CVE-2026-81092: Origin Validation Error in mark3labs mcp-go

The vulnerability in mcp-go versions before 0.56.0 involves the lack of Host header validation on HTTP transports bound to the loopback interface. Specifically, StreamableHTTPServer.ServeHTTP and SSEServer.ServeHTTP served requests regardless of the Host header value, and the SSE transport allowed any origin by default. This allowed a browser page controlled by an attacker to direct requests to the loopback address with arbitrary Host headers, potentially invoking local tools and reading resources exposed by the server under the assumption that only local clients would connect. The fix in version 0.56.0 adds server/http_localhost.go, which rejects loopback-bound requests with Host headers that are not loopback names, securing both HTTP transports.

The vulnerability allows an attacker controlling a web page to bypass origin restrictions and interact with local services on the loopback interface. This could lead to unauthorized invocation of local tools and access to sensitive resources exposed by mcp-go servers, violating the assumption that only local software can connect. The CVSS 4.0 score of 7.6 reflects high impact due to network attack vector, low attack complexity, partial user interaction, and high impact on confidentiality and integrity.

Upgrade mcp-go to version 0.56.0 or later, which includes Host header validation for loopback-bound HTTP requests. This update rejects requests with non-loopback Host headers on the loopback interface, effectively mitigating the vulnerability. No other specific mitigations are indicated by the vendor advisory or data.