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

Code Examples & CVEs

Demonstrative Examples

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

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.

BadJavaScript

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 sets object attributes based on a dot-separated path.

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.

BadJavaScript

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

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

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.

GoodJavaScript

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; 					}

Observed CVE Examples (4)

CVE-2018-3721

Prototype pollution by merging objects.

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CVE-2019-10744

Prototype pollution by setting default values to object attributes recursively.

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CVE-2019-11358

Prototype pollution by merging objects recursively.

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CVE-2020-8203

Prototype pollution by setting object attributes based on dot-separated path.

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Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-1321 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-1321, Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution'), throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Apply Input Validation controls for Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

MITRE associates mitigation with Implementation, and Architecture and Design; the listed strategies include Input Validation; documented detection approaches include Automated Static Analysis. Use these source-defined anchors to turn CWE-1321 into implementation, review, and verification checks for the affected component.

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

Triage CWE-1321 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-1321, Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution'), has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-1321

Apply MITRE's full root-cause mapping guidance when using CWE-1321, Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution'). Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-1321 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-1321, Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution'). Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')?+

CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') is a Common Weakness Enumeration (CWE) entry maintained by MITRE. 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.

What are the security consequences of Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')?+

If exploited, CWE-1321 (Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')) it can compromise Confidentiality, Integrity and Availability, leading to outcomes such as Read Application Data, Modify Application Data and DoS: Crash, Exit, or Restart.

How do you prevent or mitigate Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')?+

Recommended mitigations for CWE-1321 include: By freezing the object prototype first (for example, Object.freeze(Object.prototype)), modification of the prototype becomes impossible. By blocking modifications of attributes that resolve to object prototype, such as proto or prototype, this weakness can be mitigated. When handling untrusted objects, validating using a schema can be used.

Which programming languages are affected by Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')?+

CWE-1321 commonly affects JavaScript. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What are real-world examples of Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')?+

MITRE documents real CVEs mapped to CWE-1321, including CVE-2018-3721, CVE-2019-10744, CVE-2019-11358 and CVE-2020-8203. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-1321 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

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

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Code Examples & CVEs

Demonstrative Examples

Observed CVE Examples (4)

CWE Relationships

Parent Weaknesses

Source-backed guidance

Verify controls for CWE-1321 with SSDF evidence

Apply Input Validation controls for Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

Triage CWE-1321 against known exploitation evidence

Apply precise root-cause mapping to CWE-1321

Validate CWE-1321 with root-cause mapping checks

Frequently Asked Questions

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