MCP Registry vulnerable to stored XSS in catalogue UI via attribute-quote breakout in publisher-controlled websiteUrl

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.1 · CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:N/SI:L/SA:L · score source

EPSS

• CVE-2026-44429: 0.17% estimated exploitation activity in the next 30 days; percentile 6.04 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary The public catalogue UI served at GET / (file internal/api/handlers/v0/uiindex.html) is vulnerable to stored cross-site scripting via the server.websiteUrl field of any published server.json. Server-side validation in internal/validators/validators.go (validateWebsiteURL) only checks that the URL parses, is absolute, and uses the https scheme; it does not reject quote characters. Client-side, the value is interpolated into a double-quoted href attribute via innerHTML, using a homegrown escapeHtml helper that performs the standard textContent → innerHTML round-trip. Per the HTML serialisation algorithm, that round-trip encodes only &, <, and U+00A0 inside text nodes — it does not encode " or '. A literal " in websiteUrl therefore breaks out of the href attribute, allowing arbitrary on event handlers to be appended to the same <a element. The Content-Security-Policy on / is script-src 'self' 'unsafe-inline' https://cdn.tailwindcss.com, so the injected event handlers execute.…

Published by the source

8 May 2026, 17:18 UTC

Last modified by the source

15 May 2026, 23:46 UTC

Source last checked

26 Oct 2026, 15:22 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-rqv2-m695-f8j4

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# Patch Next advisory brief: CVE-2026-44429

Advisory: MCP Registry vulnerable to stored XSS in catalogue UI via attribute-quote breakout in publisher-controlled `websiteUrl`
IDs: CVE-2026-44429, GHSA-rqv2-m695-f8j4
Record: https://patchnext.com/advisories/cve-2026-44429 (revision 1, source checked 26 Oct 2026, 15:22 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/modelcontextprotocol/registry 1.7.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/modelcontextprotocol/registry: affected < 1.7.7; fixed in 1.7.7

## Suggested inspection (read-only)
- Go github.com/modelcontextprotocol/registry: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/modelcontextprotocol/registry.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-rqv2-m695-f8j4 (GitHub-reviewed advisory GHSA-rqv2-m695-f8j4 lists affected ranges: Go github.com/modelcontextprotocol/registry >=0 <1.7.7)

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