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CVE-2026-59951 high CVSS 7.5

Vulnerability in mcp (CVE-2026-59951)

Summary

Vulnerability in mcp (CVE-2026-59951). Risk of unauthorized operations or information disclosure. Exploitable via DELETE. Mitigation: upgrade to 2.2.0 or later.

Original description

The description as published by the source.

Summary

In affected versions, a Streamable HTTP server in stateful mode (the default) never reclaimed session state:

  • sessions did not expire;
  • a client DELETE ended the session but left its entry registered;
  • a session whose first request was refused stayed registered with a running task.

Each session also kept its parsed initialize parameters, and the number of sessions was unlimited. A client that could reach the endpoint could grow server memory without bound using ordinary short requests; only a restart reclaimed it.

Am I affected?

Yes, if you serve MCP over Streamable HTTP in stateful mode and clients you do not fully trust can reach the endpoint. Stateful mode is the default for:

  • FastMCP (1.x)
  • MCPServer and Server.streamable_http_app() (2.x)
  • StreamableHTTPSessionManager

Affected versions: 1.8.0 through 1.29.1, and 2.0.0 through 2.1.1.

Not affected:

  • servers on 1.12.0 or later passing stateless_http=True (no sessions are kept)
  • servers using only the stdio or SSE transports

Authentication limits who can open sessions, not how many a token holder can open.

Fix

Upgrade to mcp 1.30.0 (1.x) or 2.2.0 (2.x) or later. In those versions:

  • a session with no request in flight for session_idle_timeout seconds (default 1800) is closed and its ID then answers 404; open streams and in-progress requests keep it alive;
  • at most max_sessions sessions exist at once (default 10000); a request that would open another is answered `503…

Source: GitHub Security Advisories (Global) NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-59951 has been found in mcp.
Attackers can target a specific entry point like DELETE over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 7.5/10.
What to do: upgrade mcp to 2.2.0 or later.
If unsure, ask your IT team or search "mcp CVE-2026-59951" on the vendor's site.

CVE-2026-59951 (mcp) — CWE-400 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: DELETE / initialize / FastMCP / MCPServer
Patched: 2.2.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-400) exists in mcp. Attackers reach the vulnerable code path via DELETE without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of mcp. Attack surface: DELETE / initialize / FastMCP / MCPServer.

🔥 Severity

Severity: High (CVSS 7.5/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 2.2.0.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'mcp' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'mcp' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で mcp を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade mcp to 2.2.0

    ステージング環境で 2.2.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

pip mcp
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.2.0"}]}]
PyPI mcp
[{"type":"ECOSYSTEM","events":[{"introduced":"2.0.0"},{"fixed":"2.2.0"}]}]

References

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