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

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.

Timeline

1. Oct 8, 2026, 1:41 PM UTCCVE published
2. Oct 8, 2026, 1:58 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: partial

3. Oct 8, 2026, 1:58 PM UTCRecord last updated

Affected versions

• beam_mcp

  From 0.1.0 · fixed in 0.10.1

• scriptkittyos/beam_mcp

  From 083838eb8e17fe5f6fcaf761bdbe203110288b0b · fixed in 289dbdbad641943b29a3b8d1eb36506cc8cec10a

• beam_mcpHex

  From 0.1.0 · fixed in 0.10.1

References 5