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MCP guide · OAuth Auditor

MCP OAuth Auditor: token passthrough & audience risks — @guardbee/mcp-oauth-auditor

@guardbee/mcp-oauth-auditor scans for the OAuth 2.1 anti-patterns named in the MCP spec’s Security Considerations — especially token passthrough and missing audience validation.

What does it do?

Six categories: token-passthrough, token-validation (missing audience), discovery-ssrf, pkce, redirect-validation, secrets-exposure. Some rules look for an absence near a call site (audience / code_challenge).

Who is it for?

Teams building MCP servers / OAuth resource servers
DevSecOps reviewing OAuth/OIDC on every PR
Developers who want this check inside Claude or Cursor

What it is not

Generic web-app API-key scanning (use Secret Scanner)
Live token abuse / red team (different from LLM Red Team)
A formal OAuth certification

Features

Feature	Description
Spec-aligned	MCP Security Considerations focus
Absence checks	audience / PKCE proximity analysis
6 categories	passthrough → secrets
SARIF 2.1.0	CI
guardbee.yml	fail-on, exclude

MCP tools

Tool	Description
`scan_text`	Scan a snippet
`scan_file`	Scan a single file
`scan_directory`	Scan auth source
`list_patterns`	List categories

Install with npm

Global install (handy for Claude Desktop / Cursor):

bash

npm install -g @guardbee/mcp-oauth-auditor

or via npx (recommended with -y in config):

bash

npx -y @guardbee/mcp-oauth-auditor

npm: @guardbee/mcp-oauth-auditor

CLI

npx @guardbee/mcp-oauth-auditor scan ./src --fail-on=high --format=sarif > results.sarif

Claude Desktop configuration

Config file paths:

macOS~/Library/Application Support/Claude/claude_desktop_config.json
Windows%APPDATA%\Claude\claude_desktop_config.json
Linux~/.config/Claude/claude_desktop_config.json

Merge this block, then restart Claude Desktop:

claude_desktop_config.json

{
  "mcpServers": {
    "guardbee-oauth-auditor": {
      "command": "npx",
      "args": [
        "-y",
        "@guardbee/mcp-oauth-auditor"
      ]
    }
  }
}

Cursor and other MCP clients

In Cursor, add the same command / args / env under Settings → MCP. stdio MCP servers use the same contract as Claude Desktop.

Example prompts in Claude

“Is there token passthrough in this MCP auth code?”
“Does jwt.verify check audience?”
“Which OAuth anti-patterns do you support?”

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Frequently asked questions

Why MCP-specific?

The MCP spec explicitly names token passthrough and audience as resource-server risks.

API key?

No — local source scanning.

Is PKCE always required?

OAuth 2.1 expects it on authorization requests; absence is flagged.

MCP OAuth Auditor: token passthrough & audience risks