CVE-2026-88257 (GCVE-0-2026-88257)

Vulnerability from cvelistv5 – Published: 2026-10-08 13:40 – Updated: 2026-10-08 14:09
VLAI
Title
beam_mcp: nested tool argument constraints advertised but not enforced
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.
Severity
5.3 (Medium)
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
SSVC
Exploitation: none Automatable: no Technical Impact: partial
CISA Coordinator · CISA-ADP (v2.0.3)
Decision recorded 2026-10-08 13:59 UTC
CWE
  • CWE-20 - Improper Input Validation
Assigner
EEF CNA under the mitre root
CNA scorecard A 97/100 over 357 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
ScriptKittyOS beam_mcp Affected: 0.1.0 , < 0.10.1 (semver)
    cpe:2.3:a:scriptkittyos:beam_mcp:*:*:*:*:*:*:*:*
Create a notification for this product.
ScriptKittyOS beam_mcp Affected: 083838eb8e17fe5f6fcaf761bdbe203110288b0b , < 289dbdbad641943b29a3b8d1eb36506cc8cec10a (git)
    cpe:2.3:a:scriptkittyos:beam_mcp:*:*:*:*:*:*:*:*
Create a notification for this product.
Date Public
2026-09-26 00:00
Credits
Ayla Croft / Script Kitty OS Ayla Croft / Script Kitty OS Ayla Croft / Script Kitty OS Jonatan Männchen / EEF
Show details on NVD website
To clipboard 

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              "value": "**1. Advertising.** `tools/list` returns each tool\u0027s `input_schema` verbatim, including nested `properties`, `items` and constraint keywords.\n\n**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 once its own type matches, and its contents are not visited. Keywords the validator does not know are ignored.\n\n**3. Dispatch.** `BeamMCP.Server.validate_and_dispatch/3` passes the accepted arguments to `normalize_arguments/2` and then to the host\u0027s dispatch function, so an undeclared key inside a nested object, an out-of-range nested number, or an array item of the wrong type reaches host code that believed the schema excluded it. The same validator runs on `prompts/get` arguments. The fix in 0.10.1 enforces every keyword of the documented subset at every depth and refuses a schema that uses any other keyword when the catalog is loaded."
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                  "value": "1. Declare a tool whose `input_schema` has a nested object property `opts` with `\"properties\": {\"level\": {\"type\": \"integer\", \"maximum\": 3}}`, `\"required\": [\"level\"]` and `\"additionalProperties\": false`, and an array property `tags` with `\"items\": {\"type\": \"string\"}` and `\"maxItems\": 2`.\n2. Send `tools/call` with `\"arguments\": {\"opts\": {\"level\": 99, \"extra\": \"x\"}}`.\n3. On beam_mcp 0.10.0 the dispatch function receives `%{opts: %{\"extra\" =\u003e \"x\", \"level\" =\u003e 99}}`. On 0.10.1 the call is refused with `invalid arguments: unknown property: opts.extra`.\n4. Send `\"arguments\": {\"tags\": [1, 2, 3]}`. On 0.10.0 the dispatch function receives `tags: [1, 2, 3]`. On 0.10.1 the call is refused with `tags must have at most 2 items`."
                }
              ],
              "value": "* Declare a tool whose input_schema has a nested object property opts with \"properties\": {\"level\": {\"type\": \"integer\", \"maximum\": 3}}, \"required\": [\"level\"] and \"additionalProperties\": false, and an array property tags with \"items\": {\"type\": \"string\"} and \"maxItems\": 2.\n* Send tools/call with \"arguments\": {\"opts\": {\"level\": 99, \"extra\": \"x\"}}.\n* On beam_mcp 0.10.0 the dispatch function receives %{opts: %{\"extra\" =\u003e \"x\", \"level\" =\u003e 99}}. On 0.10.1 the call is refused with invalid arguments: unknown property: opts.extra.\n* Send \"arguments\": {\"tags\": [1, 2, 3]}. On 0.10.0 the dispatch function receives tags: [1, 2, 3]. On 0.10.1 the call is refused with tags must have at most 2 items."
            }
          ],
          "x_technicalAnalysis": [
            {
              "lang": "en",
              "supportingMedia": [
                {
                  "base64": false,
                  "type": "text/html",
                  "value": "\u003cp\u003e\u003cstrong\u003e1. Advertising.\u003c/strong\u003e \u003ccode\u003etools/list\u003c/code\u003e returns each tool\u0027s \u003ccode\u003einput_schema\u003c/code\u003e verbatim, including nested \u003ccode\u003eproperties\u003c/code\u003e, \u003ccode\u003eitems\u003c/code\u003e and constraint keywords.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2. Validation.\u003c/strong\u003e \u003ccode\u003eBeamMCP.Schema.validate/2\u003c/code\u003e in \u003ccode\u003elib/beam_mcp/schema.ex\u003c/code\u003e walks only the first level: \u003ccode\u003echeck_required/2\u003c/code\u003e and \u003ccode\u003echeck_additional/3\u003c/code\u003e run on the top-level object, and \u003ccode\u003echeck_properties/2\u003c/code\u003e applies \u003ccode\u003echeck_type\u003c/code\u003e, \u003ccode\u003echeck_enum\u003c/code\u003e and \u003ccode\u003echeck_range\u003c/code\u003e to each top-level value. A value of type \u003ccode\u003eobject\u003c/code\u003e or \u003ccode\u003earray\u003c/code\u003e is accepted once its own type matches, and its contents are not visited. Keywords the validator does not know are ignored.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3. Dispatch.\u003c/strong\u003e \u003ccode\u003eBeamMCP.Server.validate_and_dispatch/3\u003c/code\u003e passes the accepted arguments to \u003ccode\u003enormalize_arguments/2\u003c/code\u003e and then to the host\u0027s dispatch function, so an undeclared key inside a nested object, an out-of-range nested number, or an array item of the wrong type reaches host code that believed the schema excluded it. The same validator runs on \u003ccode\u003eprompts/get\u003c/code\u003e arguments. The fix in 0.10.1 enforces every keyword of the documented subset at every depth and refuses a schema that uses any other keyword when the catalog is loaded.\u003c/p\u003e"
                },
                {
                  "base64": false,
                  "type": "text/markdown",
                  "value": "**1. Advertising.** `tools/list` returns each tool\u0027s `input_schema` verbatim, including nested `properties`, `items` and constraint keywords.\n\n**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 once its own type matches, and its contents are not visited. Keywords the validator does not know are ignored.\n\n**3. Dispatch.** `BeamMCP.Server.validate_and_dispatch/3` passes the accepted arguments to `normalize_arguments/2` and then to the host\u0027s dispatch function, so an undeclared key inside a nested object, an out-of-range nested number, or an array item of the wrong type reaches host code that believed the schema excluded it. The same validator runs on `prompts/get` arguments. The fix in 0.10.1 enforces every keyword of the documented subset at every depth and refuses a schema that uses any other keyword when the catalog is loaded."
                }
              ],
              "value": "1. Advertising. tools/list returns each tool\u0027s input_schema verbatim, including nested properties, items and constraint keywords.\n\n2. 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 once its own type matches, and its contents are not visited. Keywords the validator does not know are ignored.\n\n3. Dispatch. BeamMCP.Server.validate_and_dispatch/3 passes the accepted arguments to normalize_arguments/2 and then to the host\u0027s dispatch function, so an undeclared key inside a nested object, an out-of-range nested number, or an array item of the wrong type reaches host code that believed the schema excluded it. The same validator runs on prompts/get arguments. The fix in 0.10.1 enforces every keyword of the documented subset at every depth and refuses a schema that uses any other keyword when the catalog is loaded."
            }
          ]
        }
      },
      "cveMetadata": {
        "assignerOrgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
        "assignerShortName": "EEF",
        "cveId": "CVE-2026-88257",
        "datePublished": "2026-10-08T13:40:55.828Z",
        "dateReserved": "2026-10-07T22:15:01.878Z",
        "dateUpdated": "2026-10-08T14:09:41.540Z",
        "state": "PUBLISHED"
      },
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}



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Nomenclature

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