Session Fixation Affecting mcp package, versions <0.9.2
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
Do your applications use this vulnerable package?
In a few clicks we can analyze your entire application and see what components are vulnerable in your application, and suggest you quick fixes.
Test your applications- Snyk IDSNYK-RUBY-MCP-15809786
- published29 Mar 2026
- disclosed27 Mar 2026
- creditsrikanthramu
Introduced: 27 Mar 2026
NewCVE-2026-33946 (opens in a new tab)How to fix?
Upgrade mcp to version 0.9.2 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 Session Fixation through the store_stream_for_session process in lib/mcp/server/transports/streamable_http_transport.rb. An attacker can intercept all subsequent Server-Sent Events (SSE) tool responses and notifications by replaying a valid session ID in a second GET request. A duplicate SSE connection could silently replace the victim’s active stream, leaving the legitimate client disconnected with no indication, while server-to-client messages are delivered to the attacker instead.
Notes
- The issue affects the Streamable HTTP transport when a session already has an active SSE stream and a second connection is accepted for the same session ID.