CVE-2026-67431 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 bind a session ID to a session owner, allowing an attacker with a stolen session ID to send tools/call requests that execute in the victim's session. 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 secure communication between various AI model contexts. The vulnerability resides within the StreamableHTTPTransport class of the mcp gem, specifically in versions prior to 0.23.0 where session management fails to properly associate session identifiers with their legitimate owners. This flaw constitutes a severe authorization bypass issue that fundamentally undermines the security model of the protocol implementation.
The technical flaw manifests as a lack of session binding verification within the transport layer, allowing malicious actors to exploit stolen session IDs through what is effectively a session hijacking attack vector. When an attacker successfully steals a valid session ID, they can impersonate the legitimate user and execute arbitrary tools or call requests within the victim's session context. This vulnerability directly violates fundamental security principles of authentication and authorization, as the system fails to validate that the session ID belongs to the requesting entity.
The operational impact of this vulnerability extends beyond simple privilege escalation, creating a persistent threat vector where attackers can maintain prolonged access to sensitive AI model contexts. The stolen session could enable unauthorized data access, execution of malicious commands, or manipulation of model responses within the victim's authenticated session. This represents a critical weakness in the authentication flow that has been classified under CWE-287, which deals with improper authentication mechanisms and session management failures.
The security implications are particularly concerning given that this vulnerability affects the core transport mechanism used for communication between Model Context Protocol servers and clients. Attackers could leverage this flaw to execute arbitrary code within the context of legitimate user sessions, potentially leading to data breaches or unauthorized access to sensitive AI processing capabilities. The fix implemented in version 0.23.0 addresses this by properly binding session IDs to their respective owners through enhanced session validation mechanisms.
Mitigation strategies should prioritize immediate upgrading to version 0.23.0 or later, which includes proper session binding enforcement. Organizations should also implement additional monitoring for suspicious session activity and consider implementing session timeout mechanisms. The vulnerability aligns with ATT&CK technique T1566 related to credential harvesting and session hijacking, emphasizing the need for robust session management practices. Security teams should conduct thorough reviews of all session handling code within their Model Context Protocol implementations to ensure similar binding mechanisms are properly enforced throughout their systems.
This vulnerability demonstrates the critical importance of proper session management in distributed systems, particularly those involving AI model contexts where the stakes for unauthorized access can be extremely high. The fix represents a necessary security hardening measure that addresses fundamental gaps in authentication verification and session ownership validation within the MCP Ruby SDK implementation.