CVE-2026-34742: Go MCP SDK Auth Bypass Vulnerability

CVE-2026-34742 Overview

CVE-2026-34742 is a DNS rebinding vulnerability in the Model Context Protocol (MCP) Go SDK affecting versions prior to 1.4.0. The vulnerability exists because HTTP-based MCP servers do not enable DNS rebinding protection by default when running on localhost without authentication. This architectural weakness allows malicious websites to exploit DNS rebinding techniques to bypass same-origin policy restrictions and send unauthorized requests to local MCP server instances.

When an HTTP-based MCP server uses StreamableHTTPHandler or SSEHandler on localhost without proper authentication, an attacker controlling a malicious website can manipulate DNS resolution to redirect requests to the local server. This could enable unauthorized invocation of tools or access to resources exposed by the MCP server, effectively allowing the attacker to act on behalf of the user.

Critical Impact

Attackers can bypass browser same-origin policy to invoke tools and access resources on local MCP servers, potentially leading to unauthorized data access or system manipulation through the MCP interface.

Affected Products

• Model Context Protocol (MCP) Go SDK versions prior to 1.4.0
• HTTP-based MCP servers using StreamableHTTPHandler
• HTTP-based MCP servers using SSEHandler

Discovery Timeline

• 2026-04-02 - CVE CVE-2026-34742 published to NVD
• 2026-04-02 - Last updated in NVD database

Technical Details for CVE-2026-34742

Vulnerability Analysis

This vulnerability stems from insecure default configuration (CWE-1188) in the Go MCP SDK's HTTP server implementation. The SDK did not implement DNS rebinding protection mechanisms for localhost-bound servers, creating a window for network-based attacks against what users typically assume to be isolated local services.

DNS rebinding attacks work by having an attacker-controlled DNS server initially return a legitimate IP address, then after the victim's browser caches the DNS entry and establishes a connection, the DNS record is changed to point to a local address (e.g., 127.0.0.1). The browser, still believing it's communicating with the original domain, will send requests to the local MCP server, bypassing same-origin policy protections.

The vulnerability specifically affects the StreamableHTTPHandler and SSEHandler components when they bind to localhost addresses without authentication. The lack of Host header validation allows the DNS rebinding attack to succeed, as the server accepts requests regardless of the requested hostname.

Root Cause

The root cause is the absence of automatic DNS rebinding protection for localhost servers in the MCP Go SDK. The SDK did not validate incoming request Host headers against expected localhost addresses, nor did it implement loopback address verification by default. This insecure default configuration allowed external websites to exploit DNS rebinding to communicate with local MCP servers.

Attack Vector

The attack requires the victim to visit a malicious website while running a local MCP server. The attacker's website initiates a DNS rebinding attack by:

1. Hosting content on an attacker-controlled domain
2. Configuring DNS with a short TTL initially pointing to the attacker's server
3. After the victim's browser caches the DNS entry, switching the DNS record to 127.0.0.1
4. Making requests to the attacker's domain, which the browser now routes to the local MCP server
5. Invoking MCP tools or accessing resources without proper authorization

The security patch introduces loopback address verification to prevent these attacks:

// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
// Use of this source code is governed by the license
// that can be found in the LICENSE file.
package util

import (
	"net"
	"net/netip"
	"strings"
)

func IsLoopback(addr string) bool {
	host, _, err := net.SplitHostPort(addr)
	if err != nil {
		// If SplitHostPort fails, it might be just a host without a port.
		host = strings.Trim(addr, "[]")
	}
	if host == "localhost" {
		return true
	}
	ip, err := netip.ParseAddr(host)
	if err != nil {
		return false
	}
	return ip.IsLoopback()
}

Source: GitHub Commit Changes

This new utility function validates whether an address is a loopback address, enabling the SDK to automatically apply DNS rebinding protection for localhost-bound servers.

Detection Methods for CVE-2026-34742

Indicators of Compromise

• Unusual HTTP requests to local MCP servers with unexpected or external Host headers
• DNS queries showing rapid TTL changes for domains that subsequently resolve to localhost addresses
• Web browser connections to localhost ports from untrusted web page contexts
• MCP tool invocations or resource access without corresponding legitimate user actions

Detection Strategies

• Monitor local HTTP server logs for requests with Host headers that do not match expected localhost values
• Implement network monitoring to detect DNS responses pointing external domains to loopback addresses
• Review MCP server audit logs for unexpected tool invocations or resource access patterns
• Deploy browser-based monitoring to detect potential DNS rebinding attempts

Monitoring Recommendations

• Enable verbose logging on MCP servers to capture all incoming request headers
• Configure network intrusion detection systems to alert on DNS rebinding patterns
• Implement application-level logging for all MCP tool invocations with source IP and Host header information
• Regularly audit MCP server configurations to ensure authentication is enabled where appropriate

How to Mitigate CVE-2026-34742

Immediate Actions Required

• Upgrade the Go MCP SDK to version 1.4.0 or later immediately
• Enable authentication on all HTTP-based MCP servers, especially those accessible via localhost
• Review existing MCP server deployments to identify instances running without authentication
• Implement Host header validation as an additional defense layer

Patch Information

The vulnerability has been addressed in Go MCP SDK version 1.4.0. The patch adds automatic DNS rebinding protection for localhost servers by implementing Host header validation and loopback address verification. The fix is available in commit 67bd3f2.

For detailed information about the security fix, see the GitHub Security Advisory GHSA-xw59-hvm2-8pj6 and the GitHub Release Version 1.4.0.

Workarounds

• Enable authentication on MCP servers to prevent unauthorized access regardless of request origin
• Bind MCP servers to Unix sockets instead of TCP ports when cross-network access is not required
• Implement a reverse proxy with Host header validation in front of MCP servers
• Use firewall rules to restrict access to MCP server ports from external network interfaces

# Configuration example: Update Go MCP SDK dependency
go get github.com/modelcontextprotocol/go-sdk@v1.4.0
go mod tidy

# Verify the installed version
go list -m github.com/modelcontextprotocol/go-sdk