COMMON WEAKNESS ENUMERATIONDraft

CWE-337

Predictable Seed in Pseudo-Random Number Generator (PRNG)

A Pseudo-Random Number Generator (PRNG) is initialized from a predictable seed, such as the process ID or system time.

Abstraction

Variant

Structure

Simple

Likelihood of Exploit

Medium

Understand the problem

What is the problem?

A Pseudo-Random Number Generator (PRNG) is initialized from a predictable seed, such as the process ID or system time.

Extended description

The use of predictable seeds significantly reduces the number of possible seeds that an attacker would need to test in order to predict which random numbers will be generated by the PRNG.

Root cause

How and when does the problem arise?

Lifecycle phase

Implementation

REALIZATION: This weakness is caused during implementation of an architectural security tactic.

Risk

What does successful exploitation lead to?

Varies by Context

Affected security scopes: Other

Defence

How is it prevented and fixed?

All phases

General mitigation and defence approach

Detail
Use non-predictable inputs for seed generation.

Architecture and Design · Requirements

Libraries or Frameworks

Detail
Use products or modules that conform to FIPS 140-2 [REF-267] to avoid obvious entropy problems, or use the more recent FIPS 140-3 [REF-1192] if possible.

Implementation

Implementation-phase defence

Detail
Use a PRNG that periodically re-seeds itself using input from high-quality sources, such as hardware devices with high entropy. However, do not re-seed too frequently, or else the entropy source might block.

Verification

How is it detected?

Automated Static Analysis

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.)