Description
MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not validate the HTTP Host or Origin request headers, which allows a malicious browser page to use DNS rebinding to reach a locally running MCP server and invoke exposed tools. This issue is fixed in version 0.23.0.
Published: 2026-07-29
Score: 6.9 Medium
EPSS: < 1% Very Low
KEV: No
Impact: n/a
Action: n/a
AI Analysis

Impact

The MCP Ruby SDK’s StreamableHTTPTransport component does not validate HTTP Host or Origin request headers. This omission permits a malicious web page to perform DNS rebinding, causing a user’s browser to resolve the address of a locally running MCP server and send requests that reach the server’s exposed tools. If an attacker can reach those tools, they may execute arbitrary commands or otherwise obtain unauthorized local access. The weakness is classified as CWE‑346 and CWE‑350.

Affected Systems

All installations of the Model Context Protocol Ruby SDK before update v0.23.0 are affected, irrespective of the host environment. The problem was addressed in release 0.23.0, which adds Host and Origin header validation.

Risk and Exploitability

With a CVSS score of 6.9 the vulnerability has moderate‑to‑high severity. The EPSS value of less than 1 % indicates a low probability of exploitation in the near term, and the vulnerability is not listed in the CISA KEV catalog. The most likely attack scenario involves a user visiting a compromised or malicious web page while a local MCP server is running, enabling the attacker to use DNS rebinding to target the server through a browser.

Generated by OpenCVE AI on August 3, 2026 at 12:57 UTC.

Remediation

No vendor fix or workaround currently provided.

OpenCVE Recommended Actions

  • Update the MCP Ruby SDK to version 0.23.0 or later, which introduces validation of the Host and Origin headers.
  • If an upgrade cannot be performed immediately, isolate the local MCP server from browsers by placing it on a separate network segment or configuring firewall rules that block external DNS rebinding traffic.
  • Add application‑level checks to ensure that incoming HTTP requests contain a Host or Origin header that matches the expected server address, providing an additional layer of defense.

Generated by OpenCVE AI on August 3, 2026 at 12:57 UTC.

Tracking

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Advisories
Source ID Title
Github GHSA Github GHSA GHSA-rjr6-rcgv-9m7m MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection
History

Wed, 29 Jul 2026 21:00:00 +0000

Type Values Removed Values Added
First Time appeared Modelcontextprotocol
Modelcontextprotocol ruby-sdk
Vendors & Products Modelcontextprotocol
Modelcontextprotocol ruby-sdk

Wed, 29 Jul 2026 20:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'poc', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Wed, 29 Jul 2026 19:30:00 +0000

Type Values Removed Values Added
Description MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not validate the HTTP Host or Origin request headers, which allows a malicious browser page to use DNS rebinding to reach a locally running MCP server and invoke exposed tools. This issue is fixed in version 0.23.0.
Title MCP Ruby SDK: Streamable HTTP transport lacks DNS-rebinding (Host/Origin) protection
Weaknesses CWE-346
CWE-350
References
Metrics cvssV4_0

{'score': 6.9, 'vector': 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:N/VA:N/SC:H/SI:N/SA:N'}

Subscriptions

Modelcontextprotocol Ruby-sdk
cve-icon MITRE

Status: PUBLISHED

Assigner: GitHub_M

Published:

Updated: 2026-07-29T19:35:10.375Z

Reserved: 2026-07-15T16:54:55.816Z

Link: CVE-2026-63118

cve-icon Vulnrichment

Updated: 2026-07-29T19:35:05.445Z

cve-icon NVD

Status : Deferred

Published: 2026-07-29T20:17:10.630

Modified: 2026-07-30T19:30:33.710

Link: CVE-2026-63118

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-08-03T13:00:07Z

Weaknesses
  • CWE-346

    Origin Validation Error

  • CWE-350

    Reliance on Reverse DNS Resolution for a Security-Critical Action