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

Severity: medium (CVSS 6.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-08-27 · updated 2026-09-23
Weakness: CWE-346

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.

Affected products

mark3labs mcp-go: before 0.56.0 — fixed in 0.56.0

How to fix

Upgrade to a fixed version: mark3labs mcp-go 0.56.0. Check the vendor advisory for mitigations.

References

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