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ID

CWE-338: Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

Weakness ID: 338
Abstraction: Base
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The product uses a Pseudo-Random Number Generator (PRNG) in a security context, but the PRNG's algorithm is not cryptographically strong.
+ Extended Description

When a non-cryptographic PRNG is used in a cryptographic context, it can expose the cryptography to certain types of attacks.

Often a pseudo-random number generator (PRNG) is not designed for cryptography. Sometimes a mediocre source of randomness is sufficient or preferable for algorithms that use random numbers. Weak generators generally take less processing power and/or do not use the precious, finite, entropy sources on a system. While such PRNGs might have very useful features, these same features could be used to break the cryptography.

+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfClassClass330Use of Insufficiently Random Values
+ Relevant to the view "Weaknesses for Simplified Mapping of Published Vulnerabilities" (CWE-1003)
NatureTypeIDName
ChildOfClassClass330Use of Insufficiently Random Values
+ Relevant to the view "Architectural Concepts" (CWE-1008)
NatureTypeIDName
MemberOfCategoryCategory1013Encrypt Data
+ Relevant to the view "Development Concepts" (CWE-699)
NatureTypeIDName
ChildOfClassClass330Use of Insufficiently Random Values
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

PhaseNote
Architecture and Design
ImplementationREALIZATION: This weakness is caused during implementation of an architectural security tactic.
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

(Language-Independent classes): (Undetermined Prevalence)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Access Control

Technical Impact: Bypass Protection Mechanism

If a PRNG is used for authentication and authorization, such as a session ID or a seed for generating a cryptographic key, then an attacker may be able to easily guess the ID or cryptographic key and gain access to restricted functionality.
+ Likelihood Of Exploit
Medium
+ Demonstrative Examples

Example 1

Both of these examples use a statistical PRNG to generate a random number:

(bad)
Example Language: Java 
Random random = new Random(System.currentTimeMillis());
int accountID = random.nextInt();
(bad)
Example Language:
srand(time());
int randNum = rand();

The random number functions used in these examples, rand() and Random.nextInt(), are not considered cryptographically strong. An attacker may be able to predict the random numbers generated by these functions. Note that these example also exhibit CWE-337 (Predictable Seed in PRNG).

+ Observed Examples
ReferenceDescription
Crypto product uses rand() library function to generate a recovery key, making it easier to conduct brute force attacks.
Random number generator can repeatedly generate the same value.
Web application generates predictable session IDs, allowing session hijacking.
SSL library uses a weak random number generator that only generates 65,536 unique keys.
+ Potential Mitigations

Phase: Implementation

Use functions or hardware which use a hardware-based random number generation for all crypto. This is the recommended solution. Use CyptGenRandom on Windows, or hw_rand() on Linux.
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
NatureTypeIDName
MemberOfViewView884CWE Cross-section
MemberOfCategoryCategory905SFP Primary Cluster: Predictability
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPNon-cryptographic PRNG
CERT C Secure CodingMSC30-CCWE More AbstractDo not use the rand() function for generating pseudorandom numbers
+ References
[REF-44] Michael Howard, David LeBlanc and John Viega. "24 Deadly Sins of Software Security". "Sin 20: Weak Random Numbers." Page 299. McGraw-Hill. 2010.
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
CLASP
Modifications
Modification DateModifierOrganizationSource
2008-07-01Eric DalciCigital
updated Time_of_Introduction
2008-09-08CWE Content TeamMITRE
updated Common_Consequences, Relationships, Other_Notes, Taxonomy_Mappings
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Common_Consequences, Observed_Examples, References, Relationships
2012-10-30CWE Content TeamMITRE
updated Demonstrative_Examples, Potential_Mitigations
2014-06-23CWE Content TeamMITRE
updated Applicable_Platforms, Description, Name, Other_Notes
2015-12-07CWE Content TeamMITRE
updated Relationships
2017-11-08CWE Content TeamMITRE
updated Demonstrative_Examples, Description, Modes_of_Introduction, Relationships, Taxonomy_Mappings
Previous Entry Names
Change DatePrevious Entry Name
2008-04-11Non-cryptographic PRNG
2014-06-23Use of Cryptographically Weak PRNG

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Page Last Updated: November 14, 2017