CVE-2020-26237: Highlight.js is a syntax highlighter written in JavaScript. Highlight.js versions before 9.18.2 and 10.1.2 are vulnerable to Prototype Pollution. A ma...

Description

Highlight.js is a syntax highlighter written in JavaScript. Highlight.js versions before 9.18.2 and 10.1.2 are vulnerable to Prototype Pollution. A malicious HTML code block can be crafted that will result in prototype pollution of the base object's prototype during highlighting. If you allow users to insert custom HTML code blocks into your page/app via parsing Markdown code blocks (or similar) and do not filter the language names the user can provide you may be vulnerable. The pollution should just be harmless data but this can cause problems for applications not expecting these properties to exist and can result in strange behavior or application crashes, i.e. a potential DOS vector. If your website or application does not render user provided data it should be unaffected. Versions 9.18.2 and 10.1.2 and newer include fixes for this vulnerability. If you are using version 7 or 8 you are encouraged to upgrade to a newer release.

Metrics

CVSS 3.1

8.7/10

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

EPSS Probability

1.30%

66.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-471

Affected Software

Vendor	Product	Versions
Highlightjs	Highlight.Js	< 9.18.2
Highlightjs	Highlight.Js	>= 10.1.0, < 10.1.2
Debian	Debian Linux	9.0
Oracle	Mysql Enterprise Monitor	<= 8.0.30

References

https://github.com/highlightjs/highlight.js/commit/7241013ae011a585983e176ddc0489a7a52f6bb0Patch, Third Party Advisory
https://github.com/highlightjs/highlight.js/pull/2636Patch, Third Party Advisory
https://github.com/highlightjs/highlight.js/security/advisories/GHSA-vfrc-7r7c-w9mxMitigation, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2020/12/msg00041.htmlMailing List, Third Party Advisory
https://www.npmjs.com/package/highlight.jsThird Party Advisory
https://www.oracle.com/security-alerts/cpujul2022.htmlThird Party Advisory
https://github.com/highlightjs/highlight.js/commit/7241013ae011a585983e176ddc0489a7a52f6bb0Patch, Third Party Advisory
https://github.com/highlightjs/highlight.js/pull/2636Patch, Third Party Advisory
https://github.com/highlightjs/highlight.js/security/advisories/GHSA-vfrc-7r7c-w9mxMitigation, Third Party Advisory
https://lists.debian.org/debian-lts-announce/2020/12/msg00041.htmlMailing List, Third Party Advisory
https://www.npmjs.com/package/highlight.jsThird Party Advisory
https://www.oracle.com/security-alerts/cpujul2022.htmlThird Party Advisory

Timeline

Published: Nov 24, 2020
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2020-26237?

Highlight.js is a syntax highlighter written in JavaScript. Highlight.js versions before 9.18.2 and 10.1.2 are vulnerable to Prototype Pollution. A malicious HTML code block can be crafted that will result in prototype pollution of the base object's prototype during highlighting. If you allow users to insert custom HTML code blocks into your page/app via parsing Markdown code blocks (or similar) and do not filter the language names the user can provide you may be vulnerable. The pollution should just be harmless data but this can cause problems for applications not expecting these properties to exist and can result in strange behavior or application crashes, i.e. a potential DOS vector. If your website or application does not render user provided data it should be unaffected. Versions 9.18.2 and 10.1.2 and newer include fixes for this vulnerability. If you are using version 7 or 8 you are encouraged to upgrade to a newer release.

How severe is CVE-2020-26237?

CVE-2020-26237 has a CVSS score of 8.7/10 (HIGH severity). The EPSS model estimates a 1.30% probability of exploitation in the next 30 days.

How do I fix CVE-2020-26237?

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.