CVE-2025-69196: Jlowin Fastmcp Auth Bypass Vulnerability

CVE-2025-69196 Overview

CVE-2025-69196 is an authorization bypass vulnerability in FastMCP, the standard framework for building MCP (Model Context Protocol) applications. Prior to version 2.14.2, the server does not properly respect the resource parameter submitted by the client in the authorization and token request. Instead of issuing the token explicitly for the MCP server, the token is issued for the base_url passed to the OAuthProxy during initialization. This improper authorization handling could allow attackers to obtain tokens with unintended scope or target.

Critical Impact

Attackers could potentially exploit this OAuth token misbinding to gain unauthorized access to resources, as tokens are issued for the base URL rather than the explicitly requested MCP server resource.

Affected Products

• FastMCP versions prior to 2.14.2
• jlowin FastMCP framework installations using OAuthProxy
• MCP applications built on vulnerable FastMCP versions

Discovery Timeline

• 2026-03-16 - CVE CVE-2025-69196 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2025-69196

Vulnerability Analysis

This vulnerability falls under CWE-863 (Incorrect Authorization), where the FastMCP server fails to properly validate and enforce the resource parameter during OAuth authorization flows. The core issue lies in how the OAuthProxy component handles token issuance requests.

When a client initiates an authorization or token request, it specifies a resource parameter to indicate which server the token should be scoped to. However, due to improper parameter handling in vulnerable versions, the server ignores this client-supplied resource value and instead issues tokens for the base_url configured during OAuthProxy initialization.

This design flaw means that regardless of what resource the client requests authorization for, the issued token will always be scoped to the pre-configured base URL, potentially granting access to unintended resources or allowing tokens meant for one MCP server to be used against another.

Root Cause

The root cause is improper authorization logic within the OAuthProxy component. The server implementation fails to bind the issued OAuth token to the specific resource parameter provided by the client during the authorization request. Instead, it defaults to using the base_url parameter from the proxy initialization, creating a mismatch between the requested and actual token scope.

Attack Vector

This vulnerability requires network access and some user interaction. An attacker could exploit this by:

1. Initiating an OAuth authorization flow against a FastMCP server
2. Specifying a crafted resource parameter targeting a different MCP server
3. Receiving a token that is incorrectly scoped to the base_url instead of the requested resource
4. Using this misbinded token to potentially access resources on the base URL server

The attack requires the attacker to have knowledge of the OAuth configuration and the ability to interact with the authorization endpoint. The vulnerability affects confidentiality and integrity as unauthorized tokens could be used to access or modify protected resources.

Detection Methods for CVE-2025-69196

Indicators of Compromise

• OAuth tokens being used against servers different from the resource parameter originally specified in authorization requests
• Audit logs showing token issuance where the resource parameter doesn't match the effective token scope
• Unusual authorization requests with mismatched resource parameters targeting your FastMCP instances

Detection Strategies

• Review OAuth token issuance logs for discrepancies between requested resource parameters and actual token bindings
• Monitor for authentication patterns where tokens issued for one endpoint are used against the base URL endpoint
• Implement token introspection to verify token scope matches expected resource bindings
• Audit OAuthProxy configurations to identify instances where base_url differs from expected resource targets

Monitoring Recommendations

• Enable detailed logging on OAuth authorization and token endpoints to capture resource parameter values
• Set up alerts for OAuth token usage patterns that don't match expected resource bindings
• Monitor FastMCP application logs for authorization-related anomalies
• Implement periodic security audits of MCP application OAuth configurations

How to Mitigate CVE-2025-69196

Immediate Actions Required

• Upgrade FastMCP to version 2.14.2 or later immediately
• Review OAuth configurations in existing FastMCP deployments to understand current exposure
• Audit access logs for any signs of exploitation prior to patching
• Consider invalidating and rotating existing OAuth tokens after upgrading

Patch Information

The vulnerability has been patched in FastMCP version 2.14.2. Users should upgrade to this version or later to remediate the issue. The fix ensures that the server properly respects the resource parameter submitted by the client during authorization and token requests, binding tokens to the correct target server.

For detailed patch information, refer to the GitHub Security Advisory GHSA-5h2m-4q8j-pqpj.

Workarounds

• If immediate upgrade is not possible, consider implementing additional authorization validation at the application layer
• Restrict network access to OAuth endpoints to trusted clients only
• Implement strict resource parameter validation in upstream reverse proxies or API gateways
• Monitor and log all OAuth token requests with resource parameter mismatches for manual review

# Upgrade FastMCP to patched version
pip install --upgrade fastmcp>=2.14.2

# Verify installed version
pip show fastmcp | grep Version