logo

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

Draft Base
Structure: Simple
Description

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

Extended Description

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.

Common Consequences 1
Scope: Access ControlOther

Impact: Bypass Protection MechanismOther

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.
Demonstrative Examples 3

ID : DX-177

The following code uses a statistical PRNG to generate account IDs.

Code Example:

Bad
Java
java
Because the program uses the same seed value for every invocation of the PRNG, its values are predictable, making the system vulnerable to attack.

ID : DX-102

Both of these examples use a statistical PRNG seeded with the current value of the system clock to generate a random number:

Code Example:

Bad
Java
java

Code Example:

Bad
C
c
An attacker can easily predict the seed used by these PRNGs, and so also predict the stream of random numbers generated. Note these examples also exhibit Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) (Use of Cryptographically Weak PRNG).

ID : DX-178

This code grabs some random bytes and uses them for a seed in a PRNG, in order to generate a new cryptographic key.

Code Example:

Bad
Python


getting 2 bytes of randomness for the seeding the PRNG *


python
Since only 2 bytes are used as a seed, an attacker will only need to guess 2^16 (65,536) values before being able to replicate the state of the PRNG.
  
Observed Examples 4
CVE-2020-7010Cloud application on Kubernetes generates passwords using a weak random number generator based on deployment time.
CVE-2019-11495server uses erlang:now() to seed the PRNG, which results in a small search space for potential random seeds
CVE-2018-12520Product's PRNG is not seeded for the generation of session IDs
CVE-2016-10180Router's PIN generation is based on rand(time(0)) seeding.
  
References 1
24 Deadly Sins of Software Security
Michael Howard, David LeBlanc, and John Viega
McGraw-Hill
2010
ID: REF-44
 
  
    
 
 
Applicable Platforms 
 
 
 
 
 
Languages:
 
 
 Not Language-Specific : Undetermined 
  
  
 
 
 
  
 
 
Modes of Introduction 
 
 
 
 
 
  Implementation  
 
 
 
          
 
 
Related Weaknesses 
 
 
 
 
  ChildOf:
 Use of Insufficiently Random Values (CWE-330) 
  ChildOf:
 Use of Insufficiently Random Values (CWE-330) 
 
 
 
 
Taxonomy Mapping 
  • PLOVER
  
Notes 
MaintenanceAs 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.