MCP Ruby SDK: Ruby SSE Session Poisoning
Priority HIGH 77CWE-284Published · Updated
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Attack requirements
- Present
- Impact type
- Security bypass
- Confidentiality · integrity · availability
- None · High · Low
- Fix status
- Fixed
Attack profile from CVSS 4.0 · CNA. Fix status from CNA and GitHub. Impact type from CWE-284.
Summary
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.
Why it matters now
Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00363 30th percentileScored 2026-11-07
Fix Fixed ruby-sdk (modelcontextprotocol): 0.23.0mcp (RubyGems): 0.23.0
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- Poc
- Automatable
- Yes
- Technical impact
- Total
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| MLflow | ruby-sdk | before 0.23.0 | 0.23.0 | Patch available | CNA |
| — | mcp (RubyGems) | before 0.23.0 | 0.23.0 | Patch available | GHSA-5p9g-j988-pcwv |
- ruby-sdk VendorMLflow Affected before 0.23.0 Fixed in 0.23.0 Patch available CNA
- mcp (RubyGems) Vendor— Affected before 0.23.0 Fixed in 0.23.0 Patch available GHSA-5p9g-j988-pcwv
Remediation
- Fix · CNA ruby-sdk (modelcontextprotocol) Fixed in 0.23.0
- Fix · GHSA-5p9g-j988-pcwv mcp (RubyGems) Fixed in 0.23.0 Advisory →
Timeline
- 2026-10-02 Threatist record created from CVE-2026-67431
- 2026-10-03 Alias added: GHSA-5p9g-j988-pcwv
References
Other 4
- https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-5p9g-j988-pcwv
- https://github.com/modelcontextprotocol/ruby-sdk/commit/35466605319a34e4c7808712ae9bb1ca1afb2356
- https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.23.0
- https://github.com/modelcontextprotocol/ruby-sdk
More MLflow vulnerabilities
- TI-2026-46366 Server-side request forgery in mlflow KEVCVSS 3.1CRITICAL 9.3
- TI-2023-03518 Absolute Path Traversal in mlflow/mlflow CVSS 3.1CRITICAL 10.0
- TI-2023-01113 Path Traversal: '\..\filename' in mlflow/mlflow CVSS 3.1CRITICAL 9.8
- TI-2025-31759 Redis Lua Use-After-Free may lead to remote code execution CVSS 3.1CRITICAL 10.0
- TI-2026-81527 Code execution via scorer deserialization in MLflow servers CVSS 3.1CRITICAL 9.8
- TI-2025-14028 Command Injection in mlflow/mlflow CVSS 3.1CRITICAL 10.0
Sources
- CVE-2026-67431 · CVE Program
- CVE-2026-67431 · NVD
- FIRST EPSS
- GHSA-5p9g-j988-pcwv · GitHub Advisory Database · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences