Vulnerabilities in pkg:npm/mcp-handler
PACKAGEGHSA-w2fm-25vw-vh7f is a HIGH (CVSS 7.1) mcp-handler has a tool response vulnerability (CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')) affecting Mcp handler libraries in the npm ecosystem. The issue involves mcp-handler has a tool response leak across concurrent client sessions ('race condition'). Remediation is available by upgrading affected packages to version 1.1.0.
pkg:npm/mcp-handler
Vulnerability advisories affecting package pkg:npm/mcp-handler. Select an advisory below to view technical details or fixes.
| Advisory ID | CVE / NVD | Severity | Summary |
|---|---|---|---|
| GHSA-w2fm-25vw-vh7f | - | HIGH | mcp-handler has a tool response leak across concurrent client sessions ('Race Condition') |
GHSA-w2fm-25vw-vh7f at a Glance
Security Impact
CVSS: 7.1
The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N).
How to Fix GHSA-w2fm-25vw-vh7f
To remediate this vulnerability, upgrade all affected Mcp handler dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.
npm install mcp-handler@1.1.0Run the following command in your project directory to verify that all resolved dependencies use the patched release:
npm ls mcp-handlerHow to Check if You Are Affected
To determine whether your Node.js application is affected by GHSA-w2fm-25vw-vh7f, inspect your project dependencies using npm:
npm ls mcp-handlerAnalyze the terminal output to locate any version of Mcp handler in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.1.0), your application is affected.
Note: The npm ls mcp-handler command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.
Technical Details
Vulnerability Mechanism & Technical Analysis
This vulnerability is classified under CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition'), where the product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently. In Mcp handler, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.
Source Advisory Statement
GHSA-w2fm-25vw-vh7f Affected Mcp handler Versions
1 component(s)| Product / Package | Ecosystem | Affected Versions | Fixed Version |
|---|---|---|---|
|
mcp-handler
|
npm |
< 1.1.0
|
1.1.0
|
References
Frequently Asked Questions
What is GHSA-w2fm-25vw-vh7f?
Which Mcp handler versions are affected?
What version fixes GHSA-w2fm-25vw-vh7f?
Is mcp-handler affected?
What is the CVSS score of GHSA-w2fm-25vw-vh7f?
What CWE is associated with GHSA-w2fm-25vw-vh7f?
How can I check npm dependencies for GHSA-w2fm-25vw-vh7f?
Machine-readable OSV JSON
OSV v1.5 Schema
{
"schema_version" : "1.4.0",
"id" : "GHSA-w2fm-25vw-vh7f",
"modified" : "2026-04-01T23:58:50Z",
"published" : "2026-04-01T23:58:50Z",
"aliases" : [ ],
"summary" : "mcp-handler has a tool response leak across concurrent client sessions ('Race Condition')",
"details" : "`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](https://nvd.nist.gov/vuln/detail/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.\n\n**Note:** This is _not_ a vulnerability in `mcp-handler` itself. The root cause is in the peer dependency `@modelcontextprotocol/sdk`. \n\n### Impact\n\nA 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.\n\n**Root Cause:** [CVE-2026-25536](https://nvd.nist.gov/vuln/detail/CVE-2026-25536) in `@modelcontextprotocol/sdk` < 1.26.0. The SDK did not prevent reuse of stateless transports across client connections.\n\n### Patches\n\nUpgrade to `mcp-handler@1.1.0`. This release raises the minimum peer dependency to `@modelcontextprotocol/sdk@>=1.26.0`, which contains the fix for CVE-2026-25536. \n\n### Workarounds\n\n- 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)\n- Alternatively, manually create fresh `McpServer` and transport instances per request in your handler code",
"severity" : [ {
"type" : "CVSS_V3",
"score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N"
} ],
"affected" : [ {
"package" : {
"ecosystem" : "npm",
"name" : "mcp-handler"
},
"ranges" : [ {
"type" : "ECOSYSTEM",
"events" : [ {
"introduced" : "0"
}, {
"fixed" : "1.1.0"
} ]
} ]
} ],
"references" : [ {
"type" : "WEB",
"url" : "https://github.com/vercel/mcp-handler/security/advisories/GHSA-w2fm-25vw-vh7f"
}, {
"type" : "ADVISORY",
"url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-25536"
}, {
"type" : "ADVISORY",
"url" : "https://github.com/advisories/GHSA-345p-7cg4-v4c7"
}, {
"type" : "PACKAGE",
"url" : "https://github.com/vercel/mcp-handler"
} ],
"database_specific" : {
"cwe_ids" : [ "CWE-1395", "CWE-362" ],
"severity" : "HIGH",
"github_reviewed" : true,
"github_reviewed_at" : "2026-04-01T23:58:50Z",
"nvd_published_at" : null
}
}