CVE-2026-33946
CVE-2026-33946 is a medium-severity vulnerability in Lfprojects Mcp Ruby Sdk with a CVSS 3.x base score of 5.9. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-384.
Key facts
- Severity: Medium (CVSS 3.x base score 5.9)
- CVSS v4: 8.2
- EPSS exploit prediction: 1% (44th percentile)
- Actively exploited: Not listed in CISA KEV
- EU (EUVD) id: EUVD-2026-16866
- Weakness: CWE-384
- Affected product: Lfprojects Mcp Ruby Sdk
- Published:
- Last modified:
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.
Frequently asked questions
- What is CVE-2026-33946?
- 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.
- How severe is CVE-2026-33946?
- CVE-2026-33946 has a CVSS 3.x base score of 5.9, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.
- Is CVE-2026-33946 being actively exploited?
- It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (44th percentile), an estimate of the probability of exploitation in the next 30 days.
- What products are affected by CVE-2026-33946?
- CVE-2026-33946 affects Lfprojects Mcp Ruby Sdk. See the affected-products list for the exact vulnerable versions.
- How do I fix CVE-2026-33946?
- Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
- Does CVE-2026-33946 have an EU (EUVD) identifier?
- Yes. CVE-2026-33946 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-16866.
- When was CVE-2026-33946 published?
- CVE-2026-33946 was published on 2026-03-27 and last updated on 2026-06-17.
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
Affected products (1)
- cpe:2.3:a:lfprojects:mcp_ruby_sdk:*:*:*:*:*:*:*:*
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