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CWE-124 Base Incompleto
Buffer Underwrite ('Buffer Underflow')
A buffer underwrite, also known as buffer underflow, happens when a program writes data to a memory location before the official start of a buffer.
Definición
What is CWE-124?
A buffer underwrite, also known as buffer underflow, happens when a program writes data to a memory location before the official start of a buffer.
This vulnerability occurs when a program miscalculates a write position, placing it outside the allocated buffer's bounds in the 'backward' direction. Common causes include decrementing a pointer or index too many times, performing incorrect pointer arithmetic that results in a negative offset, or directly using a negative index to access an array. The result is a write operation to an unintended, and often critical, memory location.
Writing to memory before a buffer can corrupt other program data, function pointers, or critical control structures. This can lead to system crashes, unpredictable behavior, or, if exploited, allow an attacker to hijack the program's execution flow. Developers should rigorously validate all pointer arithmetic and array indices, ensuring they remain within the valid, zero-based range of the allocated buffer.
Impacto en el mundo real
Real-world CVEs caused by CWE-124
-
buffer underwrite in firmware verification routine allows code execution via a crafted firmware image
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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
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 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.
- 4
In the case where the substring is not found in destBuf, strstr() will return NULL, causing the pointer arithmetic to be undefined, potentially setting the value of idx to a negative number. If idx is negative, this will result in a buffer underwrite of destBuf.
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-124
- Requirements Choose a language that is not susceptible to these issues.
- Implementation All calculated values that are used as index or for pointer arithmetic should be validated to ensure that they are within an expected range.
Señales de detección
How to detect CWE-124
Ejecuta pruebas dinámicas de seguridad de aplicaciones (DAST) contra el endpoint en vivo.
Vigila los logs en tiempo de ejecución para detectar trazas de excepción inusuales, entradas malformadas o intentos de bypass de autorización.
Revisión de código: marca cualquier código nuevo que maneje entrada desde esta superficie sin usar los helpers validados del framework.
CWE-124
Don't catalog this weakness. Prove it's reachable.
Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.
Preguntas frecuentes ¿Qué es CWE-124?
¿Qué gravedad tiene CWE-124?
¿Qué lenguajes o plataformas se ven afectados por CWE-124?
¿Cómo puedo prevenir CWE-124?
¿Cómo detecta y corrige Plexicus CWE-124?
¿Dónde puedo aprender más sobre CWE-124?
Frequently asked questions
¿Qué es CWE-124?
A buffer underwrite, also known as buffer underflow, happens when a program writes data to a memory location before the official start of a buffer.
¿Qué gravedad tiene CWE-124?
MITRE califica la probabilidad de explotación como Media — la explotación es realista pero suele requerir condiciones específicas.
¿Qué lenguajes o plataformas se ven afectados por CWE-124?
MITRE lists the following affected platforms: C, C++.
¿Cómo puedo prevenir CWE-124?
Choose a language that is not susceptible to these issues. All calculated values that are used as index or for pointer arithmetic should be validated to ensure that they are within an expected range.
¿Cómo detecta y corrige Plexicus CWE-124?
El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-124 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-124?
MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/124.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.
Debilidades relacionadas
Weaknesses related to CWE-124
CWE-786 Padre
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…
CWE-127 Hermano
Buffer Under-read
A buffer under-read occurs when a program attempts to read data from a memory location positioned before the start of an allocated buffer.
¿Listo para validar lo que importa?
Listo para validar lo que importa.
Plexicus es Proof-Driven AppSec: hallazgos validados, comprensión contextual y remediación revisada — anclada en evidencia, acotada contigo.