CVE-2026-88257 beam_mcp: nested tool argument constraints advertised but not enforced
An attacker can bypass input validation in beam_mcp by sending nested arguments that violate advertised schemas, potentially causing unexpected behavior in host applications.
- 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-2026-88257?
CVE-2026-88257 is a medium-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
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.
Weakness: CWE-20
Which versions are affected by CVE-2026-88257?
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-2026-88257?
Upgrade beam_mcp to 0.10.1 or later.
Is CVE-2026-88257 being exploited?
There is no known exploitation of CVE-2026-88257 in the wild and no public exploit code.
Sources
Also known as EEF-CVE-2026-88257, GHSA-mrg2-4747-fmpw, GHSA-pgrr-g28j-jvp6.