CWE-329 Variant Draft Medium likelihood

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,…

Definition

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.
Auswirkungen in der Praxis

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.

Wie Angreifer es ausnutzen

Angreiferpfad Schritt für Schritt

  1. 1

    Identifiziere einen Codepfad, der nicht vertrauenswürdige Eingaben ohne Validierung verarbeitet.

  2. 2

    Erzeuge eine Payload, die das unsichere Verhalten auslöst — Injection, Traversal, Overflow oder Logik-Missbrauch.

  3. 3

    Liefere die Payload über einen normalen Request aus und beobachte die Reaktion der Anwendung.

  4. 4

    Iteriere, bis die Antwort Daten preisgibt, Angreifer-Code ausführt oder Berechtigungen eskaliert.

Verwundbares Codebeispiel

Vulnerable C

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

Verwundbar 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);
Sicheres Codebeispiel

Secure pseudo

Sicher 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.
Präventions-Checkliste

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.
Erkennungssignale

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.)

Plexicus Auto-Fix

Plexicus erkennt CWE-329 automatisch und öffnet in unter 60 Sekunden einen Fix-PR.

Codex Remedium scannt jeden Commit, identifiziert genau diese Schwachstelle und liefert einen reviewer-ready Pull Request mit dem Patch. Keine Tickets. Keine Hand-offs.

Demo anfordern Plexicus kostenlos testen
Häufig gestellte Fragen

Frequently asked questions

Was ist 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.

Wie gravierend ist CWE-329?

MITRE stuft die Exploit-Wahrscheinlichkeit als mittel ein — eine Ausnutzung ist realistisch, erfordert aber meist bestimmte Bedingungen.

Welche Sprachen oder Plattformen sind von CWE-329 betroffen?

MITRE lists the following affected platforms: ICS/OT.

Wie kann ich CWE-329 verhindern?

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.

Wie erkennt und behebt Plexicus CWE-329?

Die SAST-Engine von Plexicus erkennt die Datenfluss-Signatur von CWE-329 bei jedem Commit. Bei einem Treffer öffnet unser Codex-Remedium-Agent einen Fix-PR mit korrigiertem Code, Tests und einer einzeiligen Zusammenfassung für den Reviewer.

Wo erfahre ich mehr über CWE-329?

MITRE veröffentlicht die kanonische Definition unter https://cwe.mitre.org/data/definitions/329.html. Für ergänzende Hinweise kannst du auch die OWASP- und NIST-Dokumentation heranziehen.

Verwandte Schwachstellen

Weaknesses related to CWE-329

CWE-1204 Parent

Generation of Weak Initialization Vector (IV)

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

Ressourcen

Further reading

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