# CVE-2026-42559

## Summary

- **CVE ID:** CVE-2026-42559
- **Severity:** HIGH
- **CVSS Score:** 8.8 (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H)
- **CWE:** CWE-346, CWE-350
- **Published:** May 14, 2026
- **Last Modified:** May 14, 2026

## 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.

## Affected Products

- modelcontextprotocol — rust-sdk (< 1.4.0)

## References

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

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.21%
- **EPSS Percentile:** 11.6

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._