CVE-2026-67431: MCP Ruby SDK - Ruby SSE Session Poisoning Vulnerability
Summary
CVE-2026-67431: MCP Ruby SDK: Ruby SSE Session Poisoning. Affects mcp versions < 0.23.0. Fixed in 0.23.0. How to fix ↓
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Background
The MCP Ruby SDK is a software development kit designed for building applications that utilize the Model Context Protocol (MCP). It facilitates real-time communication and data streaming between clients and servers. The SDK is widely used for its ability to handle server-sent events (SSE) and streamable HTTP transport, enabling developers to create interactive applications. However, it has a critical vulnerability that allows session hijacking due to a lack of session ownership validation.
Vulnerability Detail
The vulnerability arises from the absence of session ownership validation in the Ruby MCP SDK's Streamable and SSE HTTP transport implementation. An attacker who obtains a valid session ID can impersonate the victim and execute unauthorized actions within the victim's session. This silent attack compromises the integrity of the victim's session, allowing the attacker to manipulate state and execute tools without detection. The attack vector involves sending a POST request to the server with the stolen session ID, leading to unauthorized tool execution and responses being sent to the victim's SSE 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
0.23.0
This is the first release that contains the fix.
gem install mcp -v 0.23.0Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
As a temporary workaround, ensure that sensitive session IDs are not logged or exposed in any way that could allow attackers to capture them. Additionally, consider implementing additional security measures such as session expiration and monitoring for unusual activity.
Conclusion
The MCP Ruby SDK has a critical vulnerability (CVE-2026-67431) that allows session hijacking due to missing session ownership validation. It is essential to update to the patched version (0.23.0) to protect against unauthorized actions. Utilizing tools like Vulert can help monitor for such vulnerabilities in real-time.
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FAQ
CVE-2026-67431 is a vulnerability in the MCP Ruby SDK that allows attackers to hijack sessions due to missing session ownership validation.
You can check if your application is affected by using Vulert, which monitors open-source vulnerabilities in real-time. Visit Vulert Scanner to analyze your manifest file.
The vulnerability can lead to unauthorized actions being executed in the victim's session, compromising data integrity and potentially affecting application functionality.
Yes, a patch is available in version 0.23.0 of the MCP Ruby SDK. It is recommended to update to this version to mitigate the vulnerability.
If an immediate update is not possible, implement workarounds such as avoiding logging session IDs and monitoring for unusual activity to reduce the risk of exploitation.
References
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