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Home > CWE List > CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') (4.18)
ID
CWE Glossary Definition

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

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. Diagram for CWE-1321
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

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
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 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. 915 Improperly Controlled Modification of Dynamically-Determined Object Attributes
CanPrecede 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. 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 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
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 This weakness is often found in code that assigns object attributes based on user input, or merges or clones objects recursively.
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

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
Prototype pollution by merging objects.
Prototype pollution by setting default values to object attributes recursively.
Prototype pollution by merging objects recursively.
Prototype pollution by setting object attributes based on dot-separated path.
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 CategoryCategory - 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.
References
[REF-1148] Olivier Arteau. "Prototype pollution attack in NodeJS application". 2018年05月15日.
<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年07月30日.
<https://codeburst.io/what-is-prototype-pollution-49482fc4b638>.
Content History
Submissions
Submission Date Submitter Organization
2020年08月25日
(CWE 4.3, 2020年12月10日) 		Anonymous External Contributor
Modifications
Modification Date Modifier Organization
2025年09月09日
(CWE 4.18, 2025年09月09日) 		CWE Content Team MITRE
updated Common_Consequences, Description, Diagram, Modes_of_Introduction
2024年02月29日
(CWE 4.14, 2024年02月29日) 		CWE Content Team MITRE
updated Demonstrative_Examples
2023年06月29日 CWE Content Team MITRE
updated Mapping_Notes
2023年04月27日 CWE Content Team MITRE
updated Relationships
2023年01月31日 CWE Content Team MITRE
updated Description
2021年10月28日 CWE Content Team MITRE
updated Relationships
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Page Last Updated: September 09, 2025

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