Vulnerability-Lookup

CVE-2026-44429 (GCVE-0-2026-44429)

Vulnerability from cvelistv5 – Published: 2026-05-14 21:05 – Updated: 2026-05-15 14:20

Title

MCP Registry: Stored XSS in catalogue UI via attribute-quote breakout in publisher-controlled `websiteUrl`

Summary

The MCP Registry provides MCP clients with a list of MCP servers, like an app store for MCP servers. Prior to 1.7.7, the public catalogue UI served at GET / (file internal/api/handlers/v0/ui_index.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. Any user able to obtain a publish token (e.g. via POST /v0/auth/github-at with their own GitHub account, or POST /v0/auth/none on a deployment that has anonymous auth enabled) can plant a poisoned record visible to every visitor of the registry homepage. This vulnerability is fixed in 1.7.7.

Severity

5.1 (Medium)


                        
                          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

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-05-15 14:19 UTC

CWE

CWE-79 - Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
CWE-116 - Improper Encoding or Escaping of Output

Assigner

GitHub_M mitre root

References

1 reference

URL	Tags
https://github.com/modelcontextprotocol/registry/…	x_refsource_CONFIRM

Impacted products

1 product

Vendor	Product	Version	CPE status
modelcontextprotocol	registry	Affected: < 1.7.7	guessed

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2026-44429

Vulnerability from fkie_nvd - Published: 2026-05-14 21:16 - Updated: 2026-06-17 10:50

Severity

5.4 (Medium) - CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/modelcontextprotocol/registry/security/advisories/GHSA-rqv2-m695-f8j4	Exploit, Mitigation, Vendor Advisory
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/modelcontextprotocol/registry/security/advisories/GHSA-rqv2-m695-f8j4	Exploit, Mitigation, Vendor Advisory

Impacted products

	Vendor	Product	Version
	lfprojects	mcp_registry	*

JSON

To clipboard

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          "userInteraction": "PASSIVE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
          "version": "4.0",
          "vulnAvailabilityImpact": "NONE",
          "vulnConfidentialityImpact": "NONE",
          "vulnIntegrityImpact": "NONE",
          "vulnerabilityResponseEffort": "NOT_DEFINED"
        },
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-44429",
          "options": [
            {
              "exploitation": "poc"
            },
            {
              "automatable": "no"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-05-15T14:19:51.887565Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-05-14T21:16:46.677",
  "references": [
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Exploit",
        "Mitigation",
        "Vendor Advisory"
      ],
      "url": "https://github.com/modelcontextprotocol/registry/security/advisories/GHSA-rqv2-m695-f8j4"
    },
    {
      "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
      "tags": [
        "Exploit",
        "Mitigation",
        "Vendor Advisory"
      ],
      "url": "https://github.com/modelcontextprotocol/registry/security/advisories/GHSA-rqv2-m695-f8j4"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Analyzed",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-79"
        },
        {
          "lang": "en",
          "value": "CWE-116"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    }
  ]
}

GHSA-RQV2-M695-F8J4

Vulnerability from github – Published: 2026-05-08 17:18 – Updated: 2026-05-15 23:46

Summary

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

Details

Summary

The public catalogue UI served at GET / (file internal/api/handlers/v0/ui_index.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.

Any user able to obtain a publish token (e.g. via POST /v0/auth/github-at with their own GitHub account, or POST /v0/auth/none on a deployment that has anonymous auth enabled) can plant a poisoned record visible to every visitor of the registry homepage.

Affected component

Validator: internal/validators/validators.go — validateWebsiteURL (lines 153–199)
Sink: internal/api/handlers/v0/ui_index.html — toggleDetails(card, item) at line 432, the href attribute built around escapeHtml(server.websiteUrl)
Helper: escapeHtml defined at internal/api/handlers/v0/ui_index.html lines 494–498

Proof of concept

Obtain a Registry JWT for any namespace you control (a GitHub OAuth exchange against a throwaway account suffices):

bash TOKEN=$(curl -sS -X POST https://registry.modelcontextprotocol.io/v0/auth/github-at \ -H 'Content-Type: application/json' \ -d '{"github_token":"<gh-pat>"}' | jq -r .registry_token)

Publish a server with a poisoned websiteUrl. The literal " is preserved end-to-end:

bash curl -sS -X POST https://registry.modelcontextprotocol.io/v0/publish \ -H "Authorization: Bearer $TOKEN" \ -H 'Content-Type: application/json' \ --data-binary @- <<'EOF' { "$schema": "https://static.modelcontextprotocol.io/schemas/2025-09-29/server.schema.json", "name": "io.github.<your-account>/xss-poc", "version": "0.0.1", "description": "hover the website link", "websiteUrl": "https://example.com/\"onmouseover=alert(document.domain)//" } EOF

Visit https://registry.modelcontextprotocol.io/, search for xss-poc, click the card to expand it, then hover the Website link in the details panel. The injected onmouseover fires and alert(document.domain) runs on the registry.modelcontextprotocol.io origin.

Why server-side validation does not catch this

Go's net/url.Parse accepts literal " in the path component:

input="https://example.com/\"onmouseover=alert(1)//"  IsAbs=true  Scheme="https"  Path="/\"onmouseover=alert(1)//"

Neither the Huma format:"uri" annotation nor validateWebsiteURL's scheme/IsAbs triplet rejects this string. The architecture's existing protection — repository.url is regex-locked to ^https?://(www\.)?github\.com/[\w.-]+/[\w.-]+/?$ and therefore cannot contain quotes — does not extend to websiteUrl, which has no allowlist.

Why client-side `escapeHtml` does not catch this

function escapeHtml(text) {
    const div = document.createElement('div');
    div.textContent = text;
    return div.innerHTML;
}

Per the HTML5 spec (§13.3 Serialising HTML fragments), the only characters encoded inside the text content of an element are &, <, >, and U+00A0. " and ' are not encoded because in a text-content context they are not special. The helper is therefore safe in element-text contexts (where it is correctly used for name, version, description, etc.) but unsafe inside an attribute value, which is precisely where it is invoked for href on lines 432 and 426.

Impact

Stored XSS on the official MCP Registry homepage. The malicious entry sits in the public catalogue alongside legitimate ones; any user expanding the entry triggers the payload.
Because the page is served on the official registry.modelcontextprotocol.io origin, the injected script can:
Read and overwrite localStorage (baseUrl, customUrl), pinning the user's subsequent reads to an attacker-controlled "Custom" base URL.
Issue any same-origin or cross-origin XHR (connect-src * is granted).
Phish for Registry JWTs by injecting fake auth flows on the trusted origin.
The CSP script-src 'self' 'unsafe-inline' https://cdn.tailwindcss.com does not block this because 'unsafe-inline' permits inline event-handler attributes.

Suggested remediation (any one suffices)

Replace the homegrown escapeHtml with an attribute-safe encoder that also escapes ", ', backtick, and = — the OWASP HTML attribute-encoding rule.
Avoid building the href via string templates. Use setAttribute('href', value) instead — setAttribute is not subject to HTML tokenisation, so no breakout is possible.
Tighten validateWebsiteURL to reject any URL whose raw bytes contain ", ', <, >, , \t, or \n, or — conservatively — store the canonical re-serialised form (parsedURL.String() percent-encodes such characters in the path).
Drop 'unsafe-inline' from script-src after auditing the inline scripts on the page.

Approach (3) is the smallest server-side change and immediately neutralises the exploit for any new publishes; approaches (1) or (2) close the class of bug at the sink so future fields with similar patterns are safe by default.

Severity

5.4 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N

5.1 (Medium)


                  
                    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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "Go",
        "name": "github.com/modelcontextprotocol/registry"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.7.7"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-44429"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-116",
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-08T17:18:32Z",
    "nvd_published_at": "2026-05-14T21:16:46Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nThe public catalogue UI served at `GET /` (file `internal/api/handlers/v0/ui_index.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` \u2192 `innerHTML` round-trip. Per the HTML serialisation algorithm, that round-trip encodes only `\u0026`, `\u003c`, `\u003e` and U+00A0 inside text nodes \u2014 it does **not** encode `\"` or `\u0027`. A literal `\"` in `websiteUrl` therefore breaks out of the `href` attribute, allowing arbitrary `on*` event handlers to be appended to the same `\u003ca\u003e` element. The Content-Security-Policy on `/` is `script-src \u0027self\u0027 \u0027unsafe-inline\u0027 https://cdn.tailwindcss.com`, so the injected event handlers execute.\n\nAny user able to obtain a publish token (e.g. via `POST /v0/auth/github-at` with their own GitHub account, or `POST /v0/auth/none` on a deployment that has anonymous auth enabled) can plant a poisoned record visible to every visitor of the registry homepage.\n\n## Affected component\n\n- Validator: `internal/validators/validators.go` \u2014 `validateWebsiteURL` (lines 153\u2013199)\n- Sink: `internal/api/handlers/v0/ui_index.html` \u2014 `toggleDetails(card, item)` at line 432, the `href` attribute built around `escapeHtml(server.websiteUrl)`\n- Helper: `escapeHtml` defined at `internal/api/handlers/v0/ui_index.html` lines 494\u2013498\n\n## Proof of concept\n\n1. Obtain a Registry JWT for any namespace you control (a GitHub OAuth exchange against a throwaway account suffices):\n\n   ```bash\n   TOKEN=$(curl -sS -X POST https://registry.modelcontextprotocol.io/v0/auth/github-at \\\n        -H \u0027Content-Type: application/json\u0027 \\\n        -d \u0027{\"github_token\":\"\u003cgh-pat\u003e\"}\u0027 | jq -r .registry_token)\n   ```\n\n2. Publish a server with a poisoned `websiteUrl`. The literal `\"` is preserved end-to-end:\n\n   ```bash\n   curl -sS -X POST https://registry.modelcontextprotocol.io/v0/publish \\\n     -H \"Authorization: Bearer $TOKEN\" \\\n     -H \u0027Content-Type: application/json\u0027 \\\n     --data-binary @- \u003c\u003c\u0027EOF\u0027\n   {\n     \"$schema\": \"https://static.modelcontextprotocol.io/schemas/2025-09-29/server.schema.json\",\n     \"name\":  \"io.github.\u003cyour-account\u003e/xss-poc\",\n     \"version\": \"0.0.1\",\n     \"description\": \"hover the website link\",\n     \"websiteUrl\": \"https://example.com/\\\"onmouseover=alert(document.domain)//\"\n   }\n   EOF\n   ```\n\n3. Visit `https://registry.modelcontextprotocol.io/`, search for `xss-poc`, click the card to expand it, then hover the **Website** link in the details panel. The injected `onmouseover` fires and `alert(document.domain)` runs on the `registry.modelcontextprotocol.io` origin.\n\n## Why server-side validation does not catch this\n\nGo\u0027s `net/url.Parse` accepts literal `\"` in the path component:\n\n```\ninput=\"https://example.com/\\\"onmouseover=alert(1)//\"  IsAbs=true  Scheme=\"https\"  Path=\"/\\\"onmouseover=alert(1)//\"\n```\n\nNeither the Huma `format:\"uri\"` annotation nor `validateWebsiteURL`\u0027s scheme/`IsAbs` triplet rejects this string. The architecture\u0027s existing protection \u2014 `repository.url` is regex-locked to `^https?://(www\\.)?github\\.com/[\\w.-]+/[\\w.-]+/?$` and therefore cannot contain quotes \u2014 does not extend to `websiteUrl`, which has no allowlist.\n\n## Why client-side `escapeHtml` does not catch this\n\n```js\nfunction escapeHtml(text) {\n    const div = document.createElement(\u0027div\u0027);\n    div.textContent = text;\n    return div.innerHTML;\n}\n```\n\nPer the HTML5 spec (\u00a713.3 Serialising HTML fragments), the only characters encoded inside the text content of an element are `\u0026`, `\u003c`, `\u003e`, and U+00A0. `\"` and `\u0027` are **not** encoded because in a text-content context they are not special. The helper is therefore safe in element-text contexts (where it is correctly used for `name`, `version`, `description`, etc.) but unsafe inside an attribute value, which is precisely where it is invoked for `href` on lines 432 and 426.\n\n## Impact\n\n- Stored XSS on the official MCP Registry homepage. The malicious entry sits in the public catalogue alongside legitimate ones; any user expanding the entry triggers the payload.\n- Because the page is served on the official `registry.modelcontextprotocol.io` origin, the injected script can:\n  - Read and overwrite `localStorage` (`baseUrl`, `customUrl`), pinning the user\u0027s subsequent reads to an attacker-controlled \"Custom\" base URL.\n  - Issue any same-origin or cross-origin XHR (`connect-src *` is granted).\n  - Phish for Registry JWTs by injecting fake auth flows on the trusted origin.\n- The CSP `script-src \u0027self\u0027 \u0027unsafe-inline\u0027 https://cdn.tailwindcss.com` does not block this because `\u0027unsafe-inline\u0027` permits inline event-handler attributes.\n\n## Suggested remediation (any one suffices)\n\n1. Replace the homegrown `escapeHtml` with an attribute-safe encoder that also escapes `\"`, `\u0027`, backtick, and `=` \u2014 the OWASP HTML attribute-encoding rule.\n2. Avoid building the `href` via string templates. Use `setAttribute(\u0027href\u0027, value)` instead \u2014 `setAttribute` is not subject to HTML tokenisation, so no breakout is possible.\n3. Tighten `validateWebsiteURL` to reject any URL whose raw bytes contain `\"`, `\u0027`, `\u003c`, `\u003e`, ` `, `\\t`, or `\\n`, or \u2014 conservatively \u2014 store the canonical re-serialised form (`parsedURL.String()` percent-encodes such characters in the path).\n4. Drop `\u0027unsafe-inline\u0027` from `script-src` after auditing the inline scripts on the page.\n\nApproach (3) is the smallest server-side change and immediately neutralises the exploit for any new publishes; approaches (1) or (2) close the class of bug at the sink so future fields with similar patterns are safe by default.",
  "id": "GHSA-rqv2-m695-f8j4",
  "modified": "2026-05-15T23:46:13Z",
  "published": "2026-05-08T17:18:32Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/registry/security/advisories/GHSA-rqv2-m695-f8j4"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-44429"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/registry/pull/1249"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/registry/commit/78b7bbde07948049b916d76b4769faee461ff930"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/modelcontextprotocol/registry"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/registry/releases/tag/v1.7.7"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "MCP Registry vulnerable to stored XSS in catalogue UI via attribute-quote breakout in publisher-controlled `websiteUrl`"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

CVE-2026-44429 (GCVE-0-2026-44429)

FKIE_CVE-2026-44429

GHSA-RQV2-M695-F8J4

Summary

Affected component

Proof of concept

Why server-side validation does not catch this

Why client-side escapeHtml does not catch this

Impact

Suggested remediation (any one suffices)

Tags

Sightings

Nomenclature

Related by attack behaviour

Why client-side `escapeHtml` does not catch this