MCP Out-of-Scope Tool Action (Behavioural Scope Proof)

mcp-excessive-scope-proof is a high severity check in the ai category, written in python. Its source is templates/ai/mcp/mcp-excessive-scope-proof.py in cert-x-gen-templates.

What it checks

ACTIVE check. Calls an MCP tool that DECLARES itself read-only and proves, by observation, that it performed an action outside that declared scope - a write. Filesystem-witnessed against a hermetic lab for a local stdio server, protocol-witnessed (a path that was absent before the read and present after it) for a remote HTTP server.

There is a playbook for this check

MCP out-of-scope tool action sets out the use case, the probe flow, the oracle, and the fixture that proves it in both directions.

Header

Field	Value
Id	mcp-excessive-scope-proof
Severity	high
Language	python
Category	ai
Author	Bugb Research
Template version	1.0.0
Confidence	95
CVSS	7.6
Weakness	CWE-250, CWE-284
Tags	mcp, ai, agent, excessive-permissions, least-privilege, out-of-scope-action, behavioural, active, intrusive, cwe-250, cwe-284
Target kind	http, cli
Oracle	property, diff

Conventions of the repository, not fields the engine reads

@target_kinds and @oracles are documented in the templates repository’s contribution guide, and cxg’s annotation parser does not carry them into a template’s metadata. Nothing at scan time selects on them. The annotation header schema is every field the parser does read.

References

Declared in the header. cxg parses @references and then discards it, and nothing at scan time reads it, so this is the only place the links a template cites are surfaced.

• https://modelcontextprotocol.io/specification/2025-06-18/server/tools
• https://modelcontextprotocol.io/specification/2025-06-18/basic/security_best_practices
• https://cwe.mitre.org/data/definitions/250.html
• https://cwe.mitre.org/data/definitions/284.html

Run it

To see what the copy on your machine says about itself, and to confirm it is installed at all:

cxg template info mcp-excessive-scope-proof

The id it prints is the one to pass anywhere a template is selected. See cxg template for the rest of the subcommand, Scan a target for running a scan, and A match is not a finding for how to read what comes back.

Related

• Template catalog is the whole corpus.
• ai is the rest of this category.
• Templates by weakness class groups checks by the defect they look for.
• Contribute a template is the route from a check you needed and could not find to a merged one.