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High, CVSS 7.6amended

Model Context Protocol (MCP) Python SDK does not enable DNS rebinding protection by default

CVE-2025-66416GHSA-9h52-p55h-vw2f

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (mcp 1.23.0), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • PyPI mcp

    Affected
    < 1.23.0
    Fixed in
    1.23.0

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
7.6 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2025-66416: 0.54% estimated exploitation activity in the next 30 days; percentile 45.72 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Description The Model Context Protocol (MCP) Python SDK does not enable DNS rebinding protection by default for HTTP-based servers. When an HTTP-based MCP server is run on localhost without authentication using FastMCP with streamable HTTP or SSE transport, and has not configured TransportSecuritySettings, a malicious website could exploit DNS rebinding to bypass same-origin policy restrictions and send requests to the local MCP server. This could allow an attacker to invoke tools or access resources exposed by the MCP server on behalf of the user in those limited circumstances. Note that running HTTP-based MCP servers locally without authentication is not recommended per MCP security best practices. This issue does not affect servers using stdio transport. Servers created via FastMCP() now have DNS rebinding protection enabled by default when the host parameter is 127.0.0.1 or localhost. Users are advised to update to version 1.23.0 to receive this automatic protection. Users with…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-9h52-p55h-vw2f

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-9h52-p55h-vw2f

Agent brief

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# Patch Next advisory brief: CVE-2025-66416

Advisory: Model Context Protocol (MCP) Python SDK does not enable DNS rebinding protection by default
IDs: CVE-2025-66416, GHSA-9h52-p55h-vw2f
Record: https://patchnext.com/advisories/cve-2025-66416 (revision 3, source checked 26 Oct 2026, 13:46 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.6)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mcp 1.23.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI mcp: affected < 1.23.0; fixed in 1.23.0

## Suggested inspection (read-only)
- `python -m pip show mcp`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mcp.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-9h52-p55h-vw2f (OSV record GHSA-9h52-p55h-vw2f lists affected ranges: PyPI mcp >=0 <1.23.0 (fixed))
- Verified: https://github.com/advisories/GHSA-9h52-p55h-vw2f (GitHub-reviewed advisory GHSA-9h52-p55h-vw2f lists affected ranges: PyPI mcp >=0 <1.23.0)

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