CVE-2020-26237: Prototype Pollution in highlight.js

Severity: high (CVSS 8.7 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2020-11-24 · updated 2026-06-17
Weakness: CWE-471

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.

Affected products

Debian Linux: 9.0
highlightjs highlight.js: before 9.18.2; from 10.1.0 before 10.1.2 — fixed in 10.1.2, 9.18.2
Oracle Corporation MySQL Enterprise Monitor: through 8.0.30

How to fix

Upgrade to a fixed version: highlightjs highlight.js 10.1.2, 9.18.2. Check the vendor advisory for mitigations.

References

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