VNX-337: Predictable Seed in Pseudo-Random Number Generator (PRNG)

Field	Value
Rule ID	VNX-337
Severity	Medium
SARIF level	warning
Kind	sast
Languages	Python, Java, Kotlin, Go, C, C++, PHP, Ruby, JavaScript / TypeScript
CWE	CWE-337
CAPEC	CAPEC-97
ATT&CK	T1557
CWSS vector	CWSS:1.0/TI:M/AP:RU/AL:S/IC:N/FC:LT/RP:RU/RL:S/AV:L/AS:N/IN:A/SC:A/BI:M/DI:H/EX:M/EC:N/P:C
Tags	prng seed cwe-337
Source	internal/sast/rules/vnx-337.rego

Overview

VNX-337 maps to CWE-337: Predictable Seed in Pseudo-Random Number Generator (PRNG). A seed taken from the clock or the process id has only a few plausible values: an attacker who knows roughly when the process started, or can read its PID, tries them all and reproduces every value the generator produced.

The rule reports a seed built from time(NULL), time.time(), time.Now(), datetime.now(), System.currentTimeMillis()/nanoTime(), Instant.now(), Date.now()/new Date(), microtime(), hrtime(), Time.now, clock(), gettimeofday/tv_sec/tv_usec, getpid()/os.getpid()/os.Getpid()/getmypid()/Process.pid/process.pid, performance.now() and similar, inline or through a variable assigned one of those within eight lines above, passed to the same seeding APIs as VNX-336.

A time-seeded SecureRandom or ChaCha8 is always reported. A general-purpose PRNG is reported only when its output feeds a secret: the seed call sits in a function named for a secret, the generator is stored under a secret’s name, or the same file draws from a PRNG into a token, password, key, salt, nonce, OTP, CSRF or session value.

What Gets Flagged

// FLAGGED: the seed is the current second
srand(time(NULL));

// FLAGGED: clock and PID seeds in a file that draws session tokens
rand.Seed(time.Now().UnixNano())
var pidRand = rand.New(rand.NewSource(int64(os.Getpid())))

// FLAGGED: a SecureRandom seeded from the clock is predictable
sr.setSeed(System.currentTimeMillis());

What Is Not Flagged

• Clock seeds for jitter, sampling, games and load spreading in files that produce no secret.
• OS-seeded generators (new SecureRandom(), random.SystemRandom(), crypto/rand).
• Test files, and calls in comments or string literals.

Constant seeds are VNX-336 and seeds narrowed to a small range are VNX-339.

Remediation

1. Do not seed generators that produce secrets. Use a CSPRNG that seeds itself: secrets, SecureRandom, crypto/rand, random_bytes(), crypto.randomBytes().
2. Never call setSeed() on a SecureRandom with the time; let it seed itself from the operating system.
3. For confirmed false positives, add a # vulnetix-ignore: VNX-337 comment on the line.

References

• CWE-337: Predictable Seed in 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-337
// nosec vnx-337

Or record an ignore rule: vulnetix ignore add --rule vnx-337 --reason "...". See Suppressing findings.