CVE-2026-67430 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 expire sessions by default, so repeated initialize requests retain unbounded ServerSession objects and can exhaust process memory. 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 for implementing Model Context Protocol servers and clients within Ruby applications, serving as the primary interface for communication with AI model contexts. The vulnerability resides specifically within the StreamableHTTPTransport class of the mcp gem, which handles HTTP transport layer operations for server sessions. This flaw affects versions prior to 0230 where the session management mechanism fails to implement proper session expiration policies, creating a fundamental security and resource management weakness in the system architecture.
The technical implementation defect occurs when multiple initialize requests are processed through the StreamableHTTPTransport without proper session cleanup mechanisms. Each successful initialization creates a ServerSession object that remains active in memory indefinitely, as there is no default timeout or automatic expiration mechanism implemented. This design flaw allows for unbounded memory growth since these session objects accumulate in the process heap without being released, creating a classic resource exhaustion vulnerability that can lead to application instability and potential denial of service conditions.
The operational impact of this vulnerability extends beyond simple memory consumption issues, as it creates persistent attack vectors that can be exploited by malicious actors. An attacker who can repeatedly submit initialize requests can gradually consume all available process memory, ultimately causing the server application to crash or become unresponsive. This behavior aligns with CWE-400 which categorizes unchecked resource allocation as a common weakness leading to denial of service conditions. The vulnerability essentially creates an unlimited session accumulation scenario that violates fundamental resource management principles and can be exploited without requiring elevated privileges.
From an adversarial perspective, this vulnerability maps directly to ATT&CK technique T1499.004 which involves resource exhaustion attacks targeting availability. The flaw enables attackers to perform sustained memory consumption attacks that can be difficult to detect due to their gradual nature. The lack of automatic session cleanup means that even legitimate users who maintain long-running connections can contribute to memory degradation over time, making this issue particularly insidious in production environments where sessions may remain active for extended periods.
The mitigation strategy involves updating to version 0.23.0 or later where proper session expiration mechanisms have been implemented. Organizations should also consider implementing additional monitoring and alerting around memory usage patterns to detect potential exploitation attempts. System administrators should review existing session management policies and implement manual cleanup procedures where necessary. The fix addresses the root cause by introducing default timeout mechanisms that automatically expire inactive sessions, preventing the accumulation of stale ServerSession objects in memory. This represents a standard security practice for maintaining resource integrity in server applications and aligns with industry best practices for preventing resource exhaustion attacks through proper session lifecycle management.