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Home > CWE List > CWE-338: Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG) (4.20)  
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    CWE-338: Use of Cryptographically Weak Pseudo-Random Number Generator (PRNG)

    Weakness ID: 338
    Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
    Abstraction: Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.
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    + 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.

    + Common Consequences
    Section HelpThis table 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.
    Impact Details

    Bypass Protection Mechanism

    		Scope: Access Control

    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.
    + Potential Mitigations
    Phase(s) Mitigation

    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.
    + Relationships
    Section Help This table 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" (View-1000)
    Nature Type ID Name
    ChildOf Class Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. 330 Use of Insufficiently Random Values
    + Relevant to the view "Software Development" (View-699)
    Nature Type ID Name
    MemberOf Category Category - a CWE entry that contains a set of other entries that share a common characteristic. 310 Cryptographic Issues
    MemberOf Category Category - a CWE entry that contains a set of other entries that share a common characteristic. 1213 Random Number Issues
    + Relevant to the view "Weaknesses for Simplified Mapping of Published Vulnerabilities" (View-1003)
    Nature Type ID Name
    ChildOf Class Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource. 330 Use of Insufficiently Random Values
    + Relevant to the view "Architectural Concepts" (View-1008)
    Nature Type ID Name
    MemberOf Category Category - a CWE entry that contains a set of other entries that share a common characteristic. 1013 Encrypt Data
    + Modes Of Introduction
    Section HelpThe different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.
    Phase Note
    Architecture and Design
    Implementation REALIZATION: This weakness is caused during implementation of an architectural security tactic.
    + Applicable Platforms
    Section HelpThis listing shows 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

    Class: Not Language-Specific (Undetermined Prevalence)

    Technologies

    Class: Not Technology-Specific (Undetermined Prevalence)

    + Likelihood Of Exploit
    Medium
    + Demonstrative Examples

    Example 1


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

    (bad code)
    Example Language: Java 
    Random random = new Random(System.currentTimeMillis());
    int accountID = random.nextInt();
    (bad code)
    Example Language: C 
    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).



    + Selected Observed Examples

    Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.

    Reference Description
    PHP framework uses mt_rand() function (Marsenne Twister) when generating tokens
    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.
    + Weakness Ordinalities
    Ordinality Description
    Primary
    (where the weakness exists independent of other weaknesses)
    + Detection Methods
    Method Details

    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.)

    Effectiveness: High
    + Memberships
    Section HelpThis 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.
    Nature Type ID Name
    MemberOf ViewView - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries). 884 CWE Cross-section
    MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 905 SFP Primary Cluster: Predictability
    MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 1170 SEI CERT C Coding Standard - Guidelines 48. Miscellaneous (MSC)
    MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 1346 OWASP Top Ten 2021 Category A02:2021 - Cryptographic Failures
    MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 1414 Comprehensive Categorization: Randomness
    MemberOf CategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic. 1439 OWASP Top Ten 2025 Category A04:2025 - Cryptographic Failures
    + Vulnerability Mapping Notes
    Usage ALLOWED
    (this CWE ID may be used to map to real-world vulnerabilities)
    Reason Acceptable-Use

    Rationale

    		This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.

    Comments

    		Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.
    + Notes

    Maintenance

    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.
    + Taxonomy Mappings
    Mapped Taxonomy Name Node ID Fit Mapped Node Name
    CLASP Non-cryptographic PRNG
    CERT C Secure Coding MSC30-C CWE More Abstract Do not use the rand() function for generating pseudorandom numbers
    + References
    [REF-18] Secure Software, Inc.. "The CLASP Application Security Process". 2005.
    <https://cwe.mitre.org/documents/sources/TheCLASPApplicationSecurityProcess.pdf>. (URL validated: 2024-11-17)
    [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 Date Submitter Organization
    2006-07-19
    (CWE Draft 3, 2006-07-19)     		CLASP
    + Modifications
    Modification Date Modifier Organization
    2025-12-11
    (CWE 4.19, 2025-12-11)     		CWE Content Team MITRE
    updated Applicable_Platforms, Relationships, Weakness_Ordinalities
    2023-10-26
    (CWE 4.13, 2023-10-26)     		CWE Content Team MITRE
    updated Observed_Examples
    2023-06-29
    (CWE 4.12, 2023-06-29)     		CWE Content Team MITRE
    updated Mapping_Notes
    2023-04-27
    (CWE 4.11, 2023-04-27)     		CWE Content Team MITRE
    updated Detection_Factors, Relationships
    2021-10-28
    (CWE 4.6, 2021-10-28)     		CWE Content Team MITRE
    updated Relationships
    2021-07-20
    (CWE 4.5, 2021-07-20)     		CWE Content Team MITRE
    updated Maintenance_Notes
    2021-03-15
    (CWE 4.4, 2021-03-15)     		CWE Content Team MITRE
    updated Demonstrative_Examples
    2020-02-24
    (CWE 4.0, 2020-02-24)     		CWE Content Team MITRE
    updated References, Relationships
    2019-01-03
    (CWE 3.2, 2019-01-03)     		CWE Content Team MITRE
    updated Relationships
    2017-11-08
    (CWE 3.0, 2017-11-08)     		CWE Content Team MITRE
    updated Demonstrative_Examples, Description, Modes_of_Introduction, Relationships, Taxonomy_Mappings
    2015-12-07
    (CWE 2.9, 2015-12-07)     		CWE Content Team MITRE
    updated Relationships
    2014-06-23
    (CWE 2.7, 2014-06-23)     		CWE Content Team MITRE
    updated Applicable_Platforms, Description, Name, Other_Notes
    2012-10-30
    (CWE 2.3, 2012-10-30)     		CWE Content Team MITRE
    updated Demonstrative_Examples, Potential_Mitigations
    2012-05-11
    (CWE 2.2, 2012-05-15)     		CWE Content Team MITRE
    updated Common_Consequences, Observed_Examples, References, Relationships
    2011-06-01
    (CWE 1.13, 2011-06-01)     		CWE Content Team MITRE
    updated Common_Consequences
    2008-09-08
    (CWE 1.0, 2008-09-09)     		CWE Content Team MITRE
    updated Common_Consequences, Relationships, Other_Notes, Taxonomy_Mappings
    2008-07-01
    (CWE 1.0, 2008-09-09)     		Eric Dalci Cigital
    updated Time_of_Introduction
    + Previous Entry Names
    Change Date Previous Entry Name
    2014-06-23 Use of Cryptographically Weak PRNG
    2008-04-11 Non-cryptographic PRNG
    Page Last Updated: April 30, 2026
     

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