How severe is GHSA-5h2m-4q8j-pqpj?

No CVSS score has been assigned to GHSA-5h2m-4q8j-pqpj yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by GHSA-5h2m-4q8j-pqpj?

GHSA-5h2m-4q8j-pqpj affects the following packages: fastmcp (PyPI). Ecosystems affected: PyPI.

How do I fix GHSA-5h2m-4q8j-pqpj?

Update fastmcp to 2.14.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-5h2m-4q8j-pqpj is resolved across your whole dependency graph.

How do I detect GHSA-5h2m-4q8j-pqpj in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastmcp. 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.

How do I mitigate GHSA-5h2m-4q8j-pqpj if there is no patch (or I can't update yet)?

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 does O3 Security protect against GHSA-5h2m-4q8j-pqpj?

O3 pinpoints whether GHSA-5h2m-4q8j-pqpj is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is GHSA-5h2m-4q8j-pqpj actively exploited in the wild?

No public exploit code has been indexed for GHSA-5h2m-4q8j-pqpj yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What type of vulnerability is GHSA-5h2m-4q8j-pqpj?

GHSA-5h2m-4q8j-pqpj is classified as CWE-863 (CWE-863), CWE-1220 (CWE-1220). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was GHSA-5h2m-4q8j-pqpj published, and has it been updated?

GHSA-5h2m-4q8j-pqpj was published on March 16, 2026 and was last updated on July 13, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍 PyPI

GHSA-5h2m-4q8j-pqpj

GHSA-5h2m-4q8j-pqpj is a CWE-863 vulnerability in fastmcp. O3 Security confirms whether GHSA-5h2m-4q8j-pqpj is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

FastMCP OAuth Proxy token reuse across MCP servers

Also known asCVE-2025-69196PYSEC-2026-2474

Published

Mar 16, 2026

Updated

Jul 13, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Real-World Exposure

1 pkg affected

🐍fastmcp

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

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 victim completes an OAuth flow with this malicious MCP server, authenticating against the AS, the adversary can extract the token received at the malicious MCP server and use it to access other MCP servers (the benign ones) that also use the same AS, including the tools and resources they expose.

Steps to reproduce:

Extract the provided PoC environment.
Enter the client_id and client_secret of a GitHub App you control into the mcp-server-proxy.py script.
Start the benign MCP server using an OAuth Proxy (in this case the GitHubProvider): python3 mcp-server-proxy.py.
Start the malicious AS: python3 mal_auth_server.py.
Start the malicious MCP server: python3 attacker_server.py.
Connect the client to the malicious MCP server: python3 client.py.
Complete the OAuth flow.
Observe in the logs of the malicious MCP server that the request to the benign MCP server with the stolen token returned a 200 status code.

Impact

This vulnerability allows an adversary to steal a victim’s authentication material for a benign MCP server using the FastMCP OAuth Proxy. The severity of this issue was decreased to Medium due to the consent screen showing the name of the MCP server the OAuth Proxy was intended for. However, a victim might not see it or get otherwise convinced by the attacker to ignore it, and overall this does not act as a proper mitigation for this issue.

Mitigation

To mitigate this vulnerability, it is recommended to issue tokens specifically for the MCP server submitted in the authorization URL’s resource GET parameter. In this way, the receiving MCP server will be able to properly verify that the token was indeed issued for it, allowing it to reject tokens stolen by an attack like the one demonstrated above.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`fastmcp`	all versions	2.14.2

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastmcp. 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 fastmcp to 2.14.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-5h2m-4q8j-pqpj 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-5h2m-4q8j-pqpj 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-5h2m-4q8j-pqpj. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

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:** ```python self._jwt_issuer: JWTIssuer = JWTIssuer( issuer=str(self.base_url), audience=f"{str(self.base_url).rstrip('/'

O3 Security · Impact-Aware SCA

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