CWE-329: Generation of Predictable IV with CBC Mode

Definição

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 no mundo real

Real-world CVEs caused by CWE-329

CVE-2020-5408

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
CVE-2017-17704

messages for a door-unlocking product use a fixed IV in CBC mode, which is the same after each restart
CVE-2017-11133

application uses AES in CBC mode, but the pseudo-random secret and IV are generated using math.random, which is not cryptographically strong.
CVE-2007-3528

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.
CVE-2011-3389

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.

Como os atacantes a exploram

Trajeto do atacante passo a passo

1
Identificar um caminho de código que trata input não confiável sem validação.
2
Criar um payload que explora o comportamento inseguro — injeção, traversal, overflow ou abuso de lógica.
3
Entregar o payload através de um pedido normal e observar a reação da aplicação.
4
Iterar até que a resposta exponha dados, execute código do atacante ou escale privilégios.

Exemplo de código vulnerável

Vulnerable C

In the following examples, CBC mode is used when encrypting data:

Vulnerável C

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);

Exemplo 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 verificação de prevenção

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.

Sinais de deteção

How to detect CWE-329

Automated Static Analysis High

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

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Perguntas frequentes

Frequently asked questions

O que é o 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.

Qual a gravidade do CWE-329?

A MITRE classifica a probabilidade de exploração como Média — a exploração é realista mas normalmente requer condições específicas.

Que linguagens ou plataformas são afetadas pelo CWE-329?

MITRE lists the following affected platforms: ICS/OT.

Como posso prevenir o 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.

Como é que o Plexicus deteta e corrige o CWE-329?

O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-329 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-329?

A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/329.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.

Fraquezas relacionadas

Weaknesses related to CWE-329

CWE-1204 Pai

Generation of Weak Initialization Vector (IV)

This vulnerability occurs when software uses a weak or predictable Initialization Vector (IV) for cryptographic operations. Many…

Recursos

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Generation of Predictable IV with CBC Mode