SSRF (Server-Side Request Forgery) in fastmcp (CVE-2026-32871)
Summary
SSRF in fastmcp (CVE-2026-32871). Successful exploitation can lead to full system takeover. Exploitable via Authorization header. Mitigation: upgrade to 3.2.0 or later.
Original description
The description as published by the source.
FastMCP is a Pythonic way to build MCP servers and clients. Prior to version 3.2.0, the OpenAPIProvider in FastMCP exposes internal APIs to MCP clients by parsing OpenAPI specifications. The RequestDirector class is responsible for constructing HTTP requests to the backend service. A vulnerability exists in the _build_url() method. When an OpenAPI operation defines path parameters (e.g., /api/v1/users/{user_id}), the system directly substitutes parameter values into the URL template string without URL-encoding. Subsequently, urllib.parse.urljoin() resolves the final URL. Since urljoin() interprets ../ sequences as directory traversal, an attacker controlling a path parameter can perform path traversal attacks to escape the intended API prefix and access arbitrary backend endpoints. This results in authenticated SSRF, as requests are sent with the authorization headers configured in the MCP provider. This issue has been patched in version 3.2.0.
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-2026-32871 has been found in fastmcp.
Attackers can target a specific entry point like Authorization header over the network to misuse the product.
Successful exploitation can lead to full system takeover. CVSS score: 10.0/10.
What to do: upgrade fastmcp to 3.2.0 or later.
If unsure, ask your IT team or search "fastmcp CVE-2026-32871" on the vendor's site.
CVE-2026-32871 (fastmcp) — CWE-918 / CVSS v3 10.0
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: Authorization header / OpenAPIProvider / RequestDirector / internal_api.py
Patched: 3.2.0 — 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.
SSRF (CWE-918) exists in fastmcp. Attackers reach the vulnerable code path via Authorization header without authentication.
Check the vendor advisory for the affected versions of fastmcp. Attack surface: Authorization header / OpenAPIProvider / RequestDirector / internal_api.py.
Severity: Critical (CVSS 10.0/10). Successful exploitation can lead to full system takeover.
Update to 3.2.0.
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 'fastmcp' . 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 'fastmcp' . | 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
fastmcp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 fastmcp to 3.2.0ステージング環境で 3.2.0 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。
-
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 github.com
- advisory access.redhat.com
- advisory access.redhat.com
- advisory nvd.nist.gov
- advisory github.com
- advisory github.com
- package github.com
- package pypi.org
- patch github.com
- patch github.com
- report bugzilla.redhat.com
- web github.com
- web security.access.redhat.com