Sign in Sign up
JA · EN · FR
SecPulse
Categories
CVE-2025-69196 medium CVSS 6.5

Authorization Flaw in fastmcp (CVE-2025-69196)

Summary

Vulnerability in fastmcp (CVE-2025-69196). Confidential information can be exposed externally. Exploitable via resource. Mitigation: upgrade to 2.14.2 or later.

Original description

The description as published by the source.

While testing the OAuth Proxy implementation, it was noticed that the server does not properly respect the resource parameter submitted by the client in the authorization and token request. Instead of issuing the token explicitly for this MCP server, the token is issued for the base_url passed to the OAuthProxy during initialization.

Affected File:
https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L828

Affected Code:

self._jwt_issuer: JWTIssuer = JWTIssuer(
    issuer=str(self.base_url),
    audience=f"{str(self.base_url).rstrip('/')}/mcp",
    signing_key=jwt_signing_key,
)

Since the issued access and refresh tokens do not include information about the resource the token was issued for, it is impossible for the MCP server to properly verify whether the token was issued for it, hence violating the requirement of doing so demanded by the specification. Being able to verify whether the token was issued for the target MCP server enforces the protections offered by the steps proposed by the specification and the Resource Indicators OAuth extension.

Therefore, this misconfiguration exposes all MCP server setups using the FastMCP OAuth Proxy to an attack where an adversary creates a malicious MCP server that advertises the benign OAuth Proxy authorization server as its own authorization server. Once a vict…

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-2025-69196 has been found in fastmcp.
Attackers can target a specific entry point like resource over the network to misuse the product.
Confidential information can be exposed externally. CVSS score: 6.5/10.
What to do: upgrade fastmcp to 2.14.2 or later.
If unsure, ask your IT team or search "fastmcp CVE-2025-69196" on the vendor's site.

CVE-2025-69196 (fastmcp) — CWE-863 / CVSS v3 6.5
Attack vector: remote (network-reachable) / unauthenticated
Attack surface: resource / base_url / OAuthProxy
Patched: 2.14.2 — 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.

❓ What is the problem

Incorrect authorization (CWE-863) exists in fastmcp. Attackers reach the vulnerable code path via resource without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of fastmcp. Attack surface: resource / base_url / OAuthProxy.

🔥 Severity

Severity: Medium (CVSS 6.5/10). Confidential information can be exposed externally.

🔧 How to fix

Update to 2.14.2.

🛡️ 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 '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

  1. 1
    Identify 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。

  2. 2
    Apply patch patch 
    Upgrade fastmcp to 2.14.2

    ステージング環境で 2.14.2 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。

  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

PyPI fastmcp
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"2.14.2"}]}]

References

We use cookies for analytics and ad measurement. This setting applies to all Game co., Ltd. sites. Details