GHSA-QVQR-5CV7-WH35

Vulnerability from github – Published: 2026-03-27 18:36 – Updated: 2026-03-30 20:10
VLAI?
Summary
MCP Ruby SDK: Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay
Details

Summary

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.

Details

Root Cause The StreamableHTTPTransport implementation stores only one SSE stream object per session ID and lacks:

  • Session-to-user identity binding
  • Ownership validation when establishing SSE connections
  • Protection against multiple simultaneous connections to the same session

PoC

Vulnerable Code

File: streamable_http_transport.rb - L336-L339:

def store_stream_for_session(session_id, stream)
  @mutex.synchronize do
    if @sessions[session_id]
      @sessions[session_id][:stream] = stream  # OVERWRITES existing stream
    else
      stream.close
    end
  end
end

Attack Scenario

Step 1: Legitimate Session Establishment

POST / (initialize) → receives session_id: "abc123"
GET / with Mcp-Session-Id: abc123 → SSE stream connected

Step 2: Session ID Compromise

  • An attacker obtains the session ID through various means (out of scope for this analysis)

Step 3: Stream Hijacking

GET / with Mcp-Session-Id: abc123 
@sessions["abc123"][:stream] = attacker_stream `# Victim's stream is REPLACED (silently disconnected)

Step 4: Data Interception

  • ALL subsequent tool responses/notifications go to the attacker
  • The legitimate user receives no data and has no indication of the hijacking

Technical Details

The vulnerability happens:

Client 1 connects (GET request)

proc do |stream1|  # ← Rack server provides stream1 for client 1
 @sessions[session_id][:stream] = stream1  # Stored
end

Client 2 connects with SAME session ID (Attack!)

proc do |stream2|  # ← Rack provides stream2 for client 2
 @sessions[session_id][:stream] = stream2  # REPLACES stream1!
end

Now when the server sends notifications:

@sessions[session_id][:stream].write(data)  # Goes to stream2 (attacker!)
# stream1 (victim) receives nothing

Comparison: Python SDK Protection

The Python SDK prevents this vulnerability by rejecting duplicate SSE connections:

Refer: https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py#L680-L685

if GET_STREAM_KEY in self._request_streams:  # pragma: no cover
            response = self._create_error_response(
                "Conflict: Only one SSE stream is allowed per session",
                HTTPStatus.CONFLICT,
            )

When a duplicate connection attempt is detected, the Python SDK returns an HTTP 409 Conflict error, protecting the existing connection.

Recommended Mitigations For SDK Maintainers

  • Implement User Binding: All SDKs should bind session IDs to authenticated user identities where possible. Currently only, go-sdk and csharp-sdk do user binding.
  • Ruby SDK: Prevent Duplicate Connections: Implement checks to reject or handle multiple simultaneous connections to the same session
  • Improve Documentation: Provide clear guidance on secure session management implementation for SDK consumers

Steps To Reproduce:

Please find attached two python client files demonstrating the attack

Terminal 1: 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

Terminal 2:

python3 legitimate_client_ruby_server.py

What happens:

  • The client connects and prints the session ID
  • Press Enter to start the SSE stream
  • Notifications start appearing every 3 seconds as the client makes a tool call

Terminal 3 (while the legitimate client is running):

python3 attacker_client_ruby_server.py <SESSION_ID>

Replace <SESSION_ID> with the ID from Terminal 2.

What happens immediately:

  • Terminal 2 (Legitimate): Stops receiving notifications, shows disconnect message
  • Terminal 3 (Attacker): Starts receiving ALL the tool call responses

Impact

While the absence of user binding may not pose immediate risks if session IDs are not used to store sensitive data or state, the fundamental purpose of session IDs is to maintain stateful connections. If the SDK or its consumers utilize session IDs for sensitive operations without proper user binding controls, this creates a potential security vulnerability. For example: In the case of the Ruby SDK, the attacker was able to hijack the stream and receive all the tool responses belonging to the victim. The tool responses can be sensitive confidential data.

Additional Details

Session Hijacking Protection in MCP Implementations

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

Current Implementation Status Across SDKs

Of the 10 official MCP SDKs, only the following implementations bind session IDs to user-specific information:

  1. csharp-sdk - https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L93-L97
  2. Go-sdk - https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L281C1-L288C2

attacker_client_ruby_server.py legitimate_client_ruby_server.py The remaining SDKs do not implement session-to-user binding. Most implementations only verify that a session ID exists, without validating ownership. Additionally, SDK documentation does not provide clear guidance on implementing secure session management, leaving security responsibilities unclear for SDK consumers.

Severity ?
8.2 (High)
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.9.1"
      },
      "package": {
        "ecosystem": "RubyGems",
        "name": "mcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.9.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33946"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-384",
      "CWE-639"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-27T18:36:45Z",
    "nvd_published_at": "2026-03-27T22:16:21Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\nThe Ruby SDK\u0027s [streamable_http_transport.rb](https://github.com/modelcontextprotocol/ruby-sdk/blob/main/lib/mcp/server/transports/streamable_http_transport.rb) implementation contains a session hijacking vulnerability. An attacker who obtains a valid session ID can completely hijack the victim\u0027s Server-Sent Events (SSE) stream and intercept all real-time data.\n\n### Details\n**Root Cause**\nThe StreamableHTTPTransport implementation stores only one SSE stream object per session ID and lacks:\n\n- Session-to-user identity binding\n- Ownership validation when establishing SSE connections\n- Protection against multiple simultaneous connections to the same session\n\n### PoC\n\n#### Vulnerable Code\n\n**File**: streamable_http_transport.rb - [L336-L339](https://github.com/modelcontextprotocol/ruby-sdk/blob/main/lib/mcp/server/transports/streamable_http_transport.rb#L336-L339):\n\n```\ndef store_stream_for_session(session_id, stream)\n  @mutex.synchronize do\n    if @sessions[session_id]\n      @sessions[session_id][:stream] = stream  # OVERWRITES existing stream\n    else\n      stream.close\n    end\n  end\nend\n```\n#### Attack Scenario\n**Step 1**: Legitimate Session Establishment\n```\nPOST / (initialize) \u2192 receives session_id: \"abc123\"\nGET / with Mcp-Session-Id: abc123 \u2192 SSE stream connected\n```\nStep 2: Session ID Compromise\n\n- An attacker obtains the session ID through various means (out of scope for this analysis)\n\n**Step 3**: Stream Hijacking\n\n```\nGET / with Mcp-Session-Id: abc123 \n@sessions[\"abc123\"][:stream] = attacker_stream `# Victim\u0027s stream is REPLACED (silently disconnected)\n```\n\n**Step 4**: Data Interception\n\n- ALL subsequent tool responses/notifications go to the attacker\n- The legitimate user receives no data and has no indication of the hijacking\n\n#### Technical Details\n\nThe vulnerability happens:\n\n**Client 1 connects (GET request)**\n\n```\nproc do |stream1|  # \u2190 Rack server provides stream1 for client 1\n @sessions[session_id][:stream] = stream1  # Stored\nend\n```\n\n**Client 2 connects with SAME session ID (Attack!)**\n```\nproc do |stream2|  # \u2190 Rack provides stream2 for client 2\n @sessions[session_id][:stream] = stream2  # REPLACES stream1!\nend\n```\n\n**Now when the server sends notifications:**\n\n```\n@sessions[session_id][:stream].write(data)  # Goes to stream2 (attacker!)\n# stream1 (victim) receives nothing\n```\n\n**Comparison: Python SDK Protection**\n\nThe Python SDK prevents this vulnerability by rejecting duplicate SSE connections:\n\n**Refer**: https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py#L680-L685\n\n```\nif GET_STREAM_KEY in self._request_streams:  # pragma: no cover\n            response = self._create_error_response(\n                \"Conflict: Only one SSE stream is allowed per session\",\n                HTTPStatus.CONFLICT,\n            )\n```\n\nWhen a duplicate connection attempt is detected, the Python SDK returns an HTTP 409 Conflict error, protecting the existing connection.\n\n**Recommended Mitigations**\n**For SDK Maintainers**\n\n- Implement User Binding: All SDKs should bind session IDs to authenticated user identities where possible. Currently only, go-sdk and csharp-sdk do user binding.\n- **Ruby SDK**: Prevent Duplicate Connections: Implement checks to reject or handle multiple simultaneous connections to the same session\n- **Improve Documentation**: Provide clear guidance on secure session management implementation for SDK consumers\n\n### Steps To Reproduce:\n\nPlease find attached two python client files demonstrating the attack\n\n**Terminal 1:**\n`ruby streamable_http_server.rb`\n\nMakes use of https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb\nThis server has a tool call notification_tool which the clients call\n\n**Terminal 2:**\n\n`python3 legitimate_client_ruby_server.py`\n\n**What happens:**\n\n- The client connects and prints the session ID\n- Press Enter to start the SSE stream\n- Notifications start appearing every 3 seconds as the client makes a tool call\n\n**Terminal 3 (while the legitimate client is running):**\n\n`python3 attacker_client_ruby_server.py \u003cSESSION_ID\u003e`\n\nReplace `\u003cSESSION_ID\u003e` with the ID from Terminal 2.\n\n**What happens immediately:**\n\n- Terminal 2 (Legitimate): Stops receiving notifications, shows disconnect message\n- Terminal 3 (Attacker): Starts receiving ALL the tool call responses\n\n### Impact\nWhile the absence of user binding may not pose immediate risks if session IDs are not used to store sensitive data or state, the fundamental purpose of session IDs is to maintain stateful connections. If the SDK or its consumers utilize session IDs for sensitive operations without proper user binding controls, this creates a potential security vulnerability. For example: In the case of the Ruby SDK, the attacker was able to hijack the stream and receive all the tool responses belonging to the victim. The tool responses can be sensitive confidential data.\n\n### Additional Details\n#### Session Hijacking Protection in MCP Implementations\nThe MCP specification recommends - \"MCP servers SHOULD bind session IDs to user-specific information\".\n\n#### Current Implementation Status Across SDKs\n\nOf the 10 official MCP SDKs, only the following implementations bind session IDs to user-specific information:\n\n1. csharp-sdk - https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L93-L97\n2. Go-sdk - https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L281C1-L288C2\n\n[attacker_client_ruby_server.py](https://github.com/user-attachments/files/25408485/attacker_client_ruby_server.py)\n[legitimate_client_ruby_server.py](https://github.com/user-attachments/files/25408486/legitimate_client_ruby_server.py)\nThe remaining SDKs do not implement session-to-user binding. Most implementations only verify that a session ID exists, without validating ownership. Additionally, SDK documentation does not provide clear guidance on implementing secure session management, leaving security responsibilities unclear for SDK consumers.",
  "id": "GHSA-qvqr-5cv7-wh35",
  "modified": "2026-03-30T20:10:39Z",
  "published": "2026-03-27T18:36:45Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/security/advisories/GHSA-qvqr-5cv7-wh35"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-33946"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/commit/db40143402d65b4fb6923cec42d2d72cb89b3874"
    },
    {
      "type": "WEB",
      "url": "https://hackerone.com/reports/3556146"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/src/ModelContextProtocol.AspNetCore/SseHandler.cs#L93-L97"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/go-sdk/blob/main/mcp/streamable.go#L281C1-L288C2"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py#L680-L685"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/blob/main/examples/streamable_http_server.rb"
    },
    {
      "type": "WEB",
      "url": "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v0.9.2"
    },
    {
      "type": "WEB",
      "url": "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/mcp/CVE-2026-33946.yml"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "MCP Ruby SDK: Insufficient Session Binding Allows SSE Stream Hijacking via Session ID Replay"
}


Log in or create an account to share your comment.




Tags
Taxonomy of the tags.




Sightings

Author Source Type Date

Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



Detection rules are retrieved from Rulezet.