# CVE-2026-63671: @nuxtjs/mdc's URL sanitizer misses SVG xlink:href and data:text/html, allowing XSS from untrusted markdown at the default configuration

> Attackers can execute arbitrary JavaScript via untrusted Markdown by bypassing URL sanitization for SVG xlink:href and data:text/html URLs.

- **Severity:** High 8.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 18 Sep 2026
- **Also known as:** GHSA-mxm6-v9r6-r94c

## What is CVE-2026-63671?

CVE-2026-63671 is a high-severity cross-site scripting vulnerability in nuxt-content mdc, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

MDC is a tool to take regular Markdown and write documents interacting deeply with a Vue component. Prior to 0.22.1, @nuxtjs/mdc uses parseMarkdown with allowDangerousHtml enabled by default and relies on validateProps, validateProp, and unsafeLinkPrefix to remove executable URLs from untrusted Markdown. validateProp checks only attributes named href or src, allowing an SVG xlink:href value represented as xLinkHref to retain a javascript: URL that executes in the page origin when selected. The data:text/html denylist entries are also compared against url.protocol, which is only data:, so an iframe src containing data:text/html survives sanitization and executes in an opaque origin when loaded. Plain href javascript: URLs, srcdoc, object, script, and base elements are already blocked, making these two paths specific sibling gaps in the sanitizer. This issue is fixed in version 0.22.1.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation), CWE-184 (Incomplete List of Disallowed Inputs)

## Which versions are affected by CVE-2026-63671?

2 open source packages ship an affected version, including @nuxtjs/mdc.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `@nuxtjs/mdc` | npm | All versions | 0.22.1 |
| `@nuxtjs/mdc` | npm | All versions | 0.22.1 |

## How do I fix CVE-2026-63671?

Upgrade @nuxtjs/mdc to 0.22.1 or later.

## Is CVE-2026-63671 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-63671 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63671 and fix it if it is affected.

What it is: CVE-2026-63671 is a high-severity cross-site scripting vulnerability in nuxt-content mdc, published on 16 Sep 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-63671 exists.

Affected packages and fixed versions:
- @nuxtjs/mdc (npm): affected all versions; fixed in 0.22.1
- @nuxtjs/mdc (npm): affected all versions; fixed in 0.22.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-63671/ (Markdown: https://insights.sbomify.com/CVE-2026-63671/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63671
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63671
- OSV: https://osv.dev/vulnerability/CVE-2026-63671

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63671/
Summaries are generated and may contain mistakes; the official description above is the record.