing) is a powerful technique for generating large numbers of diverse inputs - either randomly or algorithmically - and dynamically invoking the code with those inputs. Even with random inputs, it is often capable of generating unexpected results such as crashes, memory corruption, or resource consumption. Fuzzing effectively produces repeatable test cases that clearly indicate bugs, which helps developers to diagnose the issues.
CWE-786 Base Incompleto
Access of Memory Location Before Start of Buffer
This vulnerability occurs when software attempts to read from or write to a memory location positioned before the official start of a buffer.
Definição
What is CWE-786?
This vulnerability occurs when software attempts to read from or write to a memory location positioned before the official start of a buffer.
This flaw, often called a 'buffer under-read' or 'under-write,' happens when a pointer or index is incorrectly positioned. Common triggers include decrementing a pointer beyond the buffer's first element, performing pointer arithmetic that steps back too far, or directly using a negative index value to access array-like structures.
Accessing memory before a buffer's valid range can lead to unpredictable behavior. It may read sensitive data from unrelated parts of memory, corrupt critical program state, or cause an immediate crash, creating opportunities for denial-of-service or information disclosure attacks.
Impacto no mundo real
Real-world CVEs caused by CWE-786
-
Unchecked length of SSLv2 challenge value leads to buffer underflow.
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Buffer underflow from a small size value with a large buffer (length parameter inconsistency, CWE-130)
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Buffer underflow from an all-whitespace string, which causes a counter to be decremented before the buffer while looking for a non-whitespace character.
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Buffer underflow resultant from encoded data that triggers an integer overflow.
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Product sets an incorrect buffer size limit, leading to "off-by-two" buffer underflow.
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Negative value is used in a memcpy() operation, leading to buffer underflow.
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Buffer underflow due to mishandled special characters
Como os atacantes a exploram
Trajeto do atacante passo a passo
- 1
In the following C/C++ example, a utility function is used to trim trailing whitespace from a character string. The function copies the input string to a local character string and uses a while statement to remove the trailing whitespace by moving backward through the string and overwriting whitespace with a NUL character.
- 2
However, this function can cause a buffer underwrite if the input character string contains all whitespace. On some systems the while statement will move backwards past the beginning of a character string and will call the isspace() function on an address outside of the bounds of the local buffer.
- 3
The following example asks a user for an offset into an array to select an item.
- 4
The programmer allows the user to specify which element in the list to select, however an attacker can provide an out-of-bounds offset, resulting in a buffer over-read (CWE-126).
- 5
The following is an example of code that may result in a buffer underwrite. This code is attempting to replace the substring "Replace Me" in destBuf with the string stored in srcBuf. It does so by using the function strstr(), which returns a pointer to the found substring in destBuf. Using pointer arithmetic, the starting index of the substring is found.
Exemplo de código vulnerável
Vulnerable C
In the following C/C++ example, a utility function is used to trim trailing whitespace from a character string. The function copies the input string to a local character string and uses a while statement to remove the trailing whitespace by moving backward through the string and overwriting whitespace with a NUL character.
char* trimTrailingWhitespace(char *strMessage, int length) {
char *retMessage;
char *message = malloc(sizeof(char)*(length+1));
```
// copy input string to a temporary string*
char message[length+1];
int index;
for (index = 0; index < length; index++) {
```
message[index] = strMessage[index];
}
message[index] = '\0';
```
// trim trailing whitespace*
int len = index-1;
while (isspace(message[len])) {
```
message[len] = '\0';
len--;
}
```
// return string without trailing whitespace*
retMessage = message;
return retMessage;} Exemplo de código seguro
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} 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-786
- Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
- Implementation Validate input at trust boundaries; use allowlists, not denylists.
- Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
- Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
- Operation Monitor logs for the runtime signals listed in the next section.
Sinais de deteção
How to detect CWE-786
O Plexicus deteta automaticamente o CWE-786 e abre um PR de correção em menos de 60 segundos.
O Codex Remedium analisa cada commit, identifica esta fraqueza exata e entrega um pull request pronto para revisão com o patch. Sem tickets. Sem transferências.
Perguntas frequentes O que é o CWE-786?
Qual a gravidade do CWE-786?
Que linguagens ou plataformas são afetadas pelo CWE-786?
Como posso prevenir o CWE-786?
Como é que o Plexicus deteta e corrige o CWE-786?
Onde posso saber mais sobre o CWE-786?
Frequently asked questions
O que é o CWE-786?
This vulnerability occurs when software attempts to read from or write to a memory location positioned before the official start of a buffer.
Qual a gravidade do CWE-786?
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-786?
MITRE lists the following affected platforms: C, C++.
Como posso prevenir o CWE-786?
Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.
Como é que o Plexicus deteta e corrige o CWE-786?
O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-786 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-786?
A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/786.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.
Fraquezas relacionadas
Weaknesses related to CWE-786
CWE-119 Pai
Improper Restriction of Operations within the Bounds of a Memory Buffer
This vulnerability occurs when software accesses a memory buffer but reads from or writes to a location outside its allocated boundary.…
CWE-120 Irmão
Buffer Copy without Checking Size of Input ('Classic Buffer Overflow')
This vulnerability occurs when a program copies data from one memory location to another without first verifying that the source data will…
CWE-123 Irmão
Write-what-where Condition
A write-what-where condition occurs when an attacker can control both the data written and the exact memory location where it's written,…
CWE-125 Irmão
Out-of-bounds Read
An out-of-bounds read occurs when software accesses memory outside the boundaries of a buffer, array, or similar data structure, reading…
CWE-130 Irmão
Improper Handling of Length Parameter Inconsistency
This vulnerability occurs when a program reads a structured data packet or message but fails to properly validate that the declared length…
CWE-466 Irmão
Return of Pointer Value Outside of Expected Range
This vulnerability occurs when a function returns a memory pointer that points outside the expected buffer range, potentially exposing…
CWE-787 Irmão
Out-of-bounds Write
This vulnerability occurs when software incorrectly writes data outside the boundaries of its allocated memory buffer, either beyond the…
CWE-788 Irmão
Access of Memory Location After End of Buffer
This vulnerability occurs when software attempts to read from or write to a memory buffer using an index or pointer that points past the…
CWE-805 Irmão
Buffer Access with Incorrect Length Value
This vulnerability occurs when software reads from or writes to a buffer using a loop or sequential operation, but mistakenly calculates…
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