GHSA-W2FM-25VW-VH7F

mcp-handler versions prior to 1.1.0 accepted @modelcontextprotocol/sdk < 1.26.0 as a peer dependency. That SDK version contains a vulnerability [CVE-2026-25536] that causes concurrent requests from different clients to share server-side state including authentication context and tool execution results when a StreamableHTTPServerTransport instance is reused across requests.

Note: This is not a vulnerability in mcp-handler itself. The root cause is in the peer dependency @modelcontextprotocol/sdk.

Workarounds

• Upgrade @modelcontextprotocol/sdk to >=1.26.0 (note: the SDK will throw on transport reuse, which will break mcp-handler < 1.1.0 which effectively forces the upgrade)
• Alternatively, manually create fresh McpServer and transport instances per request in your handler code

A low-privileged attacker making concurrent requests to an mcp-handler endpoint can read another client's session data, including authentication information and tool execution state. This is a confidentiality breach with potential for limited integrity impact.

Root Cause: CVE-2026-25536 in @modelcontextprotocol/sdk < 1.26.0. The SDK did not prevent reuse of stateless transports across client connections.

Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing. Typical impact: TOCTOU exploits, data corruption, or privilege escalation.

GHSA-W2FM-25VW-VH7F has a CVSS score of 7.1 (High). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.1.0); upgrading removes the vulnerable code path.

Kodem intelligence

Upgrade to [email protected]. This release raises the minimum peer dependency to @modelcontextprotocol/sdk@>=1.26.0, which contains the fix for CVE-2026-25536.

1. What is GHSA-W2FM-25VW-VH7F? GHSA-W2FM-25VW-VH7F is a high-severity race condition vulnerability in mcp-handler (npm), affecting versions < 1.1.0. It is fixed in 1.1.0. Multiple concurrent operations access a shared resource without proper synchronization, producing unpredictable results depending on timing.
2. How severe is GHSA-W2FM-25VW-VH7F? GHSA-W2FM-25VW-VH7F has a CVSS score of 7.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of mcp-handler are affected by GHSA-W2FM-25VW-VH7F? mcp-handler (npm) versions < 1.1.0 is affected.
4. Is there a fix for GHSA-W2FM-25VW-VH7F? Yes. GHSA-W2FM-25VW-VH7F is fixed in 1.1.0. Upgrade to this version or later.
5. Is GHSA-W2FM-25VW-VH7F exploitable, and should I be worried? Whether GHSA-W2FM-25VW-VH7F is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether GHSA-W2FM-25VW-VH7F is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix GHSA-W2FM-25VW-VH7F? Upgrade mcp-handler to 1.1.0 or later.