CVE-2026-33946: MCP Ruby SDK: Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay

CVE-2026-33946 · Severity: medium · CVSS 5.9 · Published 2026-03-27

MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to version 0.9.2, the Ruby SDK's streamable_http_transport.rb implementation contains a session hijacking vulnerability. An attacker who obtains a valid session ID can completely hijack the victim's Server-Sent Events (SSE) stream and intercept all real-time data. Version 0.9.2 contains a patch.

Technologies: mcp (RubyGems). Vendors: Lfprojects, RubyGems.

Quick answers

Is CVE-2026-33946 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-33946 being exploited in the wild.
How severe is CVE-2026-33946?
Medium severity, with a CVSS score of 5.9, published 27 March 2026.
Which products and versions are affected?
mcp (RubyGems) ECOSYSTEM:introduced=0,fixed=0.9.2.
Is there a fix for CVE-2026-33946?
Yes. mcp (RubyGems): upgrade to 0.9.2 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
Am I affected by CVE-2026-33946?
Check whether you run Lfprojects Mcp Ruby Sdk or mcp (RubyGems) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • mcp (RubyGems): upgrade to 0.9.2 or later
    bundle update mcp

Affected products

  • Lfprojects Mcp Ruby Sdk
  • mcp (RubyGems) ECOSYSTEM:introduced=0,fixed=0.9.2

Identifiers

  • CVE-2026-33946
  • GHSA-qvqr-5cv7-wh35

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.