CVE-2026-25536
📋 TL;DR
The CVE-2026-25536 vulnerability in the MCP TypeScript SDK allows cross-client response data leakage when a single server/transport instance is reused across multiple client connections. This affects deployments using stateless StreamableHTTPServerTransport, potentially exposing sensitive data between clients. Organizations using MCP TypeScript SDK versions 1.10.0 through 1.25.3 in multi-client environments are impacted.
💻 Affected Systems
- MCP TypeScript SDK
⚠️ Risk & Real-World Impact
Worst Case
Sensitive client data (authentication tokens, private messages, proprietary information) is exposed to unauthorized clients, leading to data breaches, privilege escalation, and compliance violations.
Likely Case
Accidental data leakage between clients sharing the same server instance, potentially exposing session data, configuration details, or partial application state.
If Mitigated
Minimal impact with proper isolation between client connections or when using single-client deployments.
🎯 Exploit Status
Exploitation requires access to the vulnerable server but doesn't require authentication. Attack complexity is low as it leverages improper session/data isolation.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.26.0
Vendor Advisory: https://github.com/modelcontextprotocol/typescript-sdk/security/advisories/GHSA-345p-7cg4-v4c7
Restart Required: Yes
Instructions:
1. Update MCP TypeScript SDK to version 1.26.0 or later using npm update @modelcontextprotocol/sdk. 2. Restart all affected services. 3. Verify no regressions in client-server communication.
🔧 Temporary Workarounds
Implement client isolation
allCreate separate server instances for each client connection instead of reusing instances
Use stateful transports
allAvoid stateless StreamableHTTPServerTransport in multi-client deployments
🧯 If You Can't Patch
- Implement strict network segmentation to isolate vulnerable servers from untrusted networks
- Deploy application-level monitoring to detect unusual cross-client data access patterns
🔍 How to Verify
Check if Vulnerable:
Check package.json for @modelcontextprotocol/sdk version between 1.10.0 and 1.25.3, and verify if server instances are reused across client connections.Check Version:
npm list @modelcontextprotocol/sdkVerify Fix Applied:
Confirm @modelcontextprotocol/sdk version is 1.26.0 or higher in package.json and test that client data remains isolated during concurrent connections.📡 Detection & Monitoring
Log Indicators:
- Multiple client sessions sharing server instance IDs
- Unexpected data appearing in client responses
- Cross-client request/response patterns
Network Indicators:
- Multiple clients receiving identical or overlapping response data
- Unusual data patterns in HTTP responses
SIEM Query:
source="application_logs" AND ("cross-client" OR "data leak" OR "session overlap")