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CVE-2026-67431 is an authorization flaw in the official MCP Ruby SDK affecting the mcp gem before version 0.23.0. The issue exists in MCP::Server::Transports::StreamableHTTPTransport, where server-side request handling validates only that a supplied session identifier exists, but does not verify that the requester is the legitimate owner of that session. Because session IDs are not bound to session ownership, an attacker who obtains a valid active session ID can submit crafted JSON-RPC requests, including tools/call operations, that are executed in the victim's session context. The flaw can also be abused to send cancellation notifications that interfere with the victim's in-progress actions. Version 0.23.0 addresses the issue by adding session request validation, including origin checking and support for application-level session ownership validation.
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An authorization flaw in MCP::Server::Transports::StreamableHTTPTransport where possession of a valid session ID is sufficient to dispatch JSON-RPC tool calls because pre-0.23.0 logic verifies only session existence and not session ownership. This can enable unauthorized backend tool execution and denial of service against a victim session.
An authorization/session-binding flaw in the MCP Ruby SDK where StreamableHTTPTransport does not bind a session ID to its owner, enabling an attacker with a stolen session ID to execute tools/call requests in the victim's session.
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Cross-references every affected SKU, including bundled OEM variants.
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