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How the mentioned CVE works:
The vulnerability exists in the MCP Python SDK where DNS rebinding protection is not enabled by default for HTTP-based servers. When an MCP server is configured using FastMCP with streamable HTTP or SSE transport, and runs on localhost without authentication, it fails to validate the Host header against allowed origins. A malicious website can exploit DNS rebinding by controlling a domain that resolves first to an attacker-controlled IP and then to 127.0.0.1. The browser, due to same-origin policy, allows scripts from the domain to run. After DNS rebinding, these scripts send requests to the local MCP server, bypassing origin restrictions. The server, lacking DNS rebinding protection, processes these requests as if they were local, allowing unauthorized tool invocation or resource access. This attack requires specific conditions: HTTP-based server, localhost binding, no authentication, and vulnerable transport modes. The issue is addressed in version 1.23.0 by enabling DNS rebinding protection by default for FastMCP servers with host set to localhost or 127.0.0.1.
Platform: MCP Python SDK
Version: Before 1.23.0
Vulnerability: DNS rebinding attack
Severity: Medium
Date: Not provided
Prediction: Patched in 1.23.0
What Undercode Say:
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Bash command to check version:
`python -c “import mcp; print(mcp.__version__)”`
Code example of vulnerable setup:
from mcp import FastMCP server = FastMCP(host="localhost", transport="http") Vulnerable in versions <1.23.0
How Exploit:
Attacker controls domain with DNS rebinding to 127.0.0.1. User visits malicious site. JavaScript sends requests to domain after rebinding, targeting local MCP server. Server processes requests due to missing protection.
Protection from this CVE:
Update to version 1.23.0. Configure TransportSecuritySettings explicitly for custom servers.
Impact:
Unauthorized tool invocation. Resource access exposure. Potential system compromise.
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Sources:
Reported By: github.com
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