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.
- 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
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.
Which versions are affected by CVE-2026-63671?
2 open source packages ship an affected version, including @nuxtjs/mdc.
| Package | 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.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-63671 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-63671 exists.
Sources
Also known as GHSA-mxm6-v9r6-r94c.