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Vulnerability record · CVE-2026-42559 · published 14 May 2026

CVE-2026-42559: Origin validation error vulnerability

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.

8.8 CVSS 3.1 High EPSS 0.24% · top 86.2% CWE-346 · Origin validation errorCWE-350 · CWE-350 Deferred
8.8CVSS 3.1 base score
0.24%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
5References
17 Jun 2026Last modified by NVD

Description

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.

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

References

LinkTags
https://github.com/modelcontextprotocol/rust-sdk/commit/8e22aa2de28df5a285eed87c11cd89bf15fa90d3
https://github.com/modelcontextprotocol/rust-sdk/issues/815
https://github.com/modelcontextprotocol/rust-sdk/issues/822
https://github.com/modelcontextprotocol/rust-sdk/pull/764
https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-89vp-x53w-74fx

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Source: NIST National Vulnerability Database (record CVE-2026-42559), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.