Vulnerability-Lookup

CVE-2025-69196 (GCVE-0-2025-69196)

Vulnerability from cvelistv5 – Published: 2026-03-16 18:07 – Updated: 2026-06-30 03:20

Title

FastMCP OAuth Proxy token reuse across MCP servers

Summary

FastMCP is the standard framework for building MCP applications. Prior to version 2.14.2, 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 the MCP server, the token is issued for the base_url passed to the OAuthProxy during initialization. This issue has been patched 2.14.2.

Severity

7.4 (High)


                        
                          CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:A/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

7.4 (High)


                        
                          CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

SSVC

Exploitation: poc Automatable: no Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-863 - Incorrect Authorization
CWE-1220 - Insufficient Granularity of Access Control

Assigner

GitHub_M

References

4 references

URL	Tags
https://github.com/PrefectHQ/fastmcp/security/adv…	x_refsource_CONFIRM
https://access.redhat.com/security/cve/CVE-2025-69196	vdb-entryx_refsource_REDHAT
https://bugzilla.redhat.com/show_bug.cgi?id=2448179	issue-trackingx_refsource_REDHAT
https://security.access.redhat.com/data/csaf/v2/v…	x_sadp-csaf-vex

Impacted products

4 products

Vendor	Product	Version
jlowin	fastmcp	Affected: < 2.14.2
Red Hat	Red Hat Satellite 6	cpe:/a:redhat:satellite:6
Red Hat	Red Hat Developer Hub	cpe:/a:redhat:rhdh:1
Red Hat	Red Hat OpenShift AI (RHOAI)	cpe:/a:redhat:openshift_ai

Show details on NVD website

JSON

To clipboard

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FKIE_CVE-2025-69196

Vulnerability from fkie_nvd - Published: 2026-03-16 19:16 - Updated: 2026-06-30 03:16

Severity

6.5 (Medium) - CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
7.4 (High) - CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Summary

References

URL	Tags
security-advisories@github.com	https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-5h2m-4q8j-pqpj	Exploit, Mitigation, Vendor Advisory
0b0ca135-0b70-47e7-9f44-1890c2a1c46c	https://access.redhat.com/security/cve/CVE-2025-69196
0b0ca135-0b70-47e7-9f44-1890c2a1c46c	https://bugzilla.redhat.com/show_bug.cgi?id=2448179
0b0ca135-0b70-47e7-9f44-1890c2a1c46c	https://security.access.redhat.com/data/csaf/v2/vex/2025/cve-2025-69196.json

Impacted products

	Vendor	Product	Version
	jlowin	fastmcp	*

JSON

To clipboard

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          "defaultStatus": "unaffected",
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        }
      ],
      "source": "0b0ca135-0b70-47e7-9f44-1890c2a1c46c"
    }
  ],
  "configurations": [
    {
      "nodes": [
        {
          "cpeMatch": [
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              "criteria": "cpe:2.3:a:jlowin:fastmcp:*:*:*:*:*:*:*:*",
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              "versionEndExcluding": "2.14.2",
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  "cveTags": [],
  "descriptions": [
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      "value": "FastMCP is the standard framework for building MCP applications. Prior to version 2.14.2, 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 the MCP server, the token is issued for the base_url passed to the OAuthProxy during initialization. This issue has been patched 2.14.2."
    },
    {
      "lang": "es",
      "value": "FastMCP es el framework est\u00e1ndar para construir aplicaciones MCP. Antes de la versi\u00f3n 2.14.2, el servidor no respeta adecuadamente el par\u00e1metro de recurso enviado por el cliente en la solicitud de autorizaci\u00f3n y de token. En lugar de emitir el token expl\u00edcitamente para el servidor MCP, el token se emite para la base_url pasada al OAuthProxy durante la inicializaci\u00f3n. Este problema ha sido parcheado en 2.14.2."
    }
  ],
  "id": "CVE-2025-69196",
  "lastModified": "2026-06-30T03:16:58.877",
  "metrics": {
    "cvssMetricV31": [
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          "confidentialityImpact": "HIGH",
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    "ssvcV203": [
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          "options": [
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              "exploitation": "poc"
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          "role": "CISA Coordinator",
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      "tags": [
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    },
    {
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      "type": "Primary"
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    {
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      ],
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      "type": "Secondary"
    }
  ]
}

GHSA-5H2M-4Q8J-PQPJ

Vulnerability from github – Published: 2026-03-16 15:14 – Updated: 2026-03-16 21:55

Summary

FastMCP OAuth Proxy token reuse across MCP servers

Details

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: 1. Extract the provided PoC environment. 2. Enter the client_id and client_secret of a GitHub App you control into the mcp-server-proxy.py script. 3. Start the benign MCP server using an OAuth Proxy (in this case the GitHubProvider): python3 mcp-server-proxy.py. 4. Start the malicious AS: python3 mal_auth_server.py. 5. Start the malicious MCP server: python3 attacker_server.py. 6. Connect the client to the malicious MCP server: python3 client.py. 7. Complete the OAuth flow. 8. 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.

Severity

7.4 (High)


                  
                    CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:A/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "fastmcp"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.14.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2025-69196"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-863"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-16T15:14:55Z",
    "nvd_published_at": "2026-03-16T19:16:14Z",
    "severity": "HIGH"
  },
  "details": "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. \n\n**Affected File:**\n*https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/auth/oauth_proxy.py#L828*\n\n**Affected Code:**\n```python\nself._jwt_issuer: JWTIssuer = JWTIssuer(\n    issuer=str(self.base_url),\n    audience=f\"{str(self.base_url).rstrip(\u0027/\u0027)}/mcp\",\n    signing_key=jwt_signing_key,\n)\n```\n\nSince 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](https://mcp.mintlify.app/specification/2025-11-25/basic/authorization#token-audience-binding-and-validation). 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.\n\nTherefore, 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.\n\n**Steps to reproduce:**\n1. Extract the provided [PoC environment](https://github.com/user-attachments/files/23839983/improper_resource_validation_fastmcp.tgz).\n2. Enter the *client_id* and *client_secret* of a GitHub App you control into the `mcp-server-proxy.py` script.\n3. Start the benign MCP server using an OAuth Proxy (in this case the *GitHubProvider*): `python3 mcp-server-proxy.py`.\n4. Start the malicious AS: `python3 mal_auth_server.py`.\n5. Start the malicious MCP server: `python3 attacker_server.py`.\n6. Connect the client to the malicious MCP server: `python3 client.py`.\n7. Complete the OAuth flow.\n8. 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.\n\n## Impact\n\nThis vulnerability allows an adversary to steal a victim\u2019s 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.\n\n## Mitigation\n\nTo mitigate this vulnerability, it is recommended to issue tokens specifically for the MCP server submitted in the authorization URL\u2019s `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.",
  "id": "GHSA-5h2m-4q8j-pqpj",
  "modified": "2026-03-16T21:55:10Z",
  "published": "2026-03-16T15:14:55Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/PrefectHQ/fastmcp/security/advisories/GHSA-5h2m-4q8j-pqpj"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-69196"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/PrefectHQ/fastmcp"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:A/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "FastMCP OAuth Proxy token reuse across MCP servers"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

CVE-2025-69196 (GCVE-0-2025-69196)

FKIE_CVE-2025-69196

GHSA-5H2M-4Q8J-PQPJ

Impact

Mitigation

Tags

Sightings

Nomenclature