CWE - CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') (4.18)

Weakness ID: 1321

Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
Abstraction: Variant Variant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.

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Description

The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.

Common Consequences

This 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

Read Application Data; Modify Application Data

Scope: Confidentiality, Integrity, Availability

Likelihood: High

This weakness is usually exploited by using a special attribute of objects called proto, constructor, or prototype. Such attributes give access to the object prototype. An attacker can inject attributes that are used in other components by adding or modifying attributes of an object prototype. This creates attributes that exist on every object, or replace critical attributes with malicious ones. This can be problematic if the product depends on existence or non-existence of certain attributes, or uses pre-defined attributes of the object prototype (such as hasOwnProperty, toString, or valueOf).

DoS: Crash, Exit, or Restart

Scope: Availability

Likelihood: High

An attacker can override existing attributes with ones that have incompatible type, which may lead to a crash.

Potential Mitigations

Phase(s)	Mitigation
Implementation	By freezing the object prototype first (for example, Object.freeze(Object.prototype)), modification of the prototype becomes impossible. Effectiveness: High Note: While this can mitigate this weakness completely, other methods are recommended when possible, especially in components used by upstream software ("libraries").
Architecture and Design	By blocking modifications of attributes that resolve to object prototype, such as proto or prototype, this weakness can be mitigated. Effectiveness: High
Implementation	Strategy: Input Validation When handling untrusted objects, validating using a schema can be used. Effectiveness: Limited
Implementation	By using an object without prototypes (via Object.create(null) ), adding object prototype attributes by accessing the prototype via the special attributes becomes impossible, mitigating this weakness. Effectiveness: High
Implementation	Map can be used instead of objects in most cases. If Map methods are used instead of object attributes, it is not possible to access the object prototype or modify it. Effectiveness: Moderate

Relationships

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	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.	915	Improperly Controlled Modification of Dynamically-Determined Object Attributes
CanPrecede	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.	471	Modification of Assumed-Immutable Data (MAID)

Relevant to the view "Weaknesses for Simplified Mapping of Published Vulnerabilities" (View-1003)

Nature	Type	ID	Name
ChildOf	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.	913	Improper Control of Dynamically-Managed Code Resources

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 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	This weakness is often found in code that assigns object attributes based on user input, or merges or clones objects recursively.

Applicable Platforms

This 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

JavaScript (Undetermined Prevalence)

Demonstrative Examples

Example 1

This function sets object attributes based on a dot-separated path.

(bad code)

Example Language: JavaScript

function setValueByPath (object, path, value) {

const pathArray = path.split(".");
const attributeToSet = pathArray.pop();
let objectToModify = object;
for (const attr of pathArray) {

if (typeof objectToModify[attr] !== 'object') {

objectToModify[attr] = {};
}

objectToModify = objectToModify[attr];
}

objectToModify[attributeToSet] = value;
return object;
}

This function does not check if the attribute resolves to the object prototype. These codes can be used to add "isAdmin: true" to the object prototype.

(bad code)

Example Language: JavaScript

setValueByPath({}, "__proto__.isAdmin", true)
setValueByPath({}, "constructor.prototype.isAdmin", true)

By using a denylist of dangerous attributes, this weakness can be eliminated.

(good code)

Example Language: JavaScript

function setValueByPath (object, path, value) {

const pathArray = path.split(".");
const attributeToSet = pathArray.pop();
let objectToModify = object;
for (const attr of pathArray) {

// Ignore attributes which resolve to object prototype
if (attr === "__proto__" || attr === "constructor" || attr === "prototype") {

continue;
}

if (typeof objectToModify[attr] !== "object") {

objectToModify[attr] = {};
}

objectToModify = objectToModify[attr];
}

objectToModify[attributeToSet] = value;
return object;
}

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
CVE-2018-3721	Prototype pollution by merging objects.
CVE-2019-10744	Prototype pollution by setting default values to object attributes recursively.
CVE-2019-11358	Prototype pollution by merging objects recursively.
CVE-2020-8203	Prototype pollution by setting object attributes based on dot-separated path.

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.

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1415	Comprehensive Categorization: Resource Control

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 Variant 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.

Related Attack Patterns

CAPEC-ID	Attack Pattern Name
CAPEC-1	Accessing Functionality Not Properly Constrained by ACLs
CAPEC-180	Exploiting Incorrectly Configured Access Control Security Levels
CAPEC-77	Manipulating User-Controlled Variables

References

[REF-1148]	Olivier Arteau. "Prototype pollution attack in NodeJS application". 2018-05-15. <https://github.com/HoLyVieR/prototype-pollution-nsec18/blob/master/paper/JavaScript_prototype_pollution_attack_in_NodeJS.pdf>.
[REF-1149]	Changhui Xu. "What is Prototype Pollution?". 2019-07-30. <https://codeburst.io/what-is-prototype-pollution-49482fc4b638>.

Content History

Submissions
Submission Date	Submitter	Organization
2020-08-25 (CWE 4.3, 2020-12-10)	Anonymous External Contributor
2020-08-25 (CWE 4.3, 2020-12-10)
Modifications
Modification Date	Modifier	Organization
2025-09-09 (CWE 4.18, 2025-09-09)	CWE Content Team	MITRE
2025-09-09 (CWE 4.18, 2025-09-09)	updated Common_Consequences, Description, Diagram, Modes_of_Introduction
2024-02-29 (CWE 4.14, 2024-02-29)	CWE Content Team	MITRE
2024-02-29 (CWE 4.14, 2024-02-29)	updated Demonstrative_Examples
2023-06-29	CWE Content Team	MITRE
2023-06-29	updated Mapping_Notes
2023-04-27	CWE Content Team	MITRE
2023-04-27	updated Relationships
2023-01-31	CWE Content Team	MITRE
2023-01-31	updated Description
2021-10-28	CWE Content Team	MITRE
2021-10-28	updated Relationships


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