CVE-2026-81092 mcp-go before 0.56.0 Missing Host Header Validation Enables DNS Rebinding

What is CVE-2026-81092?

CVE-2026-81092 is a high-severity server-side request forgery vulnerability in mark3labs mcp-go, published on 27 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

Official description

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.

Weakness: CWE-346

Which versions are affected by CVE-2026-81092?

How do I fix CVE-2026-81092?

Upgrade to a fixed version.

Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.

Is CVE-2026-81092 being exploited?

There is no known exploitation of CVE-2026-81092 in the wild and no public exploit code.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as GHSA-8g5x-gj7x-mxgm, RHSA-2026:54874.