{"id":"CVE-2026-63671","aliases":["EUVD-2026-80789","GHSA-mxm6-v9r6-r94c"],"status":"published","title":"@nuxtjs/mdc: the URL sanitizer misses SVG xlink:href and data:text/html, allowing XSS from untrusted markdown at the default configuration","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.","severity":{"version":"3.1","score":8.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N","level":"high","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-11","score":0.00474,"percentile":0.41427,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":true},"cwes":["CWE-184","CWE-79"],"published":"2026-09-16T14:25:47Z","modified":"2026-10-28T02:30:40Z","fixes":[{"ecosystem":"npm","package":"@nuxtjs/mdc","fixed_in":["0.22.1"],"sources":["ghsa","osv"]}],"affected":[{"source":"ghsa","ecosystem":"npm","package":"@nuxtjs/mdc","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.22.1"}]}]},{"source":"osv","ecosystem":"npm","package":"@nuxtjs/mdc","ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"0.22.1"}]}]}],"products":[{"vendor":"nuxt-content","product":"mdc"},{"vendor":"nuxt-modules","product":"mdc"}],"references":[{"url":"https://github.com/nuxt-content/mdc/security/advisories/GHSA-mxm6-v9r6-r94c","tags":["x_refsource_CONFIRM","x_refsource_MISC","exploit","advisory","web"]},{"url":"https://github.com/nuxt-content/mdc/pull/491","tags":["x_refsource_MISC","fix","web"]},{"url":"https://github.com/nuxt-content/mdc/commit/61d636c2983f021288e4fc5c4006733b38cf0d53","tags":["x_refsource_MISC","fix","web"]},{"url":"https://github.com/nuxt-content/mdc/releases/tag/v0.22.1","tags":["x_refsource_MISC","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2026-63671","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-mxm6-v9r6-r94c"},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63671.json","tags":["advisory"]},{"url":"https://github.com/nuxt-content/mdc","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2026-63671","title":"@nuxtjs/mdc: the URL sanitizer misses SVG xlink:href and data:text/html, allowing XSS from untrusted markdown at the default configuration","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.","status":"published","published":"2026-09-16T14:25:47.539Z","modified":"2026-09-18T17:54:37.852Z","cvss":[{"version":"3.1","score":8.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N","level":"high","assessor":"cna"}],"cwes":["CWE-79","CWE-184"],"updated_at":"2026-11-02T14:19:55.914391Z"}],"euvd":[{"id":"EUVD-2026-80789","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.","status":"published","published":"2026-09-16T14:25:47Z","modified":"2026-09-18T17:54:37Z","cvss":[{"version":"3.1","score":8.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N","level":"high"}],"updated_at":"2026-11-02T17:28:07.943492Z"}],"ghsa":[{"id":"GHSA-mxm6-v9r6-r94c","title":"@nuxtjs/mdc's URL sanitizer misses SVG xlink:href and data:text/html, allowing XSS from untrusted markdown at the default configuration","description":"## Summary\n\n`@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.\n\nTwo sibling vectors bypass that sanitizer at the default configuration:\n\n1. SVG anchor `xlink:href`. `validateProp` only scheme-checks attributes named exactly `href` or `src`:\n\n   ```\n   if (attribute === \"href\" || attribute === \"src\") return isAnchorLinkAllowed(value);\n   return true;\n   ```\n\n   An `xlink:href` (parsed to the hast property `xLinkHref`) is neither, so a `javascript:` URL on an SVG `\u003ca\u003e` 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 `\u003ca xlink:href=\"javascript:...\"\u003e`. Clicking it runs the script in the page origin. Plain `\u003ca href=\"javascript:...\"\u003e` is correctly stripped, which is what makes this the un-patched sibling.\n\n2. `\u003ciframe src=\"data:text/html,...\"\u003e`. `data:text/html` is present in `unsafeLinkPrefix`, but the check compares it against `url.protocol`:\n\n   ```\n   if (unsafeLinkPrefix.some((prefix) =\u003e url.protocol.toLowerCase().startsWith(prefix))) return false;\n   ```\n\n   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 `\u003ciframe src=\"data:text/html,\u003cscript\u003e...\u003c/script\u003e\"\u003e` 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.\n\n## Reproduction\n\nI 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.\n[nuxtjs-mdc-xss_poc.zip](https://github.com/user-attachments/files/29175603/nuxtjs-mdc-xss_poc.zip)\n\nTwo zero-argument checks:\n\n1. `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.\n2. `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.\n\nBoth 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.\n\n## Suggested fix\n\nIn `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.","status":"published","published":"2026-09-16T22:14:01Z","modified":"2026-09-16T22:14:03Z","cvss":[{"version":"3.1","score":8.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N","level":"high"}],"cwes":["CWE-79","CWE-184"],"updated_at":"2026-11-02T14:41:17.136201Z"}],"nvd":[{"id":"CVE-2026-63671","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.","status":"published","published":"2026-09-16T15:17:40.1Z","modified":"2026-09-24T21:25:27.05Z","cvss":[{"version":"3.1","score":8.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N","level":"high","assessor":"secondary"}],"cwes":["CWE-79","CWE-184"],"updated_at":"2026-11-02T14:12:48.630594Z"}],"osv":[{"id":"CVE-2026-63671","title":"@nuxtjs/mdc: the URL sanitizer misses SVG xlink:href and data:text/html, allowing XSS from untrusted markdown at the default configuration","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.","status":"published","published":"2026-09-16T14:25:47.539Z","modified":"2026-10-28T02:30:40.9261433Z","cvss":[{"version":"3.1","score":8.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N","level":"high"}],"cwes":["CWE-184","CWE-79"],"updated_at":"2026-11-02T15:45:13.306117Z"},{"id":"GHSA-mxm6-v9r6-r94c","title":"@nuxtjs/mdc's URL sanitizer misses SVG xlink:href and data:text/html, allowing XSS from untrusted markdown at the default configuration","description":"## Summary\n\n`@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.\n\nTwo sibling vectors bypass that sanitizer at the default configuration:\n\n1. SVG anchor `xlink:href`. `validateProp` only scheme-checks attributes named exactly `href` or `src`:\n\n   ```\n   if (attribute === \"href\" || attribute === \"src\") return isAnchorLinkAllowed(value);\n   return true;\n   ```\n\n   An `xlink:href` (parsed to the hast property `xLinkHref`) is neither, so a `javascript:` URL on an SVG `\u003ca\u003e` 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 `\u003ca xlink:href=\"javascript:...\"\u003e`. Clicking it runs the script in the page origin. Plain `\u003ca href=\"javascript:...\"\u003e` is correctly stripped, which is what makes this the un-patched sibling.\n\n2. `\u003ciframe src=\"data:text/html,...\"\u003e`. `data:text/html` is present in `unsafeLinkPrefix`, but the check compares it against `url.protocol`:\n\n   ```\n   if (unsafeLinkPrefix.some((prefix) =\u003e url.protocol.toLowerCase().startsWith(prefix))) return false;\n   ```\n\n   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 `\u003ciframe src=\"data:text/html,\u003cscript\u003e...\u003c/script\u003e\"\u003e` 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.\n\n## Reproduction\n\nI 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.\n[nuxtjs-mdc-xss_poc.zip](https://github.com/user-attachments/files/29175603/nuxtjs-mdc-xss_poc.zip)\n\nTwo zero-argument checks:\n\n1. `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.\n2. `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.\n\nBoth 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.\n\n## Suggested fix\n\nIn `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.","status":"published","published":"2026-09-16T22:14:01Z","modified":"2026-09-16T22:30:08.107441971Z","cvss":[{"version":"3.1","score":8.1,"vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N","level":"high"}],"cwes":["CWE-184","CWE-79"],"updated_at":"2026-11-02T15:46:52.534897Z"}]}}