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Security Advisories: GSA_kwCzR0hTQS1taDhqLTlqdmgtZ2pmNs4AA3oc

mockjs vulnerable to Prototype Pollution via the Util.extend function

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.


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

Add the following line in the Util.extend function:

js if (["proto", "constructor", "prototype"].includes(name)) continue

// 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
Source: GitHub Advisory Database
Origin: Unspecified
Severity: High
Classification: General
Published: 3 months ago
Updated: 2 months ago

CVSS Score: 8.2
CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H

Identifiers: GHSA-mh8j-9jvh-gjf6, CVE-2023-26158

Affected Packages

Versions: <= 1.1.0
No known fixed version