MCP Handle Binding and requestState Integrity (Stateless Core)

mcp-handle-binding-integrity is a high severity check in the ai category, written in python. Its source is templates/ai/mcp/mcp-handle-binding-integrity.py in cert-x-gen-templates.

What it checks

ACTIVE check. The 2026-07-28 MCP core removed sessions and replaced them with server-minted handles that travel inside model context, plus a requestState blob echoed back by the client. This template mints a handle as one identity and presents it as another, and mutates a security-relevant field of an input_required requestState before the retry. CONFIRMED only when a forged-handle control was rejected and a random-blob control was rejected, so acceptance is an integrity failure and not indifference.

There is a playbook for this check

MCP handle binding and requestState integrity sets out the use case, the probe flow, the oracle, and the fixture that proves it in both directions.

Header

Field	Value
Id	mcp-handle-binding-integrity
Severity	high
Language	python
Category	ai
Author	Bugb Research
Template version	1.0.0
Confidence	90
CVSS	8.1
Weakness	CWE-639, CWE-565
Tags	mcp, ai, agent, stateless-core, handle-binding, requeststate, broken-object-level-authorization, state-tampering, behavioural, active, intrusive, cwe-639, cwe-565
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://blog.modelcontextprotocol.io/posts/2026-07-28/
• https://modelcontextprotocol.io/specification/2025-06-18/basic/security_best_practices
• https://cwe.mitre.org/data/definitions/639.html
• https://cwe.mitre.org/data/definitions/565.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-handle-binding-integrity

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.