Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
CWE-329 Variante Borrador
Generation of Predictable IV with CBC Mode
This vulnerability occurs when software uses a predictable or reused Initialization Vector (IV) with Cipher Block Chaining (CBC) mode encryption. Predictable IVs undermine the security of CBC,…
Definición
What is CWE-329?
This vulnerability occurs when software uses a predictable or reused Initialization Vector (IV) with Cipher Block Chaining (CBC) mode encryption. Predictable IVs undermine the security of CBC, making encrypted data vulnerable to dictionary and chosen-plaintext attacks, especially when the same key is used multiple times.
CBC mode improves upon basic ECB encryption by ensuring identical plaintext blocks encrypt to different ciphertext blocks. It achieves this by XOR-ing the first plaintext block with a unique, random Initialization Vector (IV) before encryption, and then chaining each subsequent block with the previous ciphertext. This process breaks patterns in the plaintext. However, if the IV is predictable or reused, this core security mechanism fails.
When an attacker can guess or calculate the IV, they can launch practical attacks. Even if IVs aren't identical, predictable sequences allow attackers to perform chosen-plaintext attacks (CPA), where they analyze differences between known inputs and resulting ciphertexts to deduce the encryption key or decrypt data. Always generate IVs using a cryptographically secure random number generator for each encryption operation to maintain CBC's security guarantees.
Impacto en el mundo real
Real-world CVEs caused by CWE-329
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encryption functionality in an authentication framework uses a fixed null IV with CBC mode, allowing attackers to decrypt traffic in applications that use this functionality
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messages for a door-unlocking product use a fixed IV in CBC mode, which is the same after each restart
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application uses AES in CBC mode, but the pseudo-random secret and IV are generated using math.random, which is not cryptographically strong.
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Blowfish-CBC implementation constructs an IV where each byte is calculated modulo 8 instead of modulo 256, resulting in less than 12 bits for the effective IV length, and less than 4096 possible IV values.
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BEAST attack in SSL 3.0 / TLS 1.0. In CBC mode, chained initialization vectors are non-random, allowing decryption of HTTPS traffic using a chosen plaintext attack.
Cómo lo explotan los atacantes
Ruta del atacante paso a paso
- 1
Identifica una ruta de código que maneje entrada no confiable sin validación.
- 2
Crea un payload que ejercite el comportamiento inseguro — inyección, traversal, overflow o abuso de lógica.
- 3
Envía el payload a través de una solicitud normal y observa la reacción de la aplicación.
- 4
Itera hasta que la respuesta filtre datos, ejecute código del atacante o escale privilegios.
Ejemplo de código vulnerable
Vulnerable C
In the following examples, CBC mode is used when encrypting data:
EVP_CIPHER_CTX ctx;
char key[EVP_MAX_KEY_LENGTH];
char iv[EVP_MAX_IV_LENGTH];
RAND_bytes(key, b);
memset(iv,0,EVP_MAX_IV_LENGTH);
EVP_EncryptInit(&ctx,EVP_bf_cbc(), key,iv); 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-329
- Implementation NIST recommends two methods of generating unpredictable IVs for CBC mode [REF-1172]. The first is to generate the IV randomly. The second method is to encrypt a nonce with the same key and cipher to be used to encrypt the plaintext. In this case the nonce must be unique but can be predictable, since the block cipher will act as a pseudo random permutation.
Señales de detección
How to detect CWE-329
CWE-329
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-329?
¿Qué gravedad tiene CWE-329?
¿Qué lenguajes o plataformas se ven afectados por CWE-329?
¿Cómo puedo prevenir CWE-329?
¿Cómo detecta y corrige Plexicus CWE-329?
¿Dónde puedo aprender más sobre CWE-329?
Frequently asked questions
¿Qué es CWE-329?
This vulnerability occurs when software uses a predictable or reused Initialization Vector (IV) with Cipher Block Chaining (CBC) mode encryption. Predictable IVs undermine the security of CBC, making encrypted data vulnerable to dictionary and chosen-plaintext attacks, especially when the same key is used multiple times.
¿Qué gravedad tiene CWE-329?
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-329?
MITRE lists the following affected platforms: ICS/OT.
¿Cómo puedo prevenir CWE-329?
NIST recommends two methods of generating unpredictable IVs for CBC mode [REF-1172]. The first is to generate the IV randomly. The second method is to encrypt a nonce with the same key and cipher to be used to encrypt the plaintext. In this case the nonce must be unique but can be predictable, since the block cipher will act as a pseudo random permutation.
¿Cómo detecta y corrige Plexicus CWE-329?
El motor SAST de Plexicus detecta la firma de flujo de datos para CWE-329 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-329?
MITRE publica la definición canónica en https://cwe.mitre.org/data/definitions/329.html. También puedes consultar la documentación de OWASP y NIST para guías relacionadas.
Debilidades relacionadas
Weaknesses related to CWE-329
Recursos
Further reading
- MITRE — CWE-329 oficial https://cwe.mitre.org/data/definitions/329.html
- The CLASP Application Security Process https://cwe.mitre.org/documents/sources/TheCLASPApplicationSecurityProcess.pdf
- Why IND-CPA implies randomized encryption https://blog.cryptographyengineering.com/why-ind-cpa-implies-randomized-encryption/
- Recommendation for Block Cipher Modes of Operation https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38a.pdf
¿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.