CVE-2026-81092

MediumCVSS 6.8

Published: 27/08/2026(UTC)Updated: 28/08/2026(UTC)Translated: 27/08/2026(UTC)NVD NIST

Exploitation Probability (EPSS)

Low risk

0.17%

6th percentile - higher than 6% of all known CVEs

Summary

mcp-go accepted requests on its HTTP transports without checking the Host header. StreamableHTTPServer.ServeHTTP in server/streamable_http.go and SSEServer.ServeHTTP in server/sse.go served any request arriving over a loopback connection regardless of the host it named, and the SSE transport's cross-origin default allowed any origin. A page in a browser could point a controlled name at the loopback address and reach a server listening there, invoking tools and reading resources that the server exposed on the assumption that only local software could connect. No release before 0.56.0 validated the header on either transport; 0.56.0 adds server/http_localhost.go, which rejects a loopback-bound request carrying a host that is not a loopback name, and wires it into both transports.

Risk Assessment

An attacker can use a victim's browser to remotely invoke tools and read resources on the local MCP server, potentially leading to unauthorized access to data and actions in the user's context.

Recommendation

Upgrade mcp-go to version 0.56.0 or later, which validates the Host header on both HTTP transports.

Original NVD description (English source)

mcp-go accepted requests on its HTTP transports without checking the Host header. StreamableHTTPServer.ServeHTTP in server/streamable_http.go and SSEServer.ServeHTTP in server/sse.go served any request arriving over a loopback connection regardless of the host it named, and the SSE transport's cross-origin default allowed any origin. A page in a browser could therefore point a name it controlled at the loopback address and reach a server listening there, invoking tools and reading resources that the server exposed on the assumption that only local software could connect. No release before 0.56.0 validated the header on either transport; 0.56.0 adds server/http_localhost.go, which rejects a loopback-bound request carrying a host that is not a loopback name, and wires it into both transports.