MCP Ruby SDK publishes five advisories covering session ownership and memory exhaustion, all fixed in 0.23.0

The official MCP Ruby SDK published five security advisories on July 8, all affecting the mcp RubyGems package at 0.22.0 and earlier and all fixed in 0.23.0. The fix shipped July 7, the day before the advisories, the normal coordinated-disclosure ordering.

Two advisories are rated High in the advisory records. CVE-2026-67431 (CVSS 8.3), titled Ruby SSE Session Poisoning, states that an actor holding a session ID can execute tools via /messages/{session-id} with the responses delivered to the legitimate user's SSE stream, a session-ownership failure on the legacy SSE transport. CVE-2026-67432 (CVSS 7.5) covers an unbounded JSON-RPC request body: the read occurs before authentication checks, and the advisory documents a single 512 MB request driving process memory from 44 MB to 1.66 GB.

The three Moderate advisories cover the Streamable HTTP transport lacking DNS-rebinding (Host and Origin) protection (CVE-2026-63118, CVSS 6.9), an unbounded line buffer in the stdio transports attributed to IO#gets used without a limit argument (CVE-2026-63119, CVSS 6.2), and unbounded session retention in StreamableHTTPTransport (CVE-2026-67430, CVSS 5.3).

Taken together, two of the five concern binding a request to the session or principal that owns it, and three concern unbounded resource consumption. Severity labels and scores here are as the GitHub advisory records state them. Operators running the Ruby SDK's HTTP transports should upgrade to 0.23.0 or later; the SDK's subsequent releases continued through v1.1.0 in early August.