Vulnerability · Published Dec 2, 2025 · Assigned by GitHub_M
CVE-2025-66416DNS Rebinding Protection Disabled by Default in Model Context Protocol Python SDK for Servers Running on Localhost
High 7.6
In plain English
A high-severity security flaw in python-sdk from modelcontextprotocol. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 46% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| PyPI | mcp | 1.23.0 | GHSA-9h52-p55h-vw2f |
| PyPI | mcp | 1.23.0 | PYSEC-2026-1617 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| modelcontextprotocol | python-sdk | < 1.23.0 |
Weakness: CWE-1188 · Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N