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

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

• CISA KEV Not listed.
• EPSS 0.0047, the 41st percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

This record came from the full history of the GitHub Advisory Database, published 27 Mar 2026. New records show when each source confirmed them.

Sources

• GitHub Advisory Database: GHSA-qvqr-5cv7-wh35
• OSV: GHSA-qvqr-5cv7-wh35
• github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35
• nvd.nist.gov/vuln/detail/CVE-2026-33946
• github.com/modelcontextprotocol/ruby-sdk/commit/db40143402d65b4fb6923cec42d2d72cb89b3874
• hackerone.com/reports/3556146
• github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L93-L97
• github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L281C1-L288C2
• github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py#L680-L685
• github.com/modelcontextprotocol/ruby-sdk
• github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb
• github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.9.2
• github.com/rubysec/ruby-advisory-db/blob/master/gems/mcp/CVE-2026-33946.yml

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.