CVE-2023-26158: All versions of the package mockjs are vulnerable to Prototype Pollution via the Util.extend function due to missing check if the attribute resolves t...

Description

All versions of the package mockjs are vulnerable to Prototype Pollution via the Util.extend function due to missing check if the attribute resolves to the object prototype. By adding or modifying attributes of an object prototype, it is possible to create attributes that exist on every object, or replace critical attributes with malicious ones. This can be problematic if the software depends on existence or non-existence of certain attributes, or uses pre-defined attributes of object prototype (such as hasOwnProperty, toString or valueOf). User controlled inputs inside the extend() method of the Mock.Handler, Mock.Random, Mock.RE.Handler or Mock.Util, will allow an attacker to exploit this vulnerability. Workaround By using a denylist of dangerous attributes, this weakness can be eliminated. Add the following line in the Util.extend function: js js if (["__proto__", "constructor", "prototype"].includes(name)) continue js // src/mock/handler.js Util.extend = function extend() { var target = arguments[0] || {}, i = 1, length = arguments.length, options, name, src, copy, clone if (length === 1) { target = this i = 0 } for (; i < length; i++) { options = arguments[i] if (!options) continue for (name in options) { if (["__proto__", "constructor", "prototype"].includes(name)) continue src = target[name] copy = options[name] if (target === copy) continue if (copy === undefined) continue if (Util.isArray(copy) || Util.isObject(copy)) { if (Util.isArray(copy)) clone = src && Util.isArray(src) ? src : [] if (Util.isObject(copy)) clone = src && Util.isObject(src) ? src : {} target[name] = Util.extend(clone, copy) } else { target[name] = copy } } } return target }

Metrics

CVSS 3.1

8.2/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H

EPSS Probability

0.80%

51.9th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Mockjs	Mock.Js	<= 1.1.0

References

https://github.com/nuysoft/Mock/blob/00ce04b92eb464e664a4438430903f2de96efb47/dist/mock.js%23L721-L755Broken Link
https://security.snyk.io/vuln/SNYK-JS-MOCKJS-6051365Exploit, Mitigation, Third Party Advisory
https://github.com/nuysoft/Mock/blob/00ce04b92eb464e664a4438430903f2de96efb47/dist/mock.js%23L721-L755Broken Link
https://security.snyk.io/vuln/SNYK-JS-MOCKJS-6051365Exploit, Mitigation, Third Party Advisory

Timeline

Published: Dec 8, 2023
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2023-26158?

All versions of the package mockjs are vulnerable to Prototype Pollution via the Util.extend function due to missing check if the attribute resolves to the object prototype. By adding or modifying attributes of an object prototype, it is possible to create attributes that exist on every object, or replace critical attributes with malicious ones. This can be problematic if the software depends on existence or non-existence of certain attributes, or uses pre-defined attributes of object prototype (such as hasOwnProperty, toString or valueOf). User controlled inputs inside the extend() method of the Mock.Handler, Mock.Random, Mock.RE.Handler or Mock.Util, will allow an attacker to exploit this vulnerability. Workaround By using a denylist of dangerous attributes, this weakness can be eliminated. Add the following line in the Util.extend function: js js if (["__proto__", "constructor", "prototype"].includes(name)) continue js // src/mock/handler.js Util.extend = function extend() { var target = arguments[0] || {}, i = 1, length = arguments.length, options, name, src, copy, clone if (length === 1) { target = this i = 0 } for (; i < length; i++) { options = arguments[i] if (!options) continue for (name in options) { if (["__proto__", "constructor", "prototype"].includes(name)) continue src = target[name] copy = options[name] if (target === copy) continue if (copy === undefined) continue if (Util.isArray(copy) || Util.isObject(copy)) { if (Util.isArray(copy)) clone = src && Util.isArray(src) ? src : [] if (Util.isObject(copy)) clone = src && Util.isObject(src) ? src : {} target[name] = Util.extend(clone, copy) } else { target[name] = copy } } } return target }

How severe is CVE-2023-26158?

CVE-2023-26158 has a CVSS score of 8.2/10 (HIGH severity). The EPSS model estimates a 0.80% probability of exploitation in the next 30 days.

How do I fix CVE-2023-26158?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.