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.
CVE-2026-42559
HIGHNVD 8.88.8—
EchelonGraph scoreMEDIUM confidence
This high-severity CVE scores 8.8 under NVD CVSS v3. EPSS exploit probability: 0.2%, top 88% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).
Triggered by: NVD CVSS baseline
Sources: epss, nvd
8.8
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
- High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 0%CVSS: 8.8Exploit: NoneExposed: 0
No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.
- CVSS v3
- 8.8
- EG Score
- 8.8(medium)
- EPSS
- 11.6%
- KEV
- Not listed
Published
May 14, 2026
Last Modified
May 14, 2026
Advisory Details (5)
Auto-updated May 14, 2026Patch available. Sources: github.
github Patch Available
DNS rebinding vulnerability in rmcp Streamable HTTP server transport · Advisory · modelcontextprotocol/rust-sdk · GitHub
https://github.com/modelcontextprotocol/rust-sdk/security/advisories/GHSA-89vp-x53w-74fxgeneric
fix(http): add host check by jokemanfire · Pull Request #764 · modelcontextprotocol/rust-sdk · GitHub
https://github.com/modelcontextprotocol/rust-sdk/pull/764generic
Add Origin header validation for defense-in-depth in Streamable HTTP server · Issue #822 · modelcontextprotocol/rust-sdk · GitHub
https://github.com/modelcontextprotocol/rust-sdk/issues/822generic
[SECURITY] Status of DNS Rebinding Vulnerability Fix for rust-sdk · Issue #815 · modelcontextprotocol/rust-sdk · GitHub
https://github.com/modelcontextprotocol/rust-sdk/issues/815generic
fix(http): add host check (#764) · modelcontextprotocol/rust-sdk@8e22aa2 · GitHub
https://github.com/modelcontextprotocol/rust-sdk/commit/8e22aa2de28df5a285eed87c11cd89bf15fa90d3Vendor Advisories for CVE-2026-42559(1)
These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.
Affected Packages
(3 across 2 ecosystems)
crates.io(2)
| Package | Vulnerable range | Fixed in | Dependents |
|---|---|---|---|
| dynoxide-rs | — | 0.9.13 | — |
| rmcp | — | 1.4.0 | — |
npm(1)
| Package | Vulnerable range | Fixed in | Dependents |
|---|---|---|---|
| dynoxide | — | 0.9.13 | — |
Weakness Classification(2)
MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.
Frequently asked(5)
What is CVE-2026-42559?
CVE-2026-42559 is a high vulnerability published on May 14, 2026. 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/streamablehttpserver/) did not validate the incoming Host header. This allowed a malicious public website, via a DNS rebinding attack, to…
When was CVE-2026-42559 disclosed?
CVE-2026-42559 was first published in the National Vulnerability Database on May 14, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-42559 actively exploited?
CVE-2026-42559 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 11.6% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2026-42559?
CVE-2026-42559 has a CVSS v3 base score of 8.8 (NVD).
How do I remediate CVE-2026-42559?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-42559, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.
Dependency Blast Radius
See which npm, PyPI, Go, and Maven packages are affected by CVE-2026-42559
Is Your Infrastructure Affected by CVE-2026-42559?
EchelonGraph automatically scans your cloud infrastructure and maps CVE exposure using blast radius analysis.