What’s Vulnerable › Vulnerabilities

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

• Severity: medium (CVSS 5.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-03-27 · updated 2026-10-26
• Weakness: CWE-384

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.

Affected products

• modelcontextprotocol ruby-sdk: before 0.9.2 — fixed in 0.9.2

How to fix

Upgrade to a fixed version: modelcontextprotocol ruby-sdk 0.9.2. Check the vendor advisory for mitigations.

References

• https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L93-L97
• https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L281C1-L288C2
• https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py#L680-L685
• https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb
• https://github.com/modelcontextprotocol/ruby-sdk/commit/db40143402d65b4fb6923cec42d2d72cb89b3874
• https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.9.2
• https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35
• https://hackerone.com/reports/3556146
• https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35

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