beam_mcp: JSON boolean and null tool arguments reach dispatch as strings

{
  "id": "2a3423becdc93d7feab3612ca1cc976fa39b64722edbb18fa6d98d3c8fe0fc5a",
  "url": "https://osv.dev/vulnerability/EEF-CVE-2026-104634",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "beam_mcp: JSON boolean and null tool arguments reach dispatch as strings",
    "summary": "## Summary Incorrect Type Conversion or Cast vulnerability in `BeamMCP.Server` in ScriptKittyOS beam_mcp allows an MCP client's JSON `true`, `false` and `null` tool arguments to reach the host's dispatch function as the strings \"true\", \"false\" and \"nil\". After `BeamMCP.Schema.validate/2` accepted a value as a boolean, `normalize_arguments/2` passed every argument through `to_json_value/1`, whose atom clause converts `true`, `false` and `nil` to strings. A string is truthy in Elixir, so a host that tests a boolean argument, for example `if args.dry_run`, takes the opposite branch for `false`, and a guard such as `confirm: false` reads as set. The client controls the argument and could send `true` directly, so the practical impact is limited to hosts whose behaviour on `false` differs from their behaviour on `true`, and to any policy layer in front of the server that permits `false` but refuses `true`. The same normalisation applies to `prompts/get` arguments, which exist from 0.5.0. This issue affects beam_mcp: from 0.1.0 before 0.10.1. ## Details **1. Validation.** `BeamMCP.Schema.check_type/3` accepts a JSON `false` for a `boolean` property because the decoded value is the atom `false`. **2. Normalisation.** `BeamMCP.Server.normalize_arguments/2` in `lib/beam_mcp/server.ex` maps each declared key to an atom and passes each value through `to_json_value/1`. That function was written to encode host results for the wire, and its clause `when is_atom(value)` converts the value wi",
    "evidence": "osv:EEF-CVE-2026-104634",
    "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": "low",
    "structured": {
      "id": "EEF-CVE-2026-104634",
      "aliases": [
        "CVE-2026-104634",
        "EEF-CVE-2026-104634",
        "GHSA-wv7p-j6qh-3hj4"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:P/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-2026-104634.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-2026-104634.json"
          }
        }
      ],
      "modified": "2026-10-08T13:56:03.266254685Z",
      "provider": "osv",
      "references": [
        "https://github.com/ScriptKittyOS/beam_mcp/security/advisories/GHSA-wv7p-j6qh-3hj4",
        "https://cna.erlef.org/cves/CVE-2026-104634.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-704"
        ],
        "capec_ids": [
          "CAPEC-153"
        ]
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-104634",
        "evidence": "/aliases"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 99681,
  "published_at": "2026-10-08T13:41:05.745+00:00"
}