CVE-2026-81092
Received Received - Intake

mcp-go HTTP Server Host Header Validation Bypass

Publication date: 2026-08-27

Last updated on: 2026-08-27

Assigner: VulnCheck

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.

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Meta Information

Published
2026-08-27
Last Modified
2026-08-27
Generated
2026-08-27
AI Q&A
2026-08-27
EPSS Evaluated
N/A
NVD
CVE-2026-81092
EUVD
EUVD-2026-67147

Affected Vendors & Products

Showing 2 associated CPEs
Vendor Product Version / Range
mark3labs mcp-go 0.56.0
mark3labs mcp-go to 0.56.0 (exc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-346 The product does not properly verify that the source of data or communication is valid.

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Executive Summary

This vulnerability involves mcp-go versions before 0.56.0 accepting HTTP requests without validating the Host header. Attackers could exploit this by rebinding a domain they control to a loopback address (127.0.0.1 or ::1), tricking the server into processing requests as if they came from local software. This allowed access to tools and resources intended for local use only.

Detection Guidance

To detect this vulnerability, check if your mcp-go server is running a version before 0.56.0. Run: curl -s http://localhost:PORT/version or inspect the server logs for version details. If the version is below 0.56.0, the system is vulnerable.

Impact Analysis

If you run a vulnerable mcp-go server, a malicious website could interact with it via your browser, potentially accessing sensitive tools or data exposed on localhost. This could lead to unauthorized actions or data leaks on your system.

Compliance Impact

This vulnerability could violate compliance by enabling unauthorized access to sensitive data or tools, potentially breaching confidentiality requirements in GDPR or HIPAA. Unauthorized exposure of personal or health data may lead to regulatory penalties.

Mitigation Strategies

Upgrade to mcp-go version 0.56.0 or later immediately. This version adds Host header validation for loopback connections, blocking DNS rebinding attacks. No other immediate steps are required as the fix is automatic upon upgrade.

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