mockjs vulnerable to Prototype Pollution via the Util.extend function

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2023-26158: 0.80% estimated exploitation activity in the next 30 days; percentile 56.68 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

8 Dec 2023, 06:30 UTC

Last modified by the source

17 Dec 2023, 05:05 UTC

Source last checked

26 Oct 2026, 12:30 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-mh8j-9jvh-gjf6

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-mh8j-9jvh-gjf6

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# Patch Next advisory brief: CVE-2023-26158

Advisory: mockjs vulnerable to Prototype Pollution via the Util.extend function
IDs: CVE-2023-26158, GHSA-mh8j-9jvh-gjf6
Record: https://patchnext.com/advisories/cve-2023-26158 (revision 2, source checked 26 Oct 2026, 12:30 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor response not established. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm mockjs: affected <= 1.1.0

## Suggested inspection (read-only)
- `npm ls mockjs --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mockjs.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-mh8j-9jvh-gjf6 (OSV record GHSA-mh8j-9jvh-gjf6 lists affected ranges: npm mockjs >=0 <=1.1.0)
- Verified: https://github.com/advisories/GHSA-mh8j-9jvh-gjf6 (GitHub-reviewed advisory GHSA-mh8j-9jvh-gjf6 lists affected ranges: npm mockjs >=0 <=1.1.0)

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