VNX-338: Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

Field	Value
Rule ID	VNX-338
Severity	High
SARIF level	error
Kind	sast
Languages	JavaScript / TypeScript, PHP, Ruby, Go
CWE	CWE-338
CAPEC	CAPEC-66
ATT&CK	T1190
CWSS vector	CWSS:1.0/TI:H/AP:A/AL:A/IC:N/FC:LT/RP:N/RL:A/AV:I/AS:N/IN:A/SC:A/BI:H/DI:H/EX:H/EC:N/P:H
Tags	prng security cwe-338
Source	internal/sast/rules/vnx-338.rego

Overview

VNX-338 maps to CWE-338: Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG). General-purpose generators (Mersenne Twister, xorshift, LCGs) are built for speed and statistical spread, not unpredictability: a few observed outputs are enough to recover their state and compute every later value.

The rule reports JavaScript/TypeScript Math.random(); PHP rand(), mt_rand(), lcg_value(), str_shuffle() and array_rand(); Ruby Kernel#rand, Random.rand, Random.new.rand/bytes and Array#sample/#shuffle; and Go math/rand and math/rand/v2 draws (under the package’s import alias), when the draw becomes a secret: the name it is assigned to or keyed by (token, password, secret, nonce, salt, OTP, PIN, CSRF/XSRF, session id, API/signing key, reset/verification/invite code, session cookie), the string key of a setter or cookie call, a return from a function named for a secret, or a variable assigned from the draw that reaches such a name within six lines.

What Gets Flagged

// FLAGGED: Math.random() as a reset token and a session id
const resetToken = Math.random().toString(36).slice(2);
const sessionId = Math.floor(Math.random() * 1e16).toString(16);

// FLAGGED: Mersenne Twister and str_shuffle as secrets
$token = md5(mt_rand());
$_SESSION['csrf_token'] = substr(str_shuffle('abcdef0123456789abcdef0123456789'), 0, 32);

# FLAGGED: Kernel#rand as a reset token
self.reset_token = rand(36**20).to_s(36)

What Is Not Flagged

• crypto.randomBytes/randomUUID/getRandomValues, random_bytes()/random_int(), SecureRandom (including .sample(random: SecureRandom)) and crypto/rand.
• Draws for jitter, sleeps, retries, colours, sampling and counts (token_count, max_tokens, token_expiry).
• Test files, and the pattern in comments or string literals.

Python, Java, C# and C/C++ are covered by VNX-PY-005, VNX-JAVA-016, VNX-CS-006 and VNX-CRYPTO-009; VNX-GO-008 covers Go in more depth.

Remediation

1. Draw every secret from a CSPRNG: crypto.randomBytes()/crypto.randomUUID(), random_bytes()/random_int(), SecureRandom.hex/urlsafe_base64, crypto/rand.
2. For confirmed false positives, add a # vulnetix-ignore: VNX-338 comment on the line.

References

• CWE-338: Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)
• OWASP Cryptographic Storage Cheat Sheet: Secure random number generation

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-338
// nosec vnx-338

Or record an ignore rule: vulnetix ignore add --rule vnx-338 --reason "...". See Suppressing findings.