GHSA-w2fm-25vw-vh7f
HIGHmcp-handler has a tool response leak across concurrent client sessions ('Race Condition')
Blast Radius
mcp-handlerReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects npm packages — download data is not available via public APIs for these ecosystems.
Description
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 [email protected]. This release raises the minimum peer dependency to @modelcontextprotocol/sdk@>=1.26.0, which contains the fix for CVE-2026-25536.
Workarounds
- Upgrade
@modelcontextprotocol/sdkto>=1.26.0(note: the SDK will throw on transport reuse, which will breakmcp-handler< 1.1.0 which effectively forces the upgrade) - Alternatively, manually create fresh
McpServerand transport instances per request in your handler code
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | mcp-handler | all versions | 1.1.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for mcp-handler. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update mcp-handler to 1.1.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-w2fm-25vw-vh7f is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-w2fm-25vw-vh7f is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-w2fm-25vw-vh7f. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-w2fm-25vw-vh7f in your dependencies?
O3 detects GHSA-w2fm-25vw-vh7f across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.