MCP Client OAuth Issuer Binding (Authorization-Server Mix-Up / Cross-Issuer Credential Reuse)

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

What it checks

ACTIVE check. Drives an MCP CLIENT’s own OAuth login path against two synthetic issuers and proves, by observation, that it fails the 2026-07-28 MCP client authorization MUSTs - it carries credentials minted by the first authorization server to the second, skips re-registration when the issuer changes, or redeems an authorization code whose RFC 9207 iss names a different issuer than the one it asked.

There is a playbook for this check

The model writes a header line: x-mcp-header value encoding on the client sets out the use case, the probe flow, the oracle, and the fixture that proves it in both directions.

Header

Field	Value
Id	mcp-client-oauth-issuer-binding
Severity	high
Language	python
Category	ai
Author	Bugb Research
Template version	1.0.0
Confidence	92
CVSS	8.1
Weakness	CWE-346, CWE-522
Tags	mcp, ai, agent, mcp-client, agent-posture, oauth, oauth-conformance, authorization-server-mixup, issuer-binding, rfc9207, credential-reuse, behavioural, active, intrusive, cwe-346, cwe-522
Target kind	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/2026-07-28/basic/authorization
• https://modelcontextprotocol.io/specification/2026-07-28/basic/authorization/security-considerations
• https://modelcontextprotocol.io/specification/2026-07-28/basic/authorization/client-registration
• https://www.rfc-editor.org/rfc/rfc9207.html
• https://www.rfc-editor.org/rfc/rfc9728.html
• https://www.rfc-editor.org/rfc/rfc8707.html
• https://cwe.mitre.org/data/definitions/346.html
• https://cwe.mitre.org/data/definitions/522.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-client-oauth-issuer-binding

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.