CVE-2025-53366Public exploitNetwork attack vector

Service halt via uncaught request validation

Published Jul 4, 2025 · Updated Jul 8, 2025

Uncaught request-validation exceptions in MCP Python SDK before 1.9.4 allow remote attackers to halt FastMCP HTTP service. BaseSession._receive_loop lets Pydantic validation errors from JSON-RPC requests escape its task group instead of returning an invalid-parameters response. A single unauthenticated tools/call request with a string arguments value can leave every endpoint returning HTTP 500 until the process is restarted.

CVSS severity8.7
High
EPSS probability7.26%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Uncaught request-validation exceptions in MCP Python SDK before 1.9.4 allow remote attackers to halt FastMCP HTTP service. BaseSession._receive_loop lets Pydantic validation errors from JSON-RPC requests escape its task group instead of returning an invalid-parameters response. A single unauthenticated tools/call request with a string arguments value can leave every endpoint returning HTTP 500 until the process is restarted.

The record

CVE
CVE-2025-53366
Published
Jul 4, 2025
Updated
Jul 8, 2025
Vendor
Model Context Protocol
Product
MCP Python SDK
Classifications
CWE-248, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 4, 2025CVE publishedPublication date reported by the CVE source.
  2. Jul 8, 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=< 1.9.4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Model Context Protocol · MCP Python SDKversion=< 1.9.4

Published CVSS scores

8.7GitHubCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA: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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-248Uncaught Exception
T1499.004Application or System Exploitation

Public exploit references

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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