Vulnerability in mcp (CVE-2026-52869)
Summary
Vulnerability in mcp (CVE-2026-52869). Confidential information can be exposed externally. Exploitable via session_id. Mitigation: upgrade to 1.27.2 or later.
Original description
The description as published by the source.
Summary
In affected versions, the SSE and Streamable HTTP server transports routed incoming requests to an existing session based only on the session identifier, without verifying that the request was authenticated as the same principal that created the session. Anyone who learned or guessed a session ID could send JSON-RPC messages on that session, regardless of which bearer token the request carried.
Am I affected?
Only if a developer's application server uses an HTTP transport (SSE, or Streamable HTTP in stateful mode) and authenticates requests. Servers on stdio, stateless Streamable HTTP, or with no authentication configured are not affected.
Details
Both transports look up the target session by its identifier alone — the session_id query parameter for SSE (mcp.server.sse.SseServerTransport) and the Mcp-Session-Id header for Streamable HTTP (mcp.server.streamable_http_manager.StreamableHTTPSessionManager). Once the lookup succeeded, the request was handled on that session without comparing its authentication context to the credentials presented when the session was created, so a request authenticated as a different OAuth client could inject messages into the session. On the SSE transport the response is delivered to the original client's event stream; on the Streamable HTTP transport it is returned on the injecting request, so the injecting client can also read the result. The SSE transport has been affected since the first release; the Streamab…
Source: NIST National Vulnerability Database 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-2026-52869 has been found in mcp.
Attackers can target a specific entry point like session_id over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 7.1/10.
What to do: upgrade mcp to 1.27.2 or later.
If unsure, ask your IT team or search "mcp CVE-2026-52869" on the vendor's site.
CVE-2026-52869 (mcp) — CWE-639 / CVSS v3 7.1
Attack vector: remote (network-reachable) / no user interaction
Attack surface: session_id / mcp.server.sse.SseServerTransport / AccessToken.subject / sub
Patched: 1.27.2 — 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.
A vulnerability (CWE-639) exists in mcp. Attackers reach the vulnerable code path via session_id.
Check the vendor advisory for the affected versions of mcp. Attack surface: session_id / mcp.server.sse.SseServerTransport / AccessToken.subject / sub.
Severity: High (CVSS 7.1/10). Confidential information can be exposed externally.
Update to 1.27.2.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
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
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1Identify 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。 -
2Apply patch patch
Upgrade mcp to 1.27.2ステージング環境で 1.27.2 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。
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3Post-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
References
- advisory nvd.nist.gov
- advisory github.com
- package github.com
- package pypi.org
- web github.com
- web github.com
- web github.com
- web github.com
- web github.com
- web github.com