Insecure dfefault initialization of resource in MCP Python SDK - CVE-2025-66416
Published: December 18, 2025
Vulnerability identifier: #VU120189
CSH Severity: Low
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/E:U/U:Clear
CVE-ID: CVE-2025-66416
CWE-ID: CWE-1188
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vendor: Model Context Protocol
Affected software:
MCP Python SDK
MCP Python SDK
Detailed vulnerability description
The vulnerability allows a remote attacker to bypass implemented security restrictions.
The vulnerability exists due to software does not enable DNS rebinding protection by default for HTTP-based servers. A remote attacker can trick the victim into visiting a malicious website and to bypass same-origin policy restrictions by exploiting DNS rebinding and initiate requests to the local MCP server.
Successful exploitation of the vulnerability requires that an HTTP-based MCP server is running on the localhost without authentication using FastMCP with streamable HTTP or SSE transport, and has not configured TransportSecuritySettings.
How to mitigate CVE-2025-66416
Install updates from vendor's website.