CVE-2026-88257 in beam_mcp
Summary
by MITRE • 10/08/2026
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.
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Analysis
by VulDB Data Team • 10/08/2026
The vulnerability identified in ScriptKittyOS, specifically within the BeamMCP.Schema module of the beam_mcp library, represents a critical failure in input validation logic that undermines the integrity of Model Context Protocol interactions. This flaw allows an MCP client to bypass declared schema constraints and deliver arguments that violate the specifications advertised by the server during tool discovery. The core technical deficiency lies in the implementation of the BeamMCP.Schema.validate/2 function, which performs rigorous checks on top-level argument properties such as type, required fields, additionalProperties restrictions, enum values, and numeric bounds. However, this validation logic fails to extend recursively into nested objects or array items. Consequently, constraints defined for inner structures, including minItems, maxItems, minLength, maxLength, pattern matching, nested required fields, and strict additionalProperties settings, are effectively ignored during the execution phase of tools/call or prompts/get operations despite being advertised in the tool/list response.
This discrepancy between schema declaration and runtime enforcement creates a significant security gap where the host system receives data that it has explicitly stated it cannot process correctly according to its own rules. When dispatch code relies on these schemas for safety, such as assuming specific data types or structural integrity within nested payloads, receiving malformed input can lead to unpredictable behavior. The operational impact is directly dependent on how the underlying application logic handles this invalid data. In worst-case scenarios, processing out-of-range numbers or undeclared keys in nested objects could trigger unhandled exceptions, cause logical errors that bypass access controls, or potentially facilitate injection attacks if the raw values are passed into system commands or database queries without further sanitization. This aligns with CWE-20 Improper Input Validation and specifically relates to CWE-749 Exposed Dangerous Method because the vulnerability exposes internal validation mechanisms to external manipulation through a misconfigured interface contract.
From an attacker perspective, this flaw enables actions consistent with ATT&CK technique T1568 Dynamic Resolution, where adversaries exploit software vulnerabilities to execute code or achieve lateral movement by manipulating input parameters that should have been rejected. The ability to send arguments outside the advertised schema allows for fuzzing of internal logic paths and potentially bypasses security controls that depend on strict data typing. For instance, if a tool expects an integer within a specific range but receives a string due to missing nested validation, subsequent type coercion or comparison operations may behave unexpectedly, leading to privilege escalation or information disclosure depending on the application context. The vulnerability affects beam_mcp versions from 0.1.0 up to, but not including, version 0.10.1, indicating that earlier iterations lacked comprehensive recursive schema enforcement.
To mitigate this risk, developers must update the BeamMCP.Schema.validate/2 function to implement deep validation logic that recursively processes nested objects and array items according to their respective schemas. It is essential to ensure that all keywords advertised in tool/list responses are enforced during tools/call execution, including complex types like oneOf, anyOf, and $ref references which were previously ignored. Implementing a robust JSON Schema validator library capable of full recursive traversal will prevent malformed data from reaching the dispatch layer. Additionally, application-level defensive programming practices should be adopted to validate inputs at every processing stage rather than relying solely on schema declarations during initialization. Regular security audits focusing on input validation boundaries and integration testing with malicious payloads can help identify similar gaps in other parts of the system before they are exploited in production environments.