vulnerability / NVI-00015365 / Revision 1
MCP Ruby SDK: Ruby SSE Session Poisoning
GitHub lists this reviewed advisory for mcp. Published version ranges and source prerequisites are shown below; this is not an inspection of a visitor’s application.
This report has not inspected your inventory, configuration, deployed build, reachability or evidence. A catalog listing and a severity score do not establish exposure or compromise.
01 / Meaning
Why it matters
Published affected context helps an owner decide what evidence to gather. This report has not examined your application.
02 / Applicability
The affected context
Affected versions, configuration prerequisites and known exclusions are public. Confirm each one against the deployed service.
mcp
source listed<= 0.22.0
### Summary **Vulnerability**: Missing Session Ownership Validation in the Ruby MCP SDK's Streamable and SSE HTTP transport implementation. Any attacker with a stolen session ID can execute tools with the victim's session. This is a silent attack - the victim's session is compromised and being used for unauthorized actions, but it is hard to know for the victim ### Details https://github.com/modelcontextprotocol/ruby-sdk/blob/main/lib/mcp/server/transports/streamable_http_transport.rb#L260-L278 **Victim** starts a legitimate MCP session and receives session ID abc-123 **Attacker** obtains the session ID (various means - network sniffing, logs, etc. out of scope for this analysis) **Attacker** sends POST to /messages/abc-123 with a tool call **Server** accepts the request (no ownership validation!) **Server** executes the tool and sends response to victim's SSE stream **Victim** receives attacker's response, thinking it's legitimate ### PoC attacker_client.py legitimate_client.py **Prerequisites** 1. Python 3.8+ 2. Install dependenc
03 / Response
What to do next
- investigate
Identify the exact component and deployed version, examine the linked publisher’s conditions, and retain evidence of any mitigation.
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04 / Limits
What this report cannot establish
- Customer inventory, deployment, reachability and exploitation have not been inspected.
05 / Provenance
Sources you can inspect
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- GHSA-5p9g-j988-pcwv ↗
GitHub Advisory Database · vendor advisory
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Public revision history
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- Revision 1 ·
First indexed by Null; source disclosure date is shown separately.