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.
Definición
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 en el mundo real
Real-world CVEs caused by CWE-786
-
Unchecked length of SSLv2 challenge value leads to buffer underflow.
-
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.
-
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
Cómo lo explotan los atacantes
Ruta del atacante paso a paso
- 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.
Ejemplo de código vulnerable
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;} Ejemplo 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 prevención
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.
Señales de detección
How to detect CWE-786
Plexicus detecta automáticamente CWE-786 y abre un PR de corrección en menos de 60 segundos.
Codex Remedium escanea cada commit, identifica esta debilidad concreta y entrega un pull request listo para revisión con el parche. Sin tickets. Sin traspasos.
Preguntas frecuentes ¿Qué es CWE-786?
¿Qué gravedad tiene CWE-786?
¿Qué lenguajes o plataformas se ven afectados por CWE-786?
¿Cómo puedo prevenir CWE-786?
¿Cómo detecta y corrige Plexicus CWE-786?
¿Dónde puedo aprender más sobre CWE-786?
Frequently asked questions
¿Qué es 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.
¿Qué gravedad tiene CWE-786?
MITRE no ha publicado una calificación de probabilidad de explotación para esta debilidad. Trátala como de impacto medio hasta que tu modelo de amenazas demuestre lo contrario.
¿Qué lenguajes o plataformas se ven afectados por CWE-786?
MITRE lists the following affected platforms: C, C++.
¿Cómo puedo prevenir 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.
¿Cómo detecta y corrige Plexicus CWE-786?
El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-786 en cada commit. Cuando hay coincidencia, nuestro agente Codex Remedium abre un PR de corrección con el código corregido, las pruebas y un resumen de una línea para el revisor.
¿Dónde puedo aprender más sobre CWE-786?
MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/786.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.
Debilidades relacionadas
Weaknesses related to CWE-786
CWE-119 Padre
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 Hermano
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 Hermano
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 Hermano
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 Hermano
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 Hermano
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 Hermano
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 Hermano
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 Hermano
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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