CVE-2026-33946: MCP Ruby SDK Session Hijacking Vulnerability via Insufficient Session Binding
Summary
CVE-2026-33946: MCP Ruby SDK: Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay. Affects mcp versions < 0.9.2. Fixed in 0.9.2. How to fix ↓
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Background
The MCP Ruby SDK is a software development kit designed to facilitate the integration of Model Context Protocol (MCP) functionalities into Ruby applications. It enables developers to implement real-time data streaming through Server-Sent Events (SSE). However, the SDK has a critical vulnerability that allows session hijacking due to insufficient session binding, which can lead to unauthorized access to sensitive data.
Vulnerability Detail
The vulnerability arises from the StreamableHTTPTransport implementation in the Ruby SDK, which only maintains a single SSE stream object per session ID. It lacks essential security measures such as binding session IDs to user identities, validating ownership during SSE connections, and preventing multiple simultaneous connections to the same session. An attacker who acquires a valid session ID can hijack the victim's SSE stream, intercepting all real-time data without the victim's knowledge. This could lead to significant data breaches, especially if sensitive information is transmitted through the stream.
How To Fix
Latest published version: 1.8.1 as of 23 Oct 2026
gem install mcp -v 1.8.1Upgrading to it may include breaking changes; the version below is the minimum fixed release for this vulnerability.
If you need the minimum change
0.9.2
This is the first release that contains the fix.
gem install mcp -v 0.9.2Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
Currently, there are no specific workarounds mentioned for this vulnerability. However, developers can mitigate risks by ensuring that session IDs are not exposed and by implementing additional security measures in their applications.
Conclusion
The MCP Ruby SDK has a critical vulnerability (CVE-2026-33946) that allows session hijacking due to insufficient session binding. It is crucial for developers to update to the patched version and implement additional security measures to protect sensitive data. Monitoring tools like Vulert can assist in identifying vulnerabilities in real-time.
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FAQ
CVE-2026-33946 is a vulnerability in the MCP Ruby SDK that allows session hijacking due to insufficient session binding, enabling attackers to intercept real-time data streams.
You can check if your application is affected by using the Vulert playground at Vulert Scanner, which monitors and alerts for open-source vulnerabilities in your software.
The vulnerability can lead to unauthorized access to sensitive data transmitted through the SSE stream, resulting in data breaches and loss of confidentiality.
You can update the MCP Ruby SDK to the patched version by running the command 'gem update mcp' in your terminal.
Developers should implement user binding to session IDs, prevent duplicate connections, and ensure that session IDs are not exposed to unauthorized users.
References
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