7.6

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2025-66416

CVE-2025-66416: DNS Rebinding Protection Disabled by Default in Model Context Protocol Python SDK for Servers Running on Localhost

The MCP Python SDK, called mcp on PyPI, is a Python implementation of the Model Context Protocol (MCP). Prior to version 1.23.0, tThe Model Context Protocol (MCP) Python SDK does not enable DNS rebinding protection by default for HTTP-based servers. When an HTTP-based MCP server is run on localhost without authentication using FastMCP with streamable HTTP or SSE transport, and has not configured TransportSecuritySettings, a malicious website could exploit DNS rebinding to bypass same-origin policy restrictions and send requests to the local MCP server. This could allow an attacker to invoke tools or access resources exposed by the MCP server on behalf of the user in those limited circumstances. Note that running HTTP-based MCP servers locally without authentication is not recommended per MCP security best practices. This issue does not affect servers using stdio transport. This vulnerability is fixed in 1.23.0.

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2025-66416

CVE-2025-66416:

7.6

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
mcp	pip	< 1.23.0	1.23.0

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The security vulnerability exists in the mcp Python SDK, specifically within the FastMCP class constructor. The root cause is the insecure default configuration for servers running on localhost. The __init__ method of the FastMCP class did not enable DNS rebinding protection by default. An attacker could exploit this by using a malicious website to perform a DNS rebinding attack, tricking the user's browser into sending requests to the local MCP server, bypassing the Same-Origin Policy. The provided patch addresses this by modifying the FastMCP.__init__ method. It introduces a check to see if the server is being bound to a localhost address (127.0.0.1, localhost, or ::1). If it is, and no explicit transport_security settings have been provided, it automatically applies TransportSecuritySettings with DNS rebinding protection enabled. This change ensures that by default, local servers are protected from this attack vector. The analysis of the commit d3a184119e4479ea6a63590bc41f01dc06e3fa99 clearly shows the addition of this security control in the FastMCP.__init__ function, making it the central point of the vulnerability and its fix.

Vulnerable functions

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WAF Protection Rules

WAF Rule

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Reasoning

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