CVE-2026-33946

MEDIUM

MCP Ruby SDK: Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay

Title source: cna

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.

References (8)

[X_Refsource_Confirm] https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35

[X_Refsource_Misc] https://github.com/modelcontextprotocol/ruby-sdk/commit/db40143402d65b4fb6923cec42d2d72cb89b3874

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[X_Refsource_Misc] https://hackerone.com/reports/3556146

[X_Refsource_Misc] https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L93-L97

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[X_Refsource_Misc] https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L281C1-L288C2

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[X_Refsource_Misc] https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py#L680-L685

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[X_Refsource_Misc] https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb

[X_Refsource_Misc] https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.9.2

Scores

CVSS v3 5.9

EPSS 0.0004

EPSS Percentile 13.8%

Attack Vector NETWORK

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

CISA SSVC

Vulnrichment

Exploitation poc

Automatable yes

Technical Impact partial

Details

CWE

CWE-384 CWE-639

Status published

Products (3)

lfprojects/mcp_ruby_sdk < 0.9.2

modelcontextprotocol/ruby-sdk < 0.9.2

rubygems/mcp 0 - 0.9.2RubyGems

Published Mar 27, 2026

Tracked Since Mar 29, 2026