CVE-2026-63671

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.

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.

Summary

@nuxtjs/mdc renders untrusted markdown (including raw HTML) to a Vue component tree. Across two prior advisories it added a URL/attribute sanitizer to block dangerous links in that HTML: validateProps / validateProp and an unsafeLinkPrefix deny-list (dist/runtime/parser/utils/props.js). The sanitizer runs at parse time (dist/runtime/parser/compiler.js) and parseMarkdown enables raw HTML by default (allowDangerousHtml: true, dist/runtime/parser/options.js), so the sanitizer is the only barrier and it applies with no configuration required.

Two sibling vectors bypass that sanitizer at the default configuration:

1. SVG anchor xlink:href. validateProp only scheme-checks attributes named exactly href or src:

   if (attribute === "href" || attribute === "src") return isAnchorLinkAllowed(value);
   return true;
   

   An xlink:href (parsed to the hast property xLinkHref) is neither, so a javascript: URL on an SVG <a> is passed through. The renderer maps the property back to the real attribute (MDCRenderer.vue: find(html, "xLinkHref").attribute is xlink:href), so the output element is <a xlink:href="javascript:...">. Clicking it runs the script in the page origin. Plain <a href="javascript:..."> is correctly stripped, which is what makes this the un-patched sibling.

2. <iframe src="data:text/html,...">. data:text/html is present in unsafeLinkPrefix, but the check compares it against url.protocol:

   if (unsafeLinkPrefix.some((prefix) => url.protocol.toLowerCase().startsWith(prefix))) return false;
   

   For any data URI url.protocol is just "data:", so "data:".startsWith("data:text/html") is always false. Every data:text/* entry in the deny-list is therefore dead code, and <iframe src="data:text/html,<script>...</script>"> is allowed (iframe is not in the render-time dangerousTags, which is only ["script","base"]). The framed document executes script in an opaque origin. For contrast, srcdoc and object are blocked, so this is a precise gap rather than a general absence of filtering.

Reproduction

I will attach the zip file for POC, you can simply extract and run ./poc.sh to install mdc and show the poc in the html file. nuxtjs-mdc-xss_poc.zip

Two zero-argument checks:

1. sh poc/poc.sh installs @nuxtjs/mdc and runs parseMarkdown (the documented API) at default. It shows the parsed tree retains a { xLinkHref: "javascript:..." } and iframe { src: "data:text/html,..." }, while the control payloads href="javascript:..." and srcdoc=... are removed by the sanitizer. This isolates the sanitizer bypass deterministically.
2. poc/poc.sh also serves poc/poc.html over http (data: iframes and javascript: links are restricted under the file:// origin, so http is used). Open the printed URL and click the blue SVG link. The page contains the exact DOM the renderer produces for those parsed nodes; clicking the SVG link executes script in the page origin (same-origin), and the data:text/html iframe executes on load. The page prints VULNERABLE for each that fires.

Both vectors were confirmed executing in a current Chromium build: the SVG xlink:href link runs script in the document origin on click, and the data:text/html iframe runs script on load.

Suggested fix

In validateProp, scheme-check xlink:href (and the hast xLinkHref) the same way as href/src. In isAnchorLinkAllowed, compare the dangerous MIME-typed entries against the full URL (or href), not against url.protocol, so data:text/html is actually matched; or add iframe to the render-time dangerous-tag set / restrict iframe src schemes.

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.

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.

Summary

@nuxtjs/mdc renders untrusted markdown (including raw HTML) to a Vue component tree. Across two prior advisories it added a URL/attribute sanitizer to block dangerous links in that HTML: validateProps / validateProp and an unsafeLinkPrefix deny-list (dist/runtime/parser/utils/props.js). The sanitizer runs at parse time (dist/runtime/parser/compiler.js) and parseMarkdown enables raw HTML by default (allowDangerousHtml: true, dist/runtime/parser/options.js), so the sanitizer is the only barrier and it applies with no configuration required.

Two sibling vectors bypass that sanitizer at the default configuration:

1. SVG anchor xlink:href. validateProp only scheme-checks attributes named exactly href or src:

   if (attribute === "href" || attribute === "src") return isAnchorLinkAllowed(value);
   return true;
   

   An xlink:href (parsed to the hast property xLinkHref) is neither, so a javascript: URL on an SVG <a> is passed through. The renderer maps the property back to the real attribute (MDCRenderer.vue: find(html, "xLinkHref").attribute is xlink:href), so the output element is <a xlink:href="javascript:...">. Clicking it runs the script in the page origin. Plain <a href="javascript:..."> is correctly stripped, which is what makes this the un-patched sibling.

2. <iframe src="data:text/html,...">. data:text/html is present in unsafeLinkPrefix, but the check compares it against url.protocol:

   if (unsafeLinkPrefix.some((prefix) => url.protocol.toLowerCase().startsWith(prefix))) return false;
   

   For any data URI url.protocol is just "data:", so "data:".startsWith("data:text/html") is always false. Every data:text/* entry in the deny-list is therefore dead code, and <iframe src="data:text/html,<script>...</script>"> is allowed (iframe is not in the render-time dangerousTags, which is only ["script","base"]). The framed document executes script in an opaque origin. For contrast, srcdoc and object are blocked, so this is a precise gap rather than a general absence of filtering.

Reproduction

I will attach the zip file for POC, you can simply extract and run ./poc.sh to install mdc and show the poc in the html file. nuxtjs-mdc-xss_poc.zip

Two zero-argument checks:

1. sh poc/poc.sh installs @nuxtjs/mdc and runs parseMarkdown (the documented API) at default. It shows the parsed tree retains a { xLinkHref: "javascript:..." } and iframe { src: "data:text/html,..." }, while the control payloads href="javascript:..." and srcdoc=... are removed by the sanitizer. This isolates the sanitizer bypass deterministically.
2. poc/poc.sh also serves poc/poc.html over http (data: iframes and javascript: links are restricted under the file:// origin, so http is used). Open the printed URL and click the blue SVG link. The page contains the exact DOM the renderer produces for those parsed nodes; clicking the SVG link executes script in the page origin (same-origin), and the data:text/html iframe executes on load. The page prints VULNERABLE for each that fires.

Both vectors were confirmed executing in a current Chromium build: the SVG xlink:href link runs script in the document origin on click, and the data:text/html iframe runs script on load.

Suggested fix

In validateProp, scheme-check xlink:href (and the hast xLinkHref) the same way as href/src. In isAnchorLinkAllowed, compare the dangerous MIME-typed entries against the full URL (or href), not against url.protocol, so data:text/html is actually matched; or add iframe to the render-time dangerous-tag set / restrict iframe src schemes.