CWE-338 Use of Cryptographically Weak PRNG

Coverage: 25 rules in the SiteShadow rule registry target this CWE (registry v2.0.0). Regex 25 Also: Taint and heuristic analyzers may also detect related flows (see coverage for the authoritative list) Registry tagging shows intent, for sample-level behaviour and benchmarked gaps see known gaps.

What this means

SiteShadow flagged use of a non-cryptographic PRNG where security-grade randomness is required (tokens, reset links, session IDs, API keys, nonces).

Why it matters

Weak PRNGs can be predicted or replayed.

• Guessable tokens lead to account takeover and unauthorized access.
• Broken crypto: weak nonces/IVs can break encryption guarantees.
• These issues are often exploitable at scale because PRNG outputs are predictable.

Safer examples

1) Use a cryptographically secure RNG (Node + Python)

import { randomBytes } from "node:crypto";
const token = randomBytes(32).toString("hex");

import secrets
token = secrets.token_urlsafe(32)

2) Don't use Math.random() / random() for secrets

Those are fine for UI effects and simulations, not auth tokens.

3) Keep entropy sufficient

Use at least 128 bits of entropy for security tokens (often 16+ bytes).

How SiteShadow detects it (high level)

• Detects weak PRNG APIs used near security-sensitive values and flows.
• Flags "token-like" generation patterns that do not use secure RNGs.

References

• CWE-338: https://cwe.mitre.org/data/definitions/338.html

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