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CVE-2026-67431

MCP Ruby SDK: Ruby SSE Session Poisoning

8.3 HighFix available

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.

Attack profile

How easy it is to exploit

Attack vector
Network
Complexity
Low
Privileges needed
None
User interaction
None

What an attacker can affect

Confidentiality
None
Integrity
High
Availability
Low

Exploit likelihood

0.36%Up 0.1 points since Jul 30, 2026
90%50%10%1%

What each source reports

CVE List Score used 8.3

MCP Ruby SDK: Ruby SSE Session Poisoning

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.

Record
CVE-2026-67431
CVSS 4.0
8.3 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N
Weakness
CWE-284
Updated
2026-11-02
EUVDEUVD-2026-50501 8.3

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.

Record
EUVD-2026-50501
CVSS 4.0
8.3 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N
Updated
2026-11-02
GitHub AdvisoriesGHSA-5p9g-j988-pcwv 8.3

MCP Ruby SDK: Ruby SSE Session Poisoning

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 dependencies: requests

Running the Demo

  1. Terminal 1: Start the Ruby MCP Server

ruby streamable_http_server.rb

Makes use of https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb This server has a tool call notification_tool which the clients call

  1. Terminal 2: Start Victim Client

python3 legitimate_client.py

  1. Terminal 3 - Attacker Client:

Copy the session ID from Terminal 1 and run:

python3 attacker_client.py abc-123-def-456
  1. Back to Terminal 2 - Victim sees the injected response:

Impact

  • Integrity: HIGH - Attacker can execute unauthorized tools and modify state
  • Availability: LOW - Attacker can disrupt victim's session with injected responses

Additional Details

Session Hijacking Protection in MCP Implementations

The MCP specification recommends - "MCP servers SHOULD bind session IDs to user-specific information".

User Binding - Comparison other SDKs

csharp-sdk

Go-sdk

Comparison with the Stream Replacement vulnerability

Stream Replacement https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35

  • Target: SSE stream connection
  • Attack Vector: GET /mcp
  • What happens to Victim: The connection is disconnected and doesn't receive reponses

Session Poisoning

  • Target: Tool execution
  • Attack Vector: POST /mcp
  • What happens to Victim: The connection stays connected and receives responses of tool calls by attacker
Record
GHSA-5p9g-j988-pcwv
CVSS 4.0
8.3 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N
Weakness
CWE-284
Updated
2026-11-02
NVD 8.3

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.

Record
CVE-2026-67431
CVSS 4.0
8.3 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Weakness
CWE-284
Updated
2026-11-02
OSV 8.3

MCP Ruby SDK: Ruby SSE Session Poisoning

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.

Record
CVE-2026-67431
CVSS 4.0
8.3 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N
Weakness
CWE-284
Updated
2026-11-02
OSVGHSA-5p9g-j988-pcwv 8.3

MCP Ruby SDK: Ruby SSE Session Poisoning

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 dependencies: requests

Running the Demo

  1. Terminal 1: Start the Ruby MCP Server

ruby streamable_http_server.rb

Makes use of https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb This server has a tool call notification_tool which the clients call

  1. Terminal 2: Start Victim Client

python3 legitimate_client.py

  1. Terminal 3 - Attacker Client:

Copy the session ID from Terminal 1 and run:

python3 attacker_client.py abc-123-def-456
  1. Back to Terminal 2 - Victim sees the injected response:

Impact

  • Integrity: HIGH - Attacker can execute unauthorized tools and modify state
  • Availability: LOW - Attacker can disrupt victim's session with injected responses

Additional Details

Session Hijacking Protection in MCP Implementations

The MCP specification recommends - "MCP servers SHOULD bind session IDs to user-specific information".

User Binding - Comparison other SDKs

csharp-sdk

Go-sdk

Comparison with the Stream Replacement vulnerability

Stream Replacement https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35

  • Target: SSE stream connection
  • Attack Vector: GET /mcp
  • What happens to Victim: The connection is disconnected and doesn't receive reponses

Session Poisoning

  • Target: Tool execution
  • Attack Vector: POST /mcp
  • What happens to Victim: The connection stays connected and receives responses of tool calls by attacker
Record
GHSA-5p9g-j988-pcwv
CVSS 4.0
8.3 HighCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N
Weakness
CWE-284
Updated
2026-11-02

Affected products

Affected packages

Affected packages
PackageAffectedFixed
mcp
RubyGems
 through 0.22.0
0.23.0
mcp
RubyGems
 before 0.23.0, plus listed versions
0.1.00.10.00.11.00.12.00.13.0
Show 20 moreShow less
0.14.00.15.00.16.00.17.00.18.00.19.00.2.00.20.00.21.00.22.00.3.00.4.00.5.00.6.00.7.00.7.10.8.00.9.00.9.10.9.2
0.23.0

History

History
RecordedChangedSources
first indexedCVE List, EUVD, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2026-67431 is a high vulnerability (CVSS 8.3) in Modelcontextprotocol Ruby-sdk, published July 29, 2026. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 0.36%, the 30th percentile. Fixed in mcp 0.23.0.

References

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