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:

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

References

• GHSA-rjr6-rcgv-9m7m: DNS-rebinding and cross-origin request execution in MCP Ruby SDK
• CVE-2026-63118 Record
• NVD - CVE-2026-63118 Detail

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