MCP Client x-mcp-header Value-Encoding Failure (Model-Controlled HTTP Header Injection)

mcp-client-header-value-encoding is a high severity check in the ai category, written in python. Its source is templates/ai/mcp/client-conformance/mcp-client-header-value-encoding.py in cert-x-gen-templates.

What it checks

ACTIVE check. The MCP 2026-07-28 revision (SEP-2243) lets a tool PARAMETER be marked x-mcp-header, so a value the model chose is placed by the CLIENT into the outgoing HTTP header block. The spec puts the value-encoding MUST on the client. This drives an MCP client against a synthetic server whose tool schema hands it a value carrying CR LF and a decoy Authorization header, then reads the RAW BYTES the client put on the wire. Extra header lines, or unencoded control characters in the value, confirm client-side header injection.

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-header-value-encoding
Severity	high
Language	python
Category	ai
Author	Bugb Research
Template version	1.0.0
Confidence	93
CVSS	7.5
Weakness	CWE-93, CWE-113
Tags	mcp, ai, agent, mcp-client, mcp-client-conformance, agent-posture, header-injection, x-mcp-header, sep-2243, crlf-injection, request-splitting, behavioural, active, intrusive, cwe-93, cwe-113
Target kind	cli
Oracle	property

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/transports
• https://modelcontextprotocol.io/specification/2026-07-28/basic/security_best_practices
• https://www.rfc-editor.org/rfc/rfc9110.html#section-5.5
• https://cwe.mitre.org/data/definitions/93.html
• https://cwe.mitre.org/data/definitions/113.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-header-value-encoding

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.