CVE-2026-63118 in ruby-sdk
Summary
by MITRE • 07/29/2026
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.
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Analysis
by VulDB Data Team • 07/29/2026
The MCP Ruby SDK represents a critical component in implementing Model Context Protocol servers and clients, serving as the official ruby implementation for facilitating communication between various AI models and their contextual environments. The vulnerability resides within the StreamableHTTPTransport class of the mcp gem, specifically affecting versions prior to 0230. This transport mechanism handles HTTP communication for MCP server implementations, making it a potential entry point for attackers seeking unauthorized access to locally running services. The flaw demonstrates a fundamental lack of input validation in the HTTP header processing pipeline.
The technical implementation of this vulnerability stems from insufficient validation of HTTP Host and Origin headers within the StreamableHTTPTransport class. When an MCP server operates locally on a machine with network access, malicious actors can exploit this weakness through DNS rebinding techniques. The attack vector leverages the fact that the transport layer fails to enforce proper header validation, allowing external web pages to establish connections to the local server instance. This creates a dangerous scenario where legitimate HTTP requests from untrusted origins can bypass normal security boundaries that would typically prevent such cross-origin access.
The operational impact of this vulnerability extends beyond simple unauthorized access, as it enables sophisticated attacks through DNS rebinding methods that can circumvent standard network security measures. An attacker could craft a malicious webpage that, when visited by a user with a locally running MCP server, establishes a connection to the local service and invokes exposed tools without proper authentication or authorization. This represents a significant privilege escalation risk for systems running MCP servers in development or testing environments where local access might be less restricted. The vulnerability effectively allows remote code execution or data manipulation through the exposed tool interfaces.
This issue aligns with CWE-295, which addresses improper certificate validation, and specifically relates to CWE-284, concerning improper access control mechanisms. The vulnerability also maps to ATT&CK technique T1071.004 for application layer protocol usage, particularly HTTP communications. Organizations using MCP Ruby SDK versions prior to 0230 should immediately implement mitigations including updating to the fixed version 0230 or applying manual patches that enforce proper HTTP header validation. Network-level protections such as firewalls and access control lists can provide additional defense in depth, though these measures are secondary to addressing the root cause within the application itself. The fix in version 0230 implements mandatory validation of Host and Origin headers, ensuring that only requests from authorized domains or IP addresses can successfully establish connections to the MCP server transport layer.
The broader implications for software security highlight the critical importance of input validation in network-facing components, particularly those handling HTTP communications. This vulnerability demonstrates how seemingly minor oversights in header validation can create substantial security risks when combined with DNS rebinding techniques. Organizations should conduct comprehensive audits of their HTTP transport implementations and ensure all network-facing services perform proper authentication and authorization checks before processing requests from external sources. Regular security updates and dependency management practices become essential for maintaining secure configurations, especially in development environments where local servers may be accessible to untrusted users or networks.