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Fencer Intel
high 7.1

GHSA-w2fm-25vw-vh7f json

mcp-handler has a tool response leak across concurrent client sessions ('Race Condition')

published 7mo ago modified 7mo ago

cvss
7.1
version 3.1, GitHub
cisa kev
Not listed
not in CISA's catalog
epss
—
no score, EPSS covers CVEs only
public exploits
0
none known

Summary

`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.

**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](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.

### 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

Patch / fix

  • npm mcp-handler 1.1.0

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CVSS scores

assigned by version score vector
GitHub 3.1 7.1 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:L/A:N

Affected packages

ecosystem package affected fixed in
npm mcp-handler
<1.1.0
1.1.0

Classification

Weakness
  • CWE-1395 Dependency on Vulnerable Third-Party Component
  • CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

The maintainers of vercel-labs/mcp-handler published their own advisory on GitHub. Reviewed sources take precedence over it.