Incorrect Usage of Seeds in Pseudo-Random Number Generator (PRNG)

Opis

The product uses a Pseudo-Random Number Generator (PRNG) but does not correctly manage seeds.

PRNGs are deterministic and, while their output appears random, they cannot actually create entropy. They rely on cryptographically secure and unique seeds for entropy so proper seeding is critical to the secure operation of the PRNG.

Management of seeds could be broken down into two main areas:

- (1) protecting seeds as cryptographic material (such as a cryptographic key);

- (2) whenever possible, using a uniquely generated seed from a cryptographically secure source

PRNGs require a seed as input to generate a stream of numbers that are functionally indistinguishable from random numbers. While the output is, in many cases, sufficient for cryptographic uses, the output of any PRNG is directly determined by the seed provided as input. If the seed can be ascertained by a third party, the entire output of the PRNG can be made known to them. As such, the seed should be kept secret and should ideally not be able to be guessed. For example, the current time may be a poor seed. Knowing the approximate time the PRNG was seeded greatly reduces the possible key space.

Seeds do not necessarily need to be unique, but reusing seeds may open up attacks if the seed is discovered.

Skutki

• Access Control, Other

  Bypass Protection Mechanism; Other

  If a PRNG is used incorrectly, such as using the same seed for each initialization or using a predictable seed, then an attacker may be able to easily guess the seed and thus the random numbers. This could lead to unauthorized access to a system if the seed is used for authentication and authorization.

Przykłady podatności

• CVE-2020-7010 Cloud application on Kubernetes generates passwords using a weak random number generator based on deployment time.
• CVE-2019-11495 server uses erlang:now() to seed the PRNG, which results in a small search space for potential random seeds
• CVE-2018-12520 Product's PRNG is not seeded for the generation of session IDs
• CVE-2016-10180 Router's PIN generation is based on rand(time(0)) seeding.

Słabość nadrzędna

• CWE-330 Use of Insufficiently Random Values

Słabości podrzędne

• CWE-336 Same Seed in Pseudo-Random Number Generator (PRNG)
• CWE-337 Predictable Seed in Pseudo-Random Number Generator (PRNG)
• CWE-339 Small Seed Space in PRNG

Kiedy powstaje

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

Dotyczy

Not Language-Specific

Uwagi

As of CWE 4.5, terminology related to randomness, entropy, and predictability can vary widely. Within the developer and other communities, "randomness" is used heavily. However, within cryptography, "entropy" is distinct, typically implied as a measurement. There are no commonly-used definitions, even within standards documents and cryptography papers. Future versions of CWE will attempt to define these terms and, if necessary, distinguish between them in ways that are appropriate for different communities but do not reduce the usability of CWE for mapping, understanding, or other scenarios.

Źródła

1. 24 Deadly Sins of Software Security — Michael Howard, David LeBlanc, John Viega, "Sin 20: Weak Random Numbers." Page 299, McGraw-Hill, 2010