GHSA-w2fm-25vw-vh7f

Source

https://github.com/advisories/GHSA-w2fm-25vw-vh7f

Import Source

https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/04/GHSA-w2fm-25vw-vh7f/GHSA-w2fm-25vw-vh7f.json

JSON Data

https://api.osv.dev/v1/vulns/GHSA-w2fm-25vw-vh7f

Published

2026-04-01T23:58:50Z

Modified

2026-04-02T13:35:40.553290Z

Severity

7.1 (High) CVSS_V3 - CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N CVSS Calculator

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] 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.

Impact

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.

Patches

Upgrade 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.

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

Database specific

{
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-01T23:58:50Z",
    "severity": "HIGH",
    "nvd_published_at": null,
    "cwe_ids": [
        "CWE-1395",
        "CWE-362"
    ]
}

References

Affected packages

npm / mcp-handler

Package

Name: mcp-handler; View open source insights on deps.dev
Purl: pkg:npm/mcp-handler

Affected ranges

Type: SEMVER
Events: Introduced

0Unknown introduced version / All previous versions are affected

Fixed

1.1.0

Database specific

source

"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/04/GHSA-w2fm-25vw-vh7f/GHSA-w2fm-25vw-vh7f.json"