High severityNVD Advisory· Published Jul 29, 2026· Updated Jul 30, 2026
MCP Ruby SDK: Ruby SSE Session Poisoning
CVE-2026-67431
Description
MCP Ruby SDK is the official Ruby SDK for Model Context Protocol servers and clients. Prior to 0.23.0, MCP::Server::Transports::StreamableHTTPTransport in the mcp gem does not bind a session ID to a session owner, allowing an attacker with a stolen session ID to send tools/call requests that execute in the victim's session. This issue is fixed in version 0.23.0.
Affected packages
Versions sourced from the GitHub Security Advisory.
| Package | Affected versions | Patched versions |
|---|---|---|
mcpRubyGems | < 0.23.0 | 0.23.0 |
Affected products
2- Range: <0.23.0
- Range: <0.23.0
Patches
Vulnerability mechanics
References
5- github.com/advisories/GHSA-5p9g-j988-pcwvghsaADVISORY
- nvd.nist.gov/vuln/detail/CVE-2026-67431ghsaADVISORY
- github.com/modelcontextprotocol/ruby-sdk/commit/35466605319a34e4c7808712ae9bb1ca1afb2356ghsax_refsource_MISCWEB
- github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0ghsax_refsource_MISCWEB
- github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwvghsax_refsource_CONFIRMWEB
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