PYSEC-2026-2476 FastMCP: Missing Consent Verification in OAuth Proxy Callback Facilitates Confused Deputy Vulnerabilities

Summary While testing the GitHubProvider OAuth integration, which allows authentication to a FastMCP MCP server via a FastMCP OAuthProxy using GitHub OAuth, it was discovered that the FastMCP OAuthProxy does not properly validate the user's consent upon receiving the authorization code from GitHu...

CVE-2026-27124

A flaw was found in FastMCP and FastMCP OAuthProxy. The OAuthProxy, used for GitHub OAuth authentication, does not properly validate a user's consent after receiving an authorization code from GitHub. This, combined with GitHub's behavior of skipping the consent page for previously authorized...

CVE-2026-27124

FastMCP is the standard framework for building MCP applications. Prior to version 3.2.0, while testing the GitHubProvider OAuth integration, which allows authentication to a FastMCP MCP server via a FastMCP OAuthProxy using GitHub OAuth, it was discovered that the FastMCP OAuthProxy does not...