---
id: CVE-2026-81092
title: >-
  mcp-go accepted requests on its HTTP transports without checking the Host
  header
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…
severity: medium
cvss: 6.8
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N'
cwe:
  - CWE-346
published: '2026-08-27'
updated: '2026-09-23'
sourceUpdated: '2026-09-23T17:17:43.410'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-81092'
references:
  - url: 'https://github.com/mark3labs/mcp-go'
    label: disclosure@vulncheck.com
  - url: 'https://github.com/mark3labs/mcp-go/pull/921'
    label: disclosure@vulncheck.com
  - url: 'https://github.com/mark3labs/mcp-go/releases/tag/v0.56.0'
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/mcp-go-before-0.56.0-missing-host-header-validation-enables-dns-rebinding
    label: disclosure@vulncheck.com
tags:
  - nvd
epss: 0.00165
epssPercentile: 0.06207
ingestedAt: '2026-09-23T17:28:14.806Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.