VNX-329: Predictable IV derived from the key or a non-cryptographic generator

Field	Value
Rule ID	VNX-329
Severity	High
SARIF level	error
Kind	sast
Languages	Go, Java, Kotlin, JavaScript / TypeScript, Python
CWE	CWE-329
CAPEC	CAPEC-66
ATT&CK	T1190
CWSS vector	CWSS:1.0/TI:H/AP:RU/AL:S/IC:N/FC:LT/RP:RU/RL:S/AV:L/AS:N/IN:A/SC:A/BI:H/DI:H/EX:H/EC:N/P:H
Tags	crypto iv-reuse predictable-iv cwe-329
Source	internal/sast/rules/vnx-329.rego

Overview

VNX-329 maps to CWE-329: Generation of Predictable IV with CBC Mode. It reports an IV or nonce that is predictable because it is derived from the encryption key, drawn from a non-cryptographic generator, or built from the clock.

Why This Matters

A predictable CBC IV allows chosen-plaintext attacks (BEAST is the famous one). Using the key as the IV is worse: an attacker who can get one chosen ciphertext decrypted recovers the key itself.

What Gets Flagged

# FLAGGED: the IV is the key
AES.new(key, AES.MODE_CBC, key)
iv = key[:16]
# FLAGGED: the IV comes from the random module
iv = random.randbytes(16)

// FLAGGED
new IvParameterSpec(keyBytes)
byte[] ivBytes = Arrays.copyOf(secretKey, 16);
rnd.nextBytes(iv);   // rnd is a java.util.Random

// FLAGGED
cipher.NewCBCEncrypter(block, key[:aes.BlockSize])
rand.Read(iv)        // math/rand

// FLAGGED
crypto.createCipheriv('aes-256-cbc', key, key.slice(0, 16));

What Is Not Flagged

• IVs from os.urandom, secrets, get_random_bytes, SecureRandom, crypto/rand, randomBytes or getRandomValues.
• IVs read from the ciphertext (blob[:16]).
• Constant IVs, which VNX-CRYPTO-010 and VNX-JAVA-020 report.
• Names that only contain the letters (keys, keyword, keystore), comments, string literals and test files.

Remediation

Generate a fresh IV for every message from a CSPRNG and send it with the ciphertext, or use an AEAD mode with a random or counter nonce that is never reused under the same key.

References

• CWE-329: Generation of Predictable IV with CBC Mode
• NIST SP 800-38A, Appendix C

Suppressing this rule

Put a nosec comment on the line, or on line 1 to cover the whole file. Use the comment marker of the file’s language (#, //, -- or ;):

# nosec vnx-329
// nosec vnx-329

Or record an ignore rule: vulnetix ignore add --rule vnx-329 --reason "...". See Suppressing findings.