CVE-2025-69196
OAuth Token Misissuance in FastMCP Server Before
Publication date: 2026-03-16
Last updated on: 2026-03-18
Assigner: GitHub, Inc.
Description
Description
CVSS Scores
EPSS Scores
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Meta Information
Affected Vendors & Products
| Vendor | Product | Version / Range |
|---|---|---|
| jlowin | fastmcp | to 2.14.2 (exc) |
Helpful Resources
Exploitability
| CWE ID | Description |
|---|---|
| CWE-863 | The product performs an authorization check when an actor attempts to access a resource or perform an action, but it does not correctly perform the check. |
Attack-Flow Graph
AI Powered Q&A
Can you explain this vulnerability to me?
CVE-2025-69196 is a vulnerability in the FastMCP OAuth Proxy prior to version 2.14.2 where the server does not properly respect the resource parameter submitted by the client during authorization and token requests.
Instead of issuing tokens explicitly for the specific MCP server requested by the client, the OAuth Proxy issues tokens based on its own base_url configured during initialization. This means the tokens lack information about the intended MCP server resource.
As a result, MCP servers cannot verify if a token was legitimately issued for them, violating OAuth specifications and enabling attackers to set up malicious MCP servers that can steal tokens and reuse them to access other legitimate MCP servers.
How can this vulnerability impact me? :
This vulnerability allows an attacker to steal OAuth tokens by tricking users into authenticating with a malicious MCP server.
Because the tokens are not bound to a specific MCP server, the attacker can reuse stolen tokens to gain unauthorized access to other legitimate MCP servers that rely on the same OAuth Proxy.
This compromises the authentication integrity of MCP servers, potentially exposing sensitive tools and resources to unauthorized parties.
How does this vulnerability affect compliance with common standards and regulations (like GDPR, HIPAA)?:
I don't know
How can this vulnerability be detected on my network or system? Can you suggest some commands?
I don't know
What immediate steps should I take to mitigate this vulnerability?
To mitigate this vulnerability, you should upgrade FastMCP to version 2.14.2 or later, where the issue has been fixed.
The fix involves modifying the OAuth Proxy to issue tokens specifically for the MCP server indicated by the `resource` parameter in the authorization URL. This ensures tokens are bound to the intended resource and prevents token theft and reuse across MCP servers.