CVE-2026-33946

Summary

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 Software

Weaknesses

• CWE-384: CWE-384: Session Fixation
• CWE-639: CWE-639: Authorization Bypass Through User-Controlled Key

ADP Enrichment

CISA ADP Vulnrichment

• SSVC:
• Exploitation: poc
  • Automatable: yes
  • Technical Impact: partial

Additional References

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

References

• https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35
• https://github.com/modelcontextprotocol/ruby-sdk/commit/db40143402d65b4fb6923cec42d2d72cb89b3874
• https://hackerone.com/reports/3556146
• 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/releases/tag/v0.9.2

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