Bug 2525106 (CVE-2026-81092)

Summary: CVE-2026-81092 github.com/mark3labs/mcp-go: mcp-go: DNS Rebinding vulnerability due to missing Host header validation
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: mwringe
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in mcp-go. This component failed to validate the Host header in HTTP requests, specifically within its StreamableHTTPServer and SSEServer transports. A remote attacker could exploit this by using a crafted web page to perform a DNS rebinding attack. This allows the attacker to direct a controlled domain name to the loopback address, bypassing local network restrictions. Consequently, the attacker could access and interact with local services and resources, potentially leading to information disclosure or unauthorized actions.
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Description OSIDB Bzimport 2026-08-27 17:01:05 UTC 
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.