CVE-2026-42559Network attack vector

Unauthorized tool invocation via unchecked Host header

Published May 14, 2026 · Updated May 14, 2026

Origin validation error in RMCP before 1.4.0 allows remote attackers to invoke local MCP tools through DNS rebinding. The Streamable HTTP server transport accepts requests without checking whether the Host header matches an approved authority. Exploitation requires a victim to visit a malicious website while an affected HTTP transport is reachable; exposed tools can read resources or trigger actions with the server process's privileges.

CVSS severity8.8
High
EPSS probability0.21%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Origin validation error in RMCP before 1.4.0 allows remote attackers to invoke local MCP tools through DNS rebinding. The Streamable HTTP server transport accepts requests without checking whether the Host header matches an approved authority. Exploitation requires a victim to visit a malicious website while an affected HTTP transport is reachable; exposed tools can read resources or trigger actions with the server process's privileges.

The record

CVE
CVE-2026-42559
Published
May 14, 2026
Updated
May 14, 2026
Vendor
Model Context Protocol
Product
RMCP
Classifications
CWE-346, CWE-350, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 14, 2026CVE publishedPublication date reported by the CVE source.
  2. May 14, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.4.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Model Context Protocol · RMCPversion=< 1.4.0

Published CVSS scores

8.8GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-346Origin Validation Error
CWE-350Reliance on Reverse DNS Resolution for a Security-Critical Action
T1189Drive-by Compromise

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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