beam_mcp: nested tool argument constraints advertised but not enforced

{
  "id": "a613b687a9f614da2fb9ed74e8c54b089a60e3bee60788b7fe9e9edb286c5142",
  "url": "https://osv.dev/vulnerability/EEF-CVE-2026-88257",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "beam_mcp: nested tool argument constraints advertised but not enforced",
    "summary": "## Summary Improper Input Validation vulnerability in `BeamMCP.Schema` in ScriptKittyOS beam_mcp allows an MCP client to reach a tool's dispatch function with arguments that violate the input schema the server advertised. `BeamMCP.Schema.validate/2` checked `type`, `required`, `additionalProperties`, `enum` and numeric bounds on the top-level arguments object only. Constraints inside nested objects and on array items (`items`, `minItems`, `maxItems`, `minLength`, `maxLength`, `pattern`, nested `required`, `enum` and `additionalProperties: false`) were advertised by `tools/list` and never checked at `tools/call` or `prompts/get`, and keywords outside the enforced subset (`oneOf`, `anyOf`, `$ref`) were advertised and ignored. A host whose dispatch code relies on the schema it declared receives values the schema forbids, such as an out-of-range number or an undeclared key inside a nested object. What the host does with such a value decides the impact. This issue affects beam_mcp: from 0.1.0 before 0.10.1. ## Details **1. Advertising.** `tools/list` returns each tool's `input_schema` verbatim, including nested `properties`, `items` and constraint keywords. **2. Validation.** `BeamMCP.Schema.validate/2` in `lib/beam_mcp/schema.ex` walks only the first level: `check_required/2` and `check_additional/3` run on the top-level object, and `check_properties/2` applies `check_type`, `check_enum` and `check_range` to each top-level value. A value of type `object` or `array` is accepted on",
    "evidence": "osv:EEF-CVE-2026-88257",
    "products": [
      {
        "name": {
          "value": "beam_mcp",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "0.10.1",
            "evidence": "/affected/0/ranges"
          }
        ],
        "affected_versions": [
          {
            "value": "0.1.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.10.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.2.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.3.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.3.1",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.4.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.5.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.6.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.7.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.8.0",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "0.9.0",
            "evidence": "/affected/0/versions"
          }
        ]
      }
    ],
    "severity": "medium",
    "structured": {
      "id": "EEF-CVE-2026-88257",
      "aliases": [
        "CVE-2026-88257",
        "EEF-CVE-2026-88257",
        "GHSA-mrg2-4747-fmpw"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "SEMVER",
              "events": [
                {
                  "introduced": "0.1.0"
                },
                {
                  "fixed": "0.10.1"
                }
              ]
            }
          ],
          "package": {
            "name": "beam_mcp",
            "purl": "pkg:hex/beam_mcp",
            "ecosystem": "Hex"
          },
          "versions": [
            "0.1.0",
            "0.10.0",
            "0.2.0",
            "0.3.0",
            "0.3.1",
            "0.4.0",
            "0.5.0",
            "0.6.0",
            "0.7.0",
            "0.8.0",
            "0.9.0"
          ],
          "database_specific": {
            "source": "https://cna.erlef.org/osv/EEF-CVE-2026-88257.json"
          }
        },
        {
          "ranges": [
            {
              "repo": "https://github.com/ScriptKittyOS/beam_mcp",
              "type": "GIT",
              "events": [
                {
                  "introduced": "083838eb8e17fe5f6fcaf761bdbe203110288b0b"
                },
                {
                  "fixed": "289dbdbad641943b29a3b8d1eb36506cc8cec10a"
                }
              ]
            }
          ],
          "versions": [
            "v0.10.0",
            "v0.9.0",
            "v0.8.0",
            "v0.7.0",
            "v0.6.0",
            "v0.5.0",
            "v0.4.0",
            "v0.3.1",
            "v0.3.0",
            "v0.2.0",
            "v0.1.0"
          ],
          "database_specific": {
            "source": "https://cna.erlef.org/osv/EEF-CVE-2026-88257.json"
          }
        }
      ],
      "modified": "2026-10-08T14:15:02.759237242Z",
      "provider": "osv",
      "references": [
        "https://github.com/ScriptKittyOS/beam_mcp/security/advisories/GHSA-mrg2-4747-fmpw",
        "https://cna.erlef.org/cves/CVE-2026-88257.html",
        "https://github.com/ScriptKittyOS/beam_mcp/commit/083838eb8e17fe5f6fcaf761bdbe203110288b0b",
        "https://github.com/ScriptKittyOS/beam_mcp/commit/289dbdbad641943b29a3b8d1eb36506cc8cec10a",
        "https://hex.pm/packages/beam_mcp"
      ],
      "provider_metadata": {
        "cpe_ids": [
          "cpe:2.3:a:scriptkittyos:beam_mcp:*:*:*:*:*:*:*:*"
        ],
        "cwe_ids": [
          "CWE-20"
        ],
        "capec_ids": [
          "CAPEC-153"
        ]
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-88257",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 99687,
  "published_at": "2026-10-08T13:40:55.828+00:00"
}