Allocation of Resources Without Limits or Throttling Affecting mcp package, versions <0.23.0
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Start learning- Snyk IDSNYK-RUBY-MCP-18017727
- published19 Jul 2026
- disclosed17 Jul 2026
- creditHE WEI
Introduced: 17 Jul 2026
New CVE NOT AVAILABLE CWE-770 (opens in a new tab)How to fix?
Upgrade mcp to version 0.23.0 or higher.
Overview
mcp is a The official Ruby SDK for Model Context Protocol servers and clients
Affected versions of this package are vulnerable to Allocation of Resources Without Limits or Throttling in the handle_initialization path of lib/mcp/server/transports/streamable_http_transport.rb, which creates a ServerSession per initialize request and, because session_idle_timeout defaults to nil, never starts the reaper that would expire them. An attacker can exhaust server memory and cause denial of service by repeatedly sending initialize requests and disconnecting, leaving an unbounded number of ServerSession objects retained with no automatic cleanup. Exploitation requires only network TCP reach to the MCP endpoint and applies under the default configuration where session_idle_timeout is unset, so no session reaper runs and the only removal path is an explicit client-issued HTTP DELETE.
Workaround
This vulnerability can be avoided by setting the session_idle_timeout parameter to a finite value when instantiating the transport, which starts the reaper thread that expires idle sessions.