CWE-194 Variante Incompleto High likelihood

Unexpected Sign Extension

This vulnerability occurs when a signed number from a smaller data type is moved or cast to a larger type, causing its sign bit to be incorrectly extended. If the original value is negative, this…

Definición

What is CWE-194?

This vulnerability occurs when a signed number from a smaller data type is moved or cast to a larger type, causing its sign bit to be incorrectly extended. If the original value is negative, this sign extension can fill the new, higher-order bits with '1's, leading to unexpectedly large positive values and causing logic errors, buffer overflows, or security bypasses.
Sign extension is a standard behavior in programming languages like C and C++ when promoting a signed integer (e.g., a signed 8-bit `char`) to a larger signed type (e.g., a 32-bit `int`). The problem arises when developers don't account for this automatic behavior, especially when treating the resulting value as an unsigned number or using it for operations like memory allocation, array indexing, or length validation. A classic example is reading a byte value of `0xFF` (-1 as a signed char) into an unsigned integer, which becomes `0xFFFFFFFF` (a very large positive number), potentially leading to out-of-bounds access. To prevent this, developers must be explicit about data types during conversions. Always consider if the source data should be treated as signed or unsigned before widening it. Use explicit casts to the intended target type, and when working with raw byte data or protocol parsing, prefer unsigned types for counts and indices. Performing range checks on the source value before the conversion or using bit masks (e.g., `new_value = old_value & 0xFF`) can effectively strip unwanted sign-extended bits and ensure the resulting value matches the intended logic.
Impacto en el mundo real

Real-world CVEs caused by CWE-194

  • CVE-2018-10887

    Chain: unexpected sign extension (CWE-194) leads to integer overflow (CWE-190), causing an out-of-bounds read (CWE-125)

  • CVE-1999-0234

    Sign extension error produces -1 value that is treated as a command separator, enabling OS command injection.

  • CVE-2003-0161

    Product uses "char" type for input character. When char is implemented as a signed type, ASCII value 0xFF (255), a sign extension produces a -1 value that is treated as a program-specific separator value, effectively disabling a length check and leading to a buffer overflow. This is also a multiple interpretation error.

  • CVE-2007-4988

    chain: signed short width value in image processor is sign extended during conversion to unsigned int, which leads to integer overflow and heap-based buffer overflow.

  • CVE-2006-1834

    chain: signedness error allows bypass of a length check; later sign extension makes exploitation easier.

  • CVE-2005-2753

    Sign extension when manipulating Pascal-style strings leads to integer overflow and improper memory copy.

Cómo lo explotan los atacantes

Ruta del atacante paso a paso

  1. 1

    Identifica una ruta de código que maneje entrada no confiable sin validación.

  2. 2

    Crea un payload que ejercite el comportamiento inseguro — inyección, traversal, overflow o abuso de lógica.

  3. 3

    Envía el payload a través de una solicitud normal y observa la reacción de la aplicación.

  4. 4

    Itera hasta que la respuesta filtre datos, ejecute código del atacante o escale privilegios.

Ejemplo de código vulnerable

Vulnerable C

The following code reads a maximum size and performs a sanity check on that size. It then performs a strncpy, assuming it will not exceed the boundaries of the array. While the use of "short s" is forced in this particular example, short int's are frequently used within real-world code, such as code that processes structured data.

Vulnerable C 
int GetUntrustedInt () {
  	return(0x0000FFFF);
  }
  void main (int argc, char **argv) {
  		char path[256];
  		char *input;
  		int i;
  		short s;
  		unsigned int sz;
  		i = GetUntrustedInt();
  		s = i;
  		/* s is -1 so it passes the safety check - CWE-697 */
  		if (s > 256) {
  			DiePainfully("go away!\n");
  		}
  		/* s is sign-extended and saved in sz */
  		sz = s;
  		/* output: i=65535, s=-1, sz=4294967295 - your mileage may vary */
  		printf("i=%d, s=%d, sz=%u\n", i, s, sz);
  		input = GetUserInput("Enter pathname:");
  		/* strncpy interprets s as unsigned int, so it's treated as MAX_INT
  		(CWE-195), enabling buffer overflow (CWE-119) */
  		strncpy(path, input, s);
  		path[255] = '\0'; /* don't want CWE-170 */
  		printf("Path is: %s\n", path);
  }
Ejemplo de código seguro

Secure pseudo

Seguro 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-194

  • Implementation Avoid using signed variables if you don't need to represent negative values. When negative values are needed, perform validation after you save those values to larger data types, or before passing them to functions that are expecting unsigned values.
Señales de detección

How to detect CWE-194

SAST High

Ejecuta análisis estático (SAST) sobre el código buscando el patrón inseguro en el flujo de datos.

DAST Moderate

Ejecuta pruebas dinámicas de seguridad de aplicaciones (DAST) contra el endpoint en vivo.

Runtime Moderate

Vigila los logs en tiempo de ejecución para detectar trazas de excepción inusuales, entradas malformadas o intentos de bypass de autorización.

Code review Moderate

Revisión de código: marca cualquier código nuevo que maneje entrada desde esta superficie sin usar los helpers validados del framework.

Auto-corrección de Plexicus

Plexicus detecta automáticamente CWE-194 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.

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Preguntas frecuentes

Frequently asked questions

¿Qué es CWE-194?

This vulnerability occurs when a signed number from a smaller data type is moved or cast to a larger type, causing its sign bit to be incorrectly extended. If the original value is negative, this sign extension can fill the new, higher-order bits with '1's, leading to unexpectedly large positive values and causing logic errors, buffer overflows, or security bypasses.

¿Qué gravedad tiene CWE-194?

MITRE califica la probabilidad de explotación como Alta — esta debilidad se explota activamente en la práctica y debe priorizarse para su remediación.

¿Qué lenguajes o plataformas se ven afectados por CWE-194?

MITRE lists the following affected platforms: C, C++.

¿Cómo puedo prevenir CWE-194?

Avoid using signed variables if you don't need to represent negative values. When negative values are needed, perform validation after you save those values to larger data types, or before passing them to functions that are expecting unsigned values.

¿Cómo detecta y corrige Plexicus CWE-194?

El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-194 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-194?

MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/194.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.

Debilidades relacionadas

Weaknesses related to CWE-194

CWE-681 Padre

Incorrect Conversion between Numeric Types

This vulnerability occurs when a program converts a value from one numeric type to another (like a 64-bit integer to a 32-bit integer) and…

CWE-192 Hermano

Integer Coercion Error

An integer coercion error occurs when a program incorrectly converts, extends, or truncates a number between different data types, leading…

CWE-195 Hermano

Signed to Unsigned Conversion Error

This vulnerability occurs when a signed integer (which can hold negative values) is converted to an unsigned integer (which holds only…

CWE-196 Hermano

Unsigned to Signed Conversion Error

This vulnerability occurs when a program takes an unsigned integer and converts it directly to a signed integer. If the original unsigned…

CWE-197 Hermano

Numeric Truncation Error

A numeric truncation error happens when a program converts a number to a smaller data type, cutting off its higher-order bits and…

Recursos

Further reading

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