CVE-2026-104634beam_mcp: JSON boolean and null tool arguments reach dispatch as strings

Official description

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.

Affected products (as reported)

References

• GitHub Advisory related, vendor-advisory
• EEF CNA record for CVE-2026-104634 related
• OSV record EEF-CVE-2026-104634 related
• Introducing commit 083838e in ScriptKittyOS/beam_mcp related
• Fix commit 289dbdb in ScriptKittyOS/beam_mcp patch