Vulnerability · Published Oct 8, 2026 · Assigned by EEF
CVE-2026-88257beam_mcp: nested tool argument constraints advertised but not enforced
Medium 5.3
In plain English
A medium-severity improper input validation in beam_mcp from ScriptKittyOS. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 19% of all known vulnerabilities).
Official description
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.
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| ScriptKittyOS | beam_mcp | 0.1.0 – before 0.10.1 |
| ScriptKittyOS | beam_mcp | 083838eb8e17fe5f6fcaf761bdbe203110288b0b – before 289dbdbad641943b29a3b8d1eb36506cc8cec10a |
Weakness: CWE-20 · Vector: 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
References
- GitHub Advisory related, vendor-advisory
- EEF CNA record for CVE-2026-88257 related
- OSV record EEF-CVE-2026-88257 related
- Introducing commit 083838e in ScriptKittyOS/beam_mcp related
- Fix commit 289dbdb in ScriptKittyOS/beam_mcp patch