CVE-2026-104634 beam_mcp: JSON boolean and null tool arguments reach dispatch as strings
Incorrect type conversion allows boolean arguments to be treated as strings, potentially bypassing security checks in MCP server implementations.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-104634?
CVE-2026-104634 is a low-severity vulnerability in ScriptKittyOS beam_mcp, published on 8 Oct 2026. Any signed-in user can exploit it over the network, with no action from the victim.
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.
Weakness: CWE-704
Which versions are affected by CVE-2026-104634?
1 open source package ship an affected version, including beam_mcp.
| Package | Affected | Fixed in |
|---|---|---|
beam_mcp Hex | From 0.1.0 | 0.10.1 |
How do I fix CVE-2026-104634?
Upgrade beam_mcp to 0.10.1 or later.
Is CVE-2026-104634 being exploited?
There is no known exploitation of CVE-2026-104634 in the wild and no public exploit code.
Sources
Also known as EEF-CVE-2026-104634, GHSA-rcm8-2hxq-3q25, GHSA-wv7p-j6qh-3hj4.