CVE-2026-25536 Exploit & Vulnerability Analysis

1. Vulnerability Background

What is this vulnerability?

• CVE-2026-25536 is a session isolation flaw in the MCP TypeScript SDK.
• In affected versions, a single McpServer/transport instance could be reused across multiple client connections in stateless StreamableHTTPServerTransport deployments.
• This reuse allowed response data, notifications, and request mapping state from one client session to leak into another.

Why is it critical/important?

• The MCP SDK is used to build server/client integrations for the Model Context Protocol.
• Failure of session isolation undermines confidentiality and integrity of model context exchanges.
• Cross-client contamination can expose data from one client to another, especially in multi-tenant or shared hosting environments.

Affected systems/versions:

• MCP TypeScript SDK versions 1.10.0 through 1.25.3.
• Patched in version 1.26.0.
• Most relevant for deployments using StreamableHTTPServerTransport or WebStandardStreamableHTTPServerTransport in stateless mode.

2. Technical Details

Root cause analysis

• The core defect is improper transport/session reuse combined with persistent internal state.
• StreamableHTTPServerTransport and WebStandardStreamableHTTPServerTransport maintain request/stream mappings, message IDs, and in-flight request state.
• When a transport instance is reused across independent HTTP requests without a session ID generator, internal maps and handlers are not reset.
• McpServer.connect() also allowed reconnecting a protocol instance to a new transport without closing the previous connection.
• close() did not abort in-flight request handlers, allowing handlers from a closed session to continue sending messages.

Attack vector and exploitation conditions

• A vulnerable server exposes an MCP endpoint and constructs a global McpServer/transport pair.
• Multiple clients connect sequentially or concurrently to the same transport.
• In stateless mode (sessionIdGenerator undefined), the same transport object processes multiple HTTP requests.
• This results in stale stream mappings, message ID collisions, and cross-session request routing.

Security implications

• Cross-session data leakage: one client may receive responses or notifications intended for another.
• Request misrouting: message IDs can collide, causing a request from one session to be interpreted as part of another.
• Information disclosure in shared hosting or multi-tenant environments.
• Potential denial of service or session confusion when transports are reused incorrectly.

3. Patch Analysis

What code changes were made?

• Example servers were updated to create a new McpServer and transport per session/request instead of using a single long-lived instance.
  • src/examples/server/standaloneSseWithGetStreamableHttp.ts
  • src/examples/server/honoWebStandardStreamableHttp.ts
  • src/examples/server/elicitationFormExample.ts
  • src/examples/server/ssePollingExample.ts
• Core transport logic was hardened.
  • src/server/webStandardStreamableHttp.ts added _hasHandledRequest and rejects reuse of stateless transports after the first request.
• Protocol lifecycle handling was tightened.
  • src/shared/protocol.ts now rejects connect() if already connected and aborts in-flight request handlers on close().
• Tests were expanded to cover the behavior.
  • test/shared/protocol-transport-handling.test.ts
  • test/integration-tests/stateManagementStreamableHttp.test.ts
  • test/server/streamableHttp.test.ts

How do these changes fix the vulnerability?

• Ensuring per-session/per-request transport+server pairs removes shared mutable state between clients.
• The stateless transport guard enforces the intended contract: a transport without a session generator cannot be reused.
• Protocol connect() guards prevent a single protocol instance from being attached to multiple transports.
• Aborting in-flight handlers on close prevents stale handlers from emitting messages after a connection has ended.

Security improvements introduced

• Explicit fail-fast behavior for improper stateless transport reuse.
• Stronger connection lifecycle enforcement in protocol management.
• Example code now models safe patterns for stateless MCP deployments.
• Additional regression tests verify session isolation and transport handling.

4. Proof of Concept (PoC) Guide

Prerequisites for exploitation

• MCP TypeScript SDK version between 1.10.0 and 1.25.3.
• A server implementation exposing a stateless StreamableHTTPServerTransport or WebStandardStreamableHTTPServerTransport.
• Two client sessions or concurrent requests to the same transport.

Step-by-step exploitation approach

1. Deploy a vulnerable server using a global McpServer and transport.
   • Example: const server = new McpServer(...); const transport = new StreamableHTTPServerTransport(); await server.connect(transport);
2. Have one client perform initialization and open a stream or send a request.
3. Have a second client connect using the same endpoint and transport.
4. Observe whether the second client receives messages from the first client or stale responses from the prior session.

Expected behavior vs exploited behavior

• Expected: each client session is isolated; session-specific messages remain confined to the originating client.
• Exploited: one client receives notifications or responses intended for another session; request/response IDs may be misassociated; stream mappings from previous sessions may be reused.

How to verify the vulnerability exists

• Run the vulnerable server example and connect two clients.
• In a stateless deployment, open two SSE streams or two WebStandard requests.
• If the same transport is reused, the second client should observe cross-session data or shared stream events.
• A robust verification is to compare the message IDs and session IDs returned by the transport; if they overlap or are reused across independent requests, the vulnerability is present.

5. Recommendations

Mitigation strategies

• Upgrade MCP TypeScript SDK to version 1.26.0 or later.
• For stateless endpoints, instantiate a fresh transport and McpServer per request.
• If you need shared sessions, use sessionIdGenerator to assign unique session IDs and avoid global reuse.
• Do not reuse a Protocol or transport object across multiple independent client connections.

Detection methods

• Look for errors from the new guard: Stateless transport cannot be reused across requests. Create a new transport per request.
• Monitor server logs for multiple connect() calls on the same protocol instance without close().
• Add regression tests that create multiple clients against stateless transports and verify isolation.
• Use request fuzzing or instrumentation to detect cross-session deliveries.

Best practices to prevent similar issues

• Treat transport objects as connection-scoped resources, not global singletons.
• Enforce explicit lifecycle management: connect, handle request, close.
• Implement per-session state separation in server frameworks.
• Add tests for session isolation and protocol reuse semantics.
• Avoid shared mutable state in protocols and transport layers when building stateless HTTP endpoints.

Summary

• CVE-2026-25536 is a legitimate cross-session isolation flaw in the MCP SDK.
• The patch fixes both the library and example usage patterns by enforcing per-request transports and stronger connection semantics.
• Upgrading to 1.26.0 and adopting proper session/transport handling is the correct remediation.