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CVE-2026-42559

HighPublished

RMCP: DNS rebinding vulnerability in rmcp Streamable HTTP server transport

Published: May 14, 2026
Updated: May 14, 2026
Assigning CNA: GitHub_M
Evidence observed: Sep 7, 2026

Primary CVSS

8.8/ 10High

nvd · CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

EPSS probability estimate

0.2%

Low · next 30 days

Percentile: 11.9%
Model date: Sep 21, 2026

EPSS is a statistical estimate, not a certainty or a measure of impact. Combine it with CVSS, KEV status, exposure and your environment.

Summary

RMCP is an official Rust SDK for the Model Context Protocol. Prior to version 1.4.0, the rmcp crate's Streamable HTTP server transport (crates/rmcp/src/transport/streamable_http_server/) did not validate the incoming Host header. This allowed a malicious public website, via a DNS rebinding attack, to send authenticated requests to an MCP server running on the victim's loopback or private-network interface. This vulnerability is fixed in 1.4.0.

Sources

EXPLOIT-CVE-2026-42559PoC
Docker lab + Python PoC for CVE-2026-42559 - DNS rebinding via unvalidated Host header in the rmcp (Rust MCP SDK) Streamable HTTP server transport

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