The vulnerability arises from the rmcp crate's Streamable HTTP server transport not validating the Host header in incoming requests before version 1.4.0. This lack of origin validation (CWE-346) allows an attacker to perform a DNS rebinding attack, whereby a malicious public website can send authenticated requests to an MCP server on the victim's local or private network interfaces. The issue affects all versions prior to 1.4.0 of the rust-sdk. The CVSS 3.1 base score is 8.8, reflecting network attack vector, low attack complexity, no privileges required, user interaction required, unchanged scope, and high impact on confidentiality, integrity, and availability. The vulnerability is addressed by updating to version 1.4.0.

Successful exploitation allows an attacker controlling a malicious website to bypass origin checks and send authenticated requests to an MCP server running on the victim's loopback or private network interface. This can lead to full compromise of confidentiality, integrity, and availability of the MCP server. The vulnerability has a high CVSS score of 8.8, indicating severe potential impact. No known exploits in the wild have been reported to date.

Upgrade the modelcontextprotocol rust-sdk to version 1.4.0 or later, where the Host header validation issue is fixed. Since this is an open-source SDK and not a cloud service, users must apply this update themselves. Patch status is confirmed by the vendor stating the vulnerability is fixed in version 1.4.0. No additional mitigations are indicated by the vendor advisory.