CVE-2026-63118: CVE-2026-63118: DNS-Rebinding and Cross-Origin Request Execution in Model Context Protocol (MCP) Ruby SDK

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CVE-2026-63118: DNS-Rebinding and Cross-Origin Request Execution in Model Context Protocol (MCP) Ruby SDK

Vulnerability ID: CVE-2026-63118
CVSS Score: 6.9
Published: 2026-07-30

A critical vulnerability (CVE-2026-63118) in the Model Context Protocol (MCP) Ruby SDK allows attackers to execute arbitrary JSON-RPC commands and exfiltrate sensitive local data from an MCP server bound to the local loopback interface. This is achieved through DNS-rebinding and cross-origin request execution due to missing validation of the HTTP Host and Origin headers in the StreamableHTTPTransport component.

TL;DR

A DNS-rebinding flaw in the MCP Ruby SDK (< 0.23.0) allows malicious sites to bypass the browser Same-Origin Policy and run arbitrary local tools.

⚠️ Exploit Status: POC

Technical Details

CWE ID: CWE-346, CWE-350
Attack Vector: Network (AV:N)
CVSS Score: 6.9 (Medium)
Exploit Status: PoC Available
Affected Component: StreamableHTTPTransport
Fixed Version: v0.23.0
CISA KEV Status: Not Listed

Affected Systems

Model Context Protocol (MCP) Ruby SDK (mcp gem)
mcp: < 0.23.0 (Fixed in: v0.23.0)

Code Analysis

Commit: ba54308

Implement DNS rebinding protection in StreamableHTTPTransport by validating Host and Origin headers.

Mitigation Strategies

Upgrade the mcp gem to version 0.23.0 or higher.
Configure upstream Host and Origin validation in reverse proxies.
Restrict server binds strictly to the loopback interface (127.0.0.1 or ::1).

Remediation Steps:

Modify the Gemfile to require 'mcp' version 0.23.0 or higher.
Run 'bundle update mcp' to apply the update.
Optionally define 'allowed_hosts' and 'allowed_origins' parameters if binding to non-loopback interfaces.

References

Read the full report for CVE-2026-63118 on our website for more details including interactive diagrams and full exploit analysis.

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