MCP Ruby SDK Session Exhaustion via Unbounded StreamableHTTPTransport Sessions

Technical Analysis

The StreamableHTTPTransport component in the mcp Ruby gem creates a new ServerSession object on each initialize request but has no default session expiration or eviction policy. An unauthenticated or low-privilege remote client can flood the server with repeated initialize calls, causing unbounded accumulation of session state in memory. This crosses from a single protocol-layer interaction into full server availability impact, since exhausted memory can crash or degrade the MCP server process, disrupting all agents and tools relying on that server. The entry point is the standard MCP HTTP transport initialization handshake, requiring no special privileges beyond network access to the endpoint.

Affected Systems

mcp (Ruby SDK); protocols: MCP

Detection Signatures

• High rate of repeated 'initialize' requests from a single client or IP within a short window
• Steadily increasing process memory (RSS) on MCP server hosts correlated with session count
• Growing number of active ServerSession objects with no corresponding termination/close events
• Absence of session expiry/eviction log entries over long uptime periods

Remediation Steps

1. 1

   Upgrade mcp gem

   Update the mcp Ruby gem to version 0.23.0 or later, which introduces session expiration.

2. 2

   Rate limit initialize requests

   Apply per-client rate limiting or connection throttling on MCP server endpoints to prevent rapid repeated session creation.

3. 3

   Monitor resource usage

   Add alerting on MCP server memory growth and active session counts to detect exhaustion attempts before crash.

4. 4

   Enforce session caps

   Configure maximum concurrent session limits and idle timeouts as defense-in-depth even after patching.