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.

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History

Fri, 28 Aug 2026 17:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}


Fri, 28 Aug 2026 16:45:00 +0000

Type Values Removed Values Added
First Time appeared Mark3labs
Mark3labs mcp-go
Vendors & Products Mark3labs
Mark3labs mcp-go

Thu, 27 Aug 2026 16:45:00 +0000

Type Values Removed Values Added
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.
Title mcp-go before 0.56.0 Missing Host Header Validation Enables DNS Rebinding
Weaknesses CWE-346
References
Metrics cvssV3_1

{'score': 6.8, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N'}

cvssV4_0

{'score': 7.6, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N'}


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cve-icon MITRE

Status: PUBLISHED

Assigner: VulnCheck

Published:

Updated: 2026-08-28T16:00:07.892Z

Reserved: 2026-08-26T16:00:30.950Z

Link: CVE-2026-81092

cve-icon Vulnrichment

Updated: 2026-08-27T18:24:14.554Z

cve-icon NVD

Status : Received

Published: 2026-08-27T17:20:51.337

Modified: 2026-08-28T20:20:09.140

Link: CVE-2026-81092

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-08-28T16:14:37Z

Weaknesses