CVE-2026-33946

Received Received - Intake

Session Hijacking in MCP Ruby SDK SSE Stream (Pre

Publication date: 2026-03-27

Last updated on: 2026-04-02

Assigner: GitHub, Inc.

Description

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.

CVSS Scores

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Meta Information

Published

2026-03-27

Last Modified

2026-04-02

Generated

2026-07-06

AI Q&A

2026-03-28

EPSS Evaluated

2026-07-05

NVD

CVE-2026-33946

Affected Vendors & Products

Vendor	Product	Version / Range
lfprojects	mcp_ruby_sdk	to 0.9.2 (exc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-384	Authenticating a user, or otherwise establishing a new user session, without invalidating any existing session identifier gives an attacker the opportunity to steal authenticated sessions.
CWE-639	The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data.

Attack-Flow Graph

Executive Summary

The vulnerability exists in the MCP Ruby SDK's streamable_http_transport.rb implementation prior to version 0.9.2. It is a session hijacking vulnerability where an attacker who obtains a valid session ID can completely hijack the victim's Server-Sent Events (SSE) stream.

By hijacking the SSE stream, the attacker can intercept all real-time data intended for the victim.

This vulnerability was fixed in version 0.9.2 of the MCP Ruby SDK.

Impact Analysis

If exploited, this vulnerability allows an attacker to hijack a user's session and intercept all real-time data sent over the Server-Sent Events stream.

This can lead to unauthorized access to sensitive information transmitted in real-time, potentially compromising confidentiality and privacy.

Mitigation Strategies

To mitigate this vulnerability, you should upgrade the MCP Ruby SDK to version 0.9.2 or later, which contains the patch for the session hijacking issue.

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