Summary
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.
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
}
Impact
CVE-2023-26158 has a CVSS score of 8.2 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2023-26158? CVE-2023-26158 is a high-severity security vulnerability in mockjs (npm), affecting versions <= 1.1.0. No fixed version is listed yet.
- How severe is CVE-2023-26158? CVE-2023-26158 has a CVSS score of 8.2 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of mockjs are affected by CVE-2023-26158? mockjs (npm) versions <= 1.1.0 is affected.
- Is there a fix for CVE-2023-26158? No fixed version is listed for CVE-2023-26158 yet. Monitor the advisory for updates and apply mitigations in the interim.
- Is CVE-2023-26158 exploitable, and should I be worried? Whether CVE-2023-26158 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2023-26158 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.