CVE-2025-66416Network attack vector

Unauthorized tool invocation via missing DNS rebinding protection

Published Dec 2, 2025 · Updated Dec 2, 2025

DNS rebinding in FastMCP in Model Context Protocol MCP Python SDK before 1.23.0 allows remote attackers to invoke tools via malicious websites. FastMCP leaves TransportSecuritySettings unset by default, so localhost HTTP and SSE servers accept rebinding-origin requests instead of restricting Host and Origin values. Exploitation requires an unauthenticated localhost server, no explicit transport security, and browser access to an attacker-controlled site; stdio servers are unaffected.

CVSS severity7.6
High
EPSS probability0.51%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

DNS rebinding in FastMCP in Model Context Protocol MCP Python SDK before 1.23.0 allows remote attackers to invoke tools via malicious websites. FastMCP leaves TransportSecuritySettings unset by default, so localhost HTTP and SSE servers accept rebinding-origin requests instead of restricting Host and Origin values. Exploitation requires an unauthenticated localhost server, no explicit transport security, and browser access to an attacker-controlled site; stdio servers are unaffected.

The record

CVE
CVE-2025-66416
Published
Dec 2, 2025
Updated
Dec 2, 2025
Vendor
Model Context Protocol
Product
MCP Python SDK
Classifications
CWE-1188, CWE-350, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 2, 2025CVE publishedPublication date reported by the CVE source.
  2. Dec 2, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.23.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Model Context Protocol · MCP Python SDKversion=< 1.23.0

Published CVSS scores

7.6GitHubCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
8.1GitLab Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-1188Initialization of a Resource with an Insecure Default
CWE-350Reliance on Reverse DNS Resolution for a Security-Critical Action
T1189Drive-by Compromise

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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